Chapter 7 Basic Plant Pathology - Clemson Universitymedia.clemson.edu/.../ch_7_plant_pathology.pdf · Basic Plant Pathology 152 Basic Plant Pathology Introduction “What is wrong

Basic Plant Pathology ◆ 151

Anthony P. Keinath, Ph.D., Meg R. Williamson, Bob Polomski,

and R. Walker Miller, Ph.D.

Chapter 7

Basic Plant Pathology

Learning Objectives

◆ Define the term plant disease.

◆ Understand the definition and concept of the disease triangle.

◆ Be familiar with symptoms and signs of diseases.

◆ Define and describe the biotic and abiotic causes of plant diseases.

◆ Have a basic understanding of how diseases are caused by fungi.

◆ Have a basic understanding of how diseases are caused by bacteria and how bacterial caused diseases are spread.

◆ Have a basic understanding of how diseases are caused by viruses and how viruses are spread.

◆ Have a basic understanding of how diseases are caused by nematodes.

◆ Understand how to use different methods to prevent and control plant diseases.


Basic Plant Pathology

Introduction“What is wrong with my plant?” is a familiar

cry to any gardener. A variety of agents can harm plants or cause plant problems. These problems may be temporary or permanent; a one-time event (an injury) or an on-going condition (a disease). Plant problems may be negligible or, on a national or global scale, cause millions of dollars in losses and require millions of dollars to combat. One recent example is sudden oak death (abbreviated SOD), which has destroyed tens of thousands of native oaks in California and Oregon and caused millions of dollars in losses to nurseries that produce woody ornamentals.

Plant disease is a process by which living or nonliving entities interfere with a plant’s functions over a period of time. This interference may cause changes in the plant’s appearance and/or bring about a lower yield than a plant of the same variety that is not diseased. Plant disease can be indicated by:

• smaller leaves• fewer leaves• smaller root system• shorter internodes• smaller annual ring width of trees• fewer or smaller fruit• blemished plant parts• death of plant parts.The change in a plant’s appearance is relative

to a plant of the same age and variety that is not diseased. One usually must know what the plant normally looks like before determining that it does or does not have a disease.

How Disease Affects Plant Health

Disease changes plants in the following ways:1. The green plant manufactures its own food

using sunlight, water, and carbon dioxide. If a disease reduces the amount of light reaching the leaf, if part of the leaf is killed, or if the leaf falls off prematurely, photosynthesis is inhibited. Leaf spots, powdery mildew, damage due to air pollution or pesticide toxicity, and twig blights reduce photosynthesis.

2. Roots, especially root tips, absorb nutrients and water while structurally supporting the plant. Browning, blackening, and softening of roots are typical symptoms of root damage or root dis-ease. Damage to roots results in yellowing, leaf scorching, slowing of growth, wilt, and dieback of above-ground portions of the plant. Fungi, bacteria, and nematodes, as well as excessive soil moisture, soil compaction, and excavation around the roots cause root diseases.

3. Water and nutrients absorbed by roots are transported to the trunk, branches, and leaves through vascular tissue. Water moves up the plant even to the tips of the leaves through the xylem. The food produced by the leaves moves back to supply the root cells by a parallel path down the plant through the phloem. If water and nutrient transport is disrupted, leaf tips and margins burn, leaves wilt, and roots die. Stem rots, vascular wilts, cankers, galls, and mechani-cal girdling cause inhibition of nutrient and water transport.

4. Perennials must have a food reserve to over-winter and have energy to resume growth in the spring. Nutrients are stored in roots and stems. Root, stem, and crown rots destroy food reserves


and can eventually lead to plant death.5. Diversion of food from plant growth to patho-

gen growth occurs when galls, mildews, nema-todes, rusts, and viruses develop on and in the plant.

6. Plant reproduction is inhibited in flower blights, stem blights, smuts, and many abiotic diseases.

Recognizing Symptoms and Signs

A symptom is a characteristic of a plant indicat-ing that it is diseased (Figures 7.1 and 7.2). Symp-toms may be recognizable only if healthy plants are near diseased ones. For example, if all trees appear the same in an area, it gives the impression that all is normal. However, all may be stunted by the pres-ence of nematodes on the roots or by damaging lev-els of air pollution. Some symptoms are microscopic and are not visible unless highly magnified.

External symptoms are visible on the outside of a plant and are usually obvious. Internal symptoms

are inside the plant, like discolored vascular tissue for Fusarium wilts. For these symptoms to be de-tected the plant must be cut open.

Local symptoms are those affecting a small area of the plant. Systemic symptoms affect a large por-tion of the plant or the entire plant.

Signs are parts of the disease-causing agent pres-ent in, on, or near a diseased plant. Fungal spores or mycelium and bacteria-filled ooze from cankers are signs. Signs help identify pathogens.

Conditions That Favor Diseases

Plant disease occurs only through the concur-rence of three factors that can be thought of as a set of traffic signals that will either allow disease to develop or prevent it. For a plant disease to occur, all factors must be in a “green light” situation. This relationship is most easily expressed through the dis-ease triangle (Figure 7.3).

Disease can proceed only when a susceptible host plant, a plant pathogen, and a set of environ-mental conditions favorable to the pathogen all exist in the same time and place. While one fac-tor may be most important in a particular disease and less important in another, no single factor acts alone.

Among the favorable environmental factors may be moderate to warm temperatures, high rela-tive humidity, a wet plant surface, saturated soil, or

Cercospora leaf spot, a symptom of this fungal disease on privet (Ligustrum spp.).

Examples of plant symptoms.

Figure 7.2

Marginal leaf burn

Fruit rot

Wilt

Figure 7.1

Blight

MosaicLeaf spot

Ringspot

Gall

Canker

Root rot

Credit: M. R. Williamson.


specific periods of exposure to such factors. As you might suspect, the climate of the humid Southeast often is favorable for diseases.

A plant disease is a dynamic biological process. Once the requirements of the disease triangle are met, the pathogen infects the susceptible host, and the disease begins and continues until the environ-ment changes to disfavor the pathogen.

Disease management is based on eliminating any one of these factors. Breeding a resistant host, using chemicals to destroy the organism, or altering the environment help manage diseases.

Two Types of DiseaseBiotic, or infectious, diseases are caused by liv-

ing factors called pathogens. The pathogens that cause biotic diseases reproduce, spread from plant to plant, and grow. If one plant has a biotic, infectious disease, the likelihood increases that a neighboring plant will get the disease. The five major groups of biotic pathogens are fungi, bacteria, viruses, nema-todes, and vascular plants. About 70% of biotic diseases are caused by fungi.

Pathogens infect plants and cause disease. Plants infected by pathogens have a disease. Re-member: the pathogen is not the disease. The dis-ease does not exist outside or away from the plant, but the pathogen can. Although we often speak about “disease spread,” we really mean spread of pathogens from host plant to host plant.

Abiotic, or noninfectious, diseases are caused by nonliving factors. Things that cause abiotic dis-eases do not grow, reproduce, or spread from plant

to plant. If one plant has an abiotic, noninfectious disease, it does not increase the likelihood that a neighboring plant will get the disease.

Abiotic diseases, also called disorders, are caused by extremes of light, moisture, nutrients, tempera-ture, and air and water pollution. They may also be caused by pesticide and plant growth regulator tox-icity and by disruption of a plant’s root environment by soil compaction, excavation, and other human activities. Diagnosing an abiotic disease is difficult, since many different things can cause similar symp-toms.

Living (Biotic) Organisms That Cause Plant Disease

A living organism that obtains its food from another living organism is termed a parasite. If it can obtain nutrients only from living tissues, it is called an obligate parasite. The living organism that provides the food—willingly or unwillingly—is called the host. If the parasite harms the host while feeding, the parasite is said to be a pathogen. Thus, plant pathogens are organisms that feed on plants and harm them, causing plant diseases. There are five major groups of plant pathogens.

FungiFungi are organisms with threadlike structures

called hyphae (Figure 7.4). Fungi commonly are referred to as molds, mildews, mushrooms, or yeasts, but there are many other types of fungi as well. They have a true nucleus—a nuclear membrane sur-rounding the area inside the cell that contains the genetic material. Fungi contain no chlorophyll and therefore do not produce their own food through photosynthesis. They obtain nutrients either from living tissues of other organisms or from dead or-ganic matter. They produce enzymes to help them absorb their food.

An organism that obtains nutrients from dead organic matter is termed a saprophyte. Many fungi have parasitic and saprophytic stages in their life cycle. If the plant dies, these pathogens continue to develop on the dead plant. This is why it is impor-tant to remove diseased leaves, branches, and roots to prevent the buildup of pathogens.

Fungi reproduce by forming spores, mycelial fragments and sclerotia. Some types of fungi mate and form sexual spores, but the most common kind is asexual, and fertilization is not necessary for re-

Plant disease triangle. Figure 7.3

Credit: Georgia Master Gardener Handbook, 2004.


production. Spores are to fungi what seeds are to plants. A

spore is a reproductive unit capable of germination and growth, somewhat similar to a plant seed. Spores enable a fungus to reproduce, spread, and survive un-favorable conditions. A fungus can produce millions of spores.

Spores can be any shape or color and can have one cell or be multicellular (Figure 7.5). Most fungal species produce a spore that looks different from other species. A fungus can be identified by the spores it produces. The way the spores are formed and their size, color, and structure are all used in identifying fun-gi. Many fungi form spores in characteristic structures called fruiting structures or fruiting bodies.

After a spore germinates, the fungus can infect a plant through natural openings between plant cells or penetrate directly into the plant cell by digesting the cell wall. The fungus then grows into the plant, sending hyphae into cells to rob them of their nutri-ents. Some fungi produce chemicals that are toxic to plant cells and kill them.

Some fungi are spread from one area to another by pieces of their mycelium called mycelial frag-ments. This form of reproduction is similar to veg-etative reproduction in higher plants. Rhizoctonia, which causes the damping-off of young vegetable and bedding plants, is spread by mycelial fragments (Figure 7.4).

Sclerotia are hard reproductive structures pro-duced by some fungi. These sclerotia will remain in the soil for several years or until a susceptible crop is planted. The sclerotia germinate at the soil surface and grow into the stems of ornamental plants, veg-etables, peanuts, soybeans, and many other crops.

Southern blight is caused by Sclerotium rolfsii, an aggressive fungal pathogen that has been reported on over 500 annual, perennial and woody plants. The fungus forms a white fan-shaped mycelial mat and round sclerotia, about the size of a mustard seed, on the plant stem and on the soil surface. The sclerotia are white initially, but turn tan to dark brown within a few days of formation. When conditions are less favorable for fungal growth, the mycelium disappears, but the sclerotia remain and persist.

The fungus survives in the soil as sclerotia, which can last for several years. The pathogen is spread by movement of sclerotia via splashing water or by transport of infested soil or plant materials. The dis-ease occurs most often from late spring through early fall when the weather is hot and humid.

Filamentous or mycelial growth of Rhizoctonia on the sheath of turfgrass.

Figure 7.4

Spores of Stagonospora curtsii, (causal agent of red fire disease) on amaryllis (top) and rust (Puccinia) urediniospores below (40X magnifica-tion).

Figure 7.5




Diagnosis is confirmed by observation of spheri-cal, tan to brown sclerotia which are often accompa-nied by a coarse white, fan shaped mycelial mat.

Fallow affected areas for 6 months, if possible, as it’s hard to get rid of this fungus.

BacteriaBacteria are single-celled microorganisms sur-

rounded by a cell wall. Bacteria lack a true nucleus (have no membrane surrounding the genetic mate-rial). Plant parasitic bacteria obtain nutrients from living or nonliving sources.

Most plant parasitic bacteria are rod-shaped and reproduce by fission. One cell divides to give rise to two cells after the DNA (genetic material) has rep-licated. Bacteria can reproduce very quickly—in 30 minutes when conditions are favorable.

About 200 species of bacteria are pathogenic to plants. These cause some 2,000 different diseases. Bacterial leaf blight, bacterial spot, fire blight, leaf scorch, and stem canker are examples of bacterial diseases of ornamental and fruit trees (Figure 7.6).

Bacteria enter plants through natural openings (stomata, hydathodes, lenticels, and nectaries) or through wounds made by insect feeding, mechanical injury, pruning, or grafting. They can be spread by rainfall, insects, and people.

A quick way to diagnose bacterial wilt of tomato is to suspend a portion of a freshly cut stem from the base of the plant in water. Look for a white, milky stream of bacteria oozing from the cut stem (Figure 7.7).

Phytoplasmas and spiroplasmas are similar to

bacteria, but unlike bacteria they have no cell wall. They reproduce as bacteria do and are usually found in the water- and food-conducting vessels of an infected plant. Phytoplasmas and spiroplasmas are obligate parasites.

Symptoms caused by these organisms include abnormal growth, yellowing, very short internodes, and distortion of leaf and flower tissues. Leafhop-pers transmit phytoplasmas.

Plant Parasitic NematodesNematodes are small, nonsegmented round-

worms, generally transparent and colorless; most are slender, with bodies ranging in size from 1/50 to 1/8 inch long (Figure 7.8). Nematodes reproduce by eggs. Some kinds deposit eggs in the soil or in plant

Newsworthy Plant Diseases and Year Dis-covered in the USA

Asian soybean rust (fungus) - 2004Sudden oak death (fungus) - 1995 on native oaks; 2001 on cultivated woody ornamentals in nurseriesDaylily rust (fungus) - 2000Plum pox (virus) - 1999Tomato yellow leaf curl (virus) - 1997

Note: Only one of these diseases originated in the USA (sudden oak death); the other pathogens spread here from other countries.--A. P. Keinath.

Bacteria (Ralstonia solancearum) streaming from tomato with bacterial wilt.

Bacterial spot on immature peach fruit caused by the bacterium Xanthomonas arboricola.

Figure 7.6

Figure 7.7




Plant Pathology Terms

Bacterium: a single-celled, microscopic organism with cell walls that reproduces by fission.Blight: rapid and extensive discoloration, wilting, and death of plant tissues.Canker: dead places on bark and cortex of twigs, stems, or trunks; often discolored and either

raised or sunken.Disease: abnormal and harmful physiological condition, brought about by living agents, such as

fungi, bacteria, nematodes, or viruses, or by nonliving agents such as nutrient deficiencies or mechanical injuries. Living agents cause infectious diseases.

Exclusion: a process by which certain plants are not allowed in an area to protect againstpathogens originating from other areas.Fungus: an organism with no chlorophyll that reproduces by means of structures called spores,

mycelial fragments or schlerotia and usually has filamentous growth; examples are molds, mil-dews, yeasts and mushrooms.

Gall: an abnormal, localized swelling on a leaf, stem or root.Host: the plant afflicted with a disease.Infection: the condition reached when the pathogen has invaded plant tissue and establishes a

parasitic relationship with the host.Inoculation: introduction of the pathogen to host plant tissue.Inoculum: spores or other structure of a pathogen that can cause disease.Myceli-um/-a: mass(es) of fungal threads (hyphae) that compose the vegetative body of a fungus.Necrosis: death of tissue.Nematode: a microscopic roundworm, usually living in the soil, many of which feed on plant

roots and can be plant pathogens.Nonhost: a plant that is not at all susceptible to a particular pathogen.Parasite: A living organism that obtains its food from another living organism.Parasitic plant: a higher plant that lives parasitically on other plants. Some parasitic plants, such

as mistletoe (Phoradendron serotinum), contain chlorophyll; others, such as dodder (Cuscuta spp.), do not.

Pathogen: a disease-producing organism.Phytoplasma: a microscopic, bacteria-like organism that lacks a cell wall; formerly called myco-

plasma-like organisms.Quarantine: regulation forbidding sale or movement of plant, plant parts or soil, usually to prevent

disease, insect, nematode or weed invasion of an area.Resistance: certain qualities in a host plant that allow it to defend itself from infection by a patho-

gen (or insect).Sanitation: any process used to remove or destroy pathogens or sources of pathogens.Saprophyte: an organism that obtains nutrients from dead organic matter. Sclerotia: hard reproductive structures, produced by some soil fungi, that also function as survival

structures.Sign: a part of the disease-causing agent present in, on, or around a diseased plant.Spore: reproductive body of fungi and lower plants, containing one or more cells.Spot: circular or irregular lesion on above-ground tissue.Virus: a submicroscopic, infectious agent too small to be seen with a compound microscope; mul-

tiplies only in living cells; contains RNA or DNA surrounded by a protein coat.Wilt: lack of freshness or turgor and drooping of leaves from lack of water; a vascular disease that

interrupts the normal uptake and distribution of water by a plant.


tissue as they are passing through. Other kinds of nematodes keep their eggs in a jellylike mass that is attached to or inside the female’s body which be-comes a tough protective capsule called a cyst when she completes her life cycle and dies. Each female produces a few dozen to over 500 eggs in her life-time. Eggs of some species survive for years without hatching but then hatch quickly when a host plant grows near them.

Rates of growth and reproduction increase as soil temperature rises, from about 50 to about 95 oF. Under ideal conditions (when the temperature is between 80 oF and 86 oF), as little as 4 weeks is all that is required for many kinds of nematodes to complete the life cycle from a newly deposited egg to an egg-depositing adult.

Only about 10% of all the known kinds of nem-atodes are parasites on plants. These nematodes are all obligate parasites, meaning they must feed on living tissues. Plant parasitic nematodes feed with a

hollow stylet or oral spear with which they puncture cell walls, inject digestive juices into cells and draw liquid contents from cells.

Some common names of plant damaging nema-todes include the “sting” (Belonolaimus spp.), “ring” (Criconemella spp., Criconemoides spp.), “stubby root” (Trichodorus spp., Paratrichodorus spp.), “root-knot” (Meloidogyne spp.) and “lance” (Hoplolaimus spp.) nematodes.

-How Nematodes Injure PlantsMost, if not all, plant nematodes inject saliva

into plant cells as they feed on them. The saliva ap-parently contains digestive enzymes to increase the amount of food that the nematode can take in. In some cases, the saliva may also contain:

• toxins that kill the cell and even other nearby cells, sometimes causing root elongation to stop, root tips to swell, and/or proliferation of lateral roots

• growth-regulating chemicals that change the way the root tissues develop and grow, which can lead to knotting or galling of roots or other changes in patterns of root growth.Some nematodes can store and transmit some

plant viruses.As nematodes move through root tissues, direct

injury to the cells causes open wounds and allows invasion by rot and wilt disease organisms. Nema-tode infection sometimes reduces plant resistance to disease.

-When to Suspect Nematodes as Plant PestsAbove-ground symptoms (secondary symptoms)

of nematode infestations are:• premature wilting or other evidence of unusual

sensitivity to heat or moisture stress• stunting or abnormally slow growth • chlorosis (yellowing) of leaves• premature loss of leaves (older first) and fruit,

especially during stress periods• thinning out of turf and its failure to compete

with weeds• irregular shape and distribution of the affected

areas (that is, not all plants are equally affected).

Below-ground symptoms (primary symptoms) of nematode infestations are:

• galls or “knots’ anywhere along a root (typical of root-knot nematode infection of many plant

The Most Famous Plant Disease

Late blight of Irish potato is arguably the most famous plant disease of all time. It was the disease that precipitated the Irish Potato Famine that lasted from 1846 to 1851. Late blight destroyed the potato crops in Ireland in 1845 and 1846, but the hunger, malnutrition, disease, mass emigrations, and social unrest that resulted lasted several more years.

A German plant pathologist, Anton DeBary, later proved that the fungus Phytophthora infestans caused late blight. This was the first case study of a plant pathogen. Because of his discovery, DeBary is considered to be the father of plant pathology. --A. Keinath.

Lance nematode larva (highly magnified). Figure 7.8



species)• abbreviated roots, often with swollen tips or lat-

eral root proliferation near the tips• discolored (dark colored) roots• very small, round bodies produced by cyst nem-

atodes and attached to the roots; ranging in col-or from white to yellowish or golden to brown, about the size of a period on this page, and may be visible to a careful observer, especially with the help of a hand lens or low-power microscope

• general stunting of the roots, which otherwise appear normal. Severe nematode damage is more common in

coarse-textured sandy soils than fine-textured clay soils.

-Laboratory AnalysisSymptoms only indicate nematodes as one pos-

sible cause of damage. A laboratory analysis of soil and/or plant tissues is the only way to determine the kinds of plant parasitic nematodes present, their approximate population levels, and to estimate the relative risk that nematodes pose to the plant that is presently on the site or is to be planted. Contact a Clemson Extension agent for information about Clemson University’s Nematode Assay Laboratory.

-Managing Nematode PestsIf a nematode problem has occurred on a site in

the recent past, it is likely to recur when a plant or crop susceptible to that kind of nematode is planted again on that site. Chemical controls are limited, especially in residential areas, so give plants opti-mum care from the start and for as long as you want them to perform well.

“Optimum” does not mean “maximum.” Fertil-ize as needed to maintain healthy growth, not to produce excessive, succulent growth that invites at-tack by nematodes and other pests. Water deeply, to encourage development of a deep root system that can exploit a large volume of soil for water and nu-trients. Frequent shallow watering causes plants to develop a shallow root system. A large root system can better withstand a small amount of nematode damage than a shallow, already minimal root system.

Do not allow maintenance to lapse. Sudden dry periods or pest outbreaks can weaken plants in an incredibly short time. Even under normal condi-tions, erratic or inadequate watering can weaken a plant so that it can no longer tolerate a modest nematode population that had existed with it for

years.Finally, keep the plant root zone mulched

to keep roots cool in hot weather and minimize evaporation of water from the soil surface. Organic mulches also contribute organic matter to the soil, thus enhancing the capacity of the soil to retain water and nutrients. Mulches reduce stress on the plant as a whole and the root system specifically, improving the plant’s chances to do well in spite of some nematode damage to roots. Greater organic matter content in soil also stimulates the activity of natural enemies such as certain fungi, predatory nematodes, and other organisms, which may help suppress harmful nematode populations.

VirusesToo small to see, even with a light microscope,

a virus is simply a chemical molecule consisting of a strand of nucleic acid (the genetic blueprint of all life; either RNA or DNA) surrounded by a protec-tive protein coat. Their genetic blueprint replicates by commandeering the plant cell metabolic ma-chinery. It is during this activity that plant disease develops. Viruses can multiply only inside a living cell and so are obligate parasites.

Few viruses persist outside the host plant. Some viruses, such as cucumber mosaic, die quickly if outside a cell or if the cell dies, while other viruses, such as tobacco mosaic, are able to infect for years after the infected plant part dies. In fact, many vi-ruses depend on a second organism to move about the environment and invade a plant. These other organisms—often insects—are known as vectors for the virus.

Vascular Plants That Parasitize Other PlantsDodder, mistletoe, witchweed, Indian pipes, and

beech drops are seed-bearing plants that gain all or some of their nutrients by parasitizing other seed-bearing plants (Figure 7.9).

An alga (Cephaleuros virescens) causes algal leaf spot that commonly affects evergreen magnolias and camellias. It is not particularly harmful, and estab-lished trees can tolerate the infection. These small to large leaf spots are raised slightly and grayish-green to dark, reddish-brown in color. They can have a velvety appearance. Heavy infections can cause yellowing and premature defoliation of leaves. Twigs and stems also can become infected.

The pathogen reproduces by algal spores that are spread by wind and rain from infected to healthy


leaves or plants. Hot, humid conditions favor repro-duction.

On smaller plants, prune and destroy infected leaves and limbs. Try to reduce humidity around the affected plant by thinning it and/or by pruning nearby plants to promote more air movement and quicker drying. Chemical control usually isn’t war-ranted.

Common Symptoms and Their CausesSymptoms are fairly distinct for some diseases,

and one can identify the disease reasonably ac-curately based on symptoms alone. In other cases, several different diseases, caused by pathogens in different groups, cause similar symptoms. In these cases, additional information, e.g. signs, or display tests, such as biochemical or DNA tests, is needed to identify the pathogen.

Leaf, Flower, and Fruit Spots, Blights, and Scorch• Spots may be large or small areas of damaged

tissue. Spots may be yellow at first and then turn tan, brown, or black. Sometimes they are surrounded by a darker border. On leaves, their centers may drop out, leaving a shot-hole ap-pearance. When leaf spots merge, entire leaves are often killed. When fruit spots merge, entire fruits may rot.

• Leaf or flower blight is the rapid killing of the entire leaf or flower.

• Leaf scorch or marginal leaf burn refer to dead areas between the veins or along the margin of the leaf.

BlightsBlight, the rapid killing of the leaves and

branches on a plant, is usually caused by bacteria or fungi. Since cankers on stems and branches, as well as root rots, can also result in the rapid death of leaves higher on the plant, affected plants must be examined carefully to determine where the initial infection occurred.

Important blights include fire blight on plants in the Rosaceae family (hawthorn, apple, pear), late blight of potato caused by the fungus Phytophthora, bacterial blight of geraniums, and Botrytis blight caused by the fungus Botrytis.

RustThe term “rust” refers to diseases caused by

many different species of fungi that have dry, pow-dery, reddish-orange spores. All rusts are obligate parasites. There are leaf rusts, gall rusts, and canker rusts. Some rust fungi require two different spe-cies of host plants that must both be present in an area for the fungus to complete its life cycle. In the Southeast, these include cedar-apple, cedar-quince, and cedar-hawthorn rusts.

Other rusts infect only one host plant. Impor-tant rusts commonly found in the Southeast are hollyhock rust, snapdragon rust, orange rust on rasp-berry, and bean rust.

SmutsSmuts are caused by fungi related to rust fungi.

Smuts invade flowers of grasses and cereals and pre-vent the seed or grain from forming. Over the cen-turies, smuts have caused major losses of grain and cereal crops including barley, corn, and wheat.

A common smut is corn smut. It is character-ized by large white swellings of tissue, mostly on ears but also on stalks, leaves, and tassels. The black smut spores released late in the season from the galled tissue survive in the soil on corn debris. Growing resistant varieties is the best control for corn smut. Smutted tissue should be destroyed be-fore spores are released.

GallsMany different plant problems, including insect

feeding, callus tissue from wound-healing, fungi, and bacteria, can cause galls. One important gall-causing organism is the bacterium Agrobacterium tumefaciens, which carries a plasmid—a small, genetic-information-carrying entity. The plasmid is

Dodder parasitizing Japanese clover in a home lawn.

Figure 7.9

Credit: B. Polomski.


transferred from the bacterium to the plant, where it directs the plant to produce an unusual number of cells. Breaking the gall off does not cure the plant; most plants cannot be cured of galls. Galled plants, if particularly unsightly, should be removed.

Vascular WiltsMost fungi and bacteria that attack the food-

and water-conducting tissues of plants enter through the roots and grow into the vascular tissue. Other wilt pathogens are spread by insects or pass from plant to plant via root grafts. These pathogens sur-vive from year to year primarily in infected plants. However, some survive for long periods in the soil. Once inside the plant, the organisms causing vascu-lar wilts are found almost exclusively in the vascular tissue.

How quickly symptoms develop after infection varies with host plant, pathogen, and environmental conditions. Vascular diseases may kill a tree in one year or may weaken the tree but never kill it. Her-baceous plants usually die by the end of the growing season.

The first symptoms of vascular disease are yellowing, wilting, or defoliation of one or two branches. As the disease progresses, larger and more branches show symptoms. Discoloration of vascular tissue in the outer wood is often seen.

Some important vascular diseases include Fusar-ium wilt of tomato and banana; Verticillium wilt of eggplant, tomato, and a large number of deciduous trees and shrubs; Dutch elm disease; and bacterial wilts of cucurbits and tomatoes. Varieties of tomato are available that are resistant to Fusarium and Ver-ticillium wilts, but these varieties are not resistant to bacterial wilt.

CankersLocalized areas of dead bark and underlying

wood on twigs, larger branches, and trunks are called cankers. Cankers can be caused either by liv-ing organisms (including fungi and bacteria) or by other factors such as excessive low or high tempera-tures, hail, or other environmental factors.

Many fungi that cause tree or shrub cankers normally inhabit the plant’s surface, gain entrance through natural or human-made wounds, and only cause disease when the plant is under stress. Some fungi, however, aggressively attack trees and cause cankers.

There are three general types of tree cankers:

1. Annual cankers are caused by fungi not nor-mally able to cause disease unless the tree is un-der stress and low in vigor. Infection can occur during the host’s dormant season. During the growing season, host callus tissues close off the canker and prevent it from spreading further. Although annual cankers do not persist, contin-ued stress makes it likely that more cankers will form (e.g., Fusarium canker).

2. Perennial cankers are seldom lethal to the tree, but they weaken its structure and detract from its appearance. The fungus invades wounds and branch stubs. The host forms callus around the infection site during the growing season, but the fungus invades more tissue. As this interac-tion continues, multiple ridges of callus create a target-spot canker. Examples include Nectria canker and Eutypella canker.

3. Diffuse cankers are elongated, with little or no callus growth. Because the fungus invades so rapidly, the tree tissue at the edge of the advanc-ing fungus is killed quickly. Branches or whole trees are girdled, sometimes in a single season. These diffuse cankers often kill the tree. Exam-ples are Chestnut blight, Botryosphaeria canker, Phytophthora dieback, and Cytospora canker.In the nursery, deciduous woody ornamen-

tals should be examined for cankers. Great care should be taken to prevent injury to the trunk and branches. Pruning should be done late in the dor-mant season, close to the trunk, without damaging surrounding bark. Pruning should be done while unwanted branches are small and wounds will heal quickly. Do not prune during wet weather, and do not prune from mid-August to leaf drop. Many can-ker and wood-decay fungi form and release spores during that period.

Promote tree vigor so that the tree can express its natural resistance to disease and wound healing can begin promptly and develop rapidly.

When inspecting branches and young trees for cankers, look for:

1. Localized areas of roughened or cracked bark, especially around wounds and branch stubs;

2. Callus formation in multiple layers or ridges; and

3. Small pimple-like, fungal spore-forming struc-tures either in the centers or around the edges of 1 and 2 above. These may be red, dark brown, or black.If these three things occur on branches, prune


them off well back from the canker. Symptoms on the main trunk of saplings indicate that the entire tree should be removed. Perennial and diffuse can-kers do not go away. Such cankers are present for the rest of the tree’s life, which may be shortened considerably.

Damping-OffDamping-off is the infection of young seedlings

by a variety of different fungi, most often species of Rhizoctonia, Pythium, and Fusarium. These soil-dwelling fungi produce spores that are spread by air, water, soil, and contaminated tools and pots. Exces-sively wet soil, excessively cool temperatures before germination or warm ones after, and densely planted seedbeds all favor the growth of damping off fungi. Damping-off principally affects young seedlings. As seedlings age, the development of secondary stem tissue creates a protective barrier that thwarts pen-etration by the fungus.

Seedlings affected by preemergence damping-off begin to decay before they can reach the soil surface. They become soft and mushy and turn brown to black. Seedlings that have emerged can be attacked just above or below the soil line. Slightly darkened water-soaked lesions occur on the stems. The in-fected stem tissue may be colorless to dark brown. As the decayed area enlarges, it girdles the stem, causing the seedlings to collapse and fall over. This disease is known as post-emergence damping-off.

Damping-off disease is easier to prevent than cure. Once seedlings are infected little can be done beyond removing them before the disease spreads further. Prevention starts with good sanitation. Always use clean containers and tools. Disinfest them if necessary with a 10% bleach solution (1 part bleach to 9 parts water) or soak them in hot soapy water (160 oF) for at least a half hour.

Rather than using sterilized soil, use a soilless growing mixture containing peat, perlite, and ver-

miculite. While not technically sterile, these media are typically free of fungus spores provided they have not had contact with open ground.

Some seed has been pretreated with fungicide, but rapid seed germination, emergence, and devel-opment also reduce the likelihood of damping off. Sow the seeds thinly to avoid overcrowding and to ensure good air movement and adequate light. Some growers cover seed flats with a thin layer of milled sphagnum peat moss. The low pH of the peat moss inhibits fungi naturally.

Prior to germination 70 to 75 oF is generally optimal. When the seedlings emerge, remove any plastic covering or lid that was used to increase humidity levels, and give the seedlings ten degree cooler temperatures.

Proper watering is also critical. Allow the sur-face of the potting medium to dry before watering again. When watering seeds by hand, use a watering can or clean nozzle that has not touched the ground. Alternatively, seed flats can be bottom-watered, but make sure that seedling trays are not standing in water.

Finally, avoid overfertilization. A water-soluble fertilizer applied at half-strength once the second true leaves have appeared is sufficient.

Root and Crown RotsRoot rot can be caused by several different spe-

cies of fungi, including Pythium, Rhizoctonia, Fusar-ium, and Phytophthora. These are common in field soil, sand, and dead roots of previous crops. Almost all plants are susceptible to root rots at some stage of development. It is important to remember that anything killing roots will cause similar symptoms above ground. Yellow leaves can be a secondary symptom of root rot. Symptoms are:

• the base of cuttings are brown or black and be-come soft;

• plants are stunted;

“Tulipomania”

Tulips were introduced to The Netherlands from Turkey in the 1590s. Among the tulips were some that were multicolored (“broken”). These unusually colored tulips were highly prized and so much in demand that prices for them soared to astonishing heights. One rare bulb sold for six times the cost of a sailing ship in 1636, the height of “Tulipomania,” the name given to this historical phenomenon, which is considered the first instance of trading (and speculation) in stock futures.

Not until the 1930s was it discovered that tulip breaking virus was responsible for the beautiful color patterns in the variegated tulip flowers. This is one of the oldest recorded plant viruses, as broken tulips were pictured in Dutch paintings from the early 1600s. Today, variegated tulips are the product of many years of traditional plant breeding.--A. P. Keinath


• root tips are brown and dead and may slough-off when plants are pulled from the soil (Figure 7.10);

• plants wilt at midday and recover at night;• plants yellow and die;• soft or brown tissue on the outer portion of the

root pulls off easily, leaving a bare strand of vascular tissue exposed; and

• stems rot at the soil line and have a dry, shred-ded appearance. (Stem or crown rot is not always accompanied by root rot.)

Root and cutting rot is difficult to control once root rot has begun. Efforts should be directed to-wards preventing the disease before it begins.

• Pasteurize potting soil and sand with heat treat-ments, or purchase a soilless potting mix since these are generally free of pests.

• Disinfest all work surfaces, tools, and equip-ment that will contact the potting mix.

• Encourage vigorous growth since older plants are more resistant.

• Remove and destroy infected plants, getting the entire root system.

• Practice cultural/nonchemical management: avoid planting too deeply; provide excellent drainage; and add organic materials, such as composted pine bark, to the planting area to enhance drainage in clay soils due to improved soil structure.

Virus DiseasesMany different viruses can infect plants. De-

pending on which virus is involved, the virus may be spread from infected plants to healthy plants in a number of ways. Once inside a plant, vegetative propagation perpetuates virus diseases. Cuttings taken from an infected plant usually are also infect-ed even if no symptoms are immediately exhibited by the cutting. The virus particles are found in all parts of the plant except the few cells at the tips of the growing points. It is these few cells that are re-moved and grown into a healthy plant free of virus by the process called tissue culturing.

Some viruses are carried on or inside pollen or seed. When this occurs, usually less than 10% of the seeds from an infected plant carry the virus.

Many viruses are spread by insects, especially aphids, whiteflies, and thrips. There can be a com-plex relationship between a virus and the insect that transmits it. Nonpersistent viruses are acquired quickly by insects and are only transmissible for

a few hours. Aphids probing a plant for 5 to 10 seconds without actually feeding can pick up these viruses. Insecticides are of little help in this case, since the insect is not killed rapidly enough to pre-vent it from moving to other plants.

Persistent viruses are acquired by an insect and can be transmitted over several days. The virus is not transmitted immediately, however, but must remain inside the insect for a period of time before-hand. Insecticides help control the spread of persis-tent viruses.

Symptoms vary with the virus involved, the species of plant infected, and environmental condi-tions. Certain environmental conditions bring out symptoms, while others mask or hide them. Symp-toms associated with virus infections are:

• reduced growth resulting in stunting;• mosaic pattern of light and dark green or yellow

on leaves or fruit (Figure 7.11);• distortion and malformation of leaves or grow-

ing points;• yellow streaking of leaves (especially mono-

cots);• yellow spotting on leaves;• ring spots or line patterns on leaves (Figure

7.12);• cup-shaped leaves;• uniform yellowing, bronzing, or reddening of

foliage; and• breaking of flower color.

Some of these symptoms can also be caused by high temperatures, phytoplasmas, insect feeding,

Sloughing-off of the cortical tissue on magnolia roots due to root rot.

Figure 7.10



growth regulators, herbicides, mineral deficiencies, and mineral excesses. Most plants are susceptible to several different viruses. Virus diseases cannot be diagnosed on the basis of symptoms alone.

There are no chemicals that cure a virus-infect-ed plant or any that protect plants from becoming infected. To guard against virus diseases:

• Purchase virus-free plants.• Maintain strict insect control.• Control weeds, since they may harbor viruses,

mites, nematodes, and insects.• Destroy virus-infected plants.

Cases of Mistaken IdentityThe variation among ornamental species that

we so highly prize can be a source of confusion. For example, one common expression of mutation (to which all plants are subject) is the loss of green pig-ment in foliage that causes variegation. Variegation that occurs on a part of a plant can resemble the symptoms produced by a viral pathogen. Similarly, the loss of variegation in a normally variegated shrub may be thought to be a result of disease when in fact it is just a mutation back to the original ap-pearance of the plant.

Squash mosaic (top) and tomato spotted wilt (bottom).

Figure 10

Different bark characteristics among plants may also prove confusing. Certain woody species of elm, sweet gum, and euonymus may exhibit wings of cork around the stem, which could be mistaken for a can-ker-type disease. This bark characteristic is highly prized in the nursery trade and is a trait in these species for which breeding programs select. Some plants like crapemyrtle and sycamore shed their outer bark, a characteristic that might be alarming if you did not know it was natural for those plants.

Sooty mold is a black fungus that develops on surfaces of trees and shrubs during the summer months. The appearance of sooty mold indicates that insects are present. Excrement from aphids, scales, and certain other insects contains sugars and nutrient-rich materials. The excrement, called hon-eydew, drips onto needles, leaves, and twigs. The sooty mold then grows on this nutrient-rich source. If the insect activity is eliminated, sooty mold will not be a problem. Control the insect rather than the fungus.

Perhaps the most distinctive plant in Lowcoun-try landscapes is the epiphytic plant Spanish moss. Although it appears to parasitize landscape plants, Spanish moss simply uses them as support structures for growth. Sunlight, along with water and nutri-ents from the air, permit the plant to grow as easily from telephone wires as in live oak trees. Occasion-ally, Spanish moss will become dense enough to cause twig breakage when wet or to reduce sunlight to tree foliage. Neither condition is irreversible or lethal.

Another epiphyte, lichens, is often wrongly identified as disease-causing agents. In fact, a li-chen is composed of a fungus and an alga that have


Impatiens necrotic spot on hosta. Figure 7.11



coevolved into a complex and symbiotic life-form. The fungus contributes support, nutrients, and wa-ter, while the algal component photosynthesizes complex foods to support the life form. Lichens are commonplace on tree trunks, but when they develop on branches and twigs, they are blamed for decline. Lichens can only colonize interior twigs and branches after the leaf canopy has thinned. They follow the sunlight into the plant interior, and though they take advantage of declining plants, they do not cause decline. Increased lichen density should be taken as an indication that a particular plant is under stress and has lost leaf cover. Real causes of this decline could include nematodes, root diseases, or environmental or cultural stress.

Some of the diverse, native saprophytic fungi also are blamed for causing plant diseases. Perhaps the most obvious are the stinkhorn fungi, Clathrus, Mutinus, and Phallus spp. These large and offensive-smelling fungi are often found near thickly mulched areas, old stumps, or declining trees. They do not cause plant disease but rather colonize deposits of organic matter. Their colonies are often extensive in an area and may arise from deep within the soil. Their odor attracts carrion flies that spread the fun-gus spores throughout the environment. If you can-not tolerate stinkhorns, remove them before they mature. No controls are needed for these fungi.

Slime molds, a group of organisms that may not be fungi at all, usually are grouped with the true fungi. Slime molds are seen in South Carolina in the spring on hardwood bark mulch, logs, or turf as a wet, brightly colored mass. One common slime mold is known as “fried egg slime,” because it is initially bright yellow. Within a short time, it turns brown and dusty, as the body turns into mil-lions of spores. Slime molds are saprophytes, not pathogens, even when they are found covering live plants. They can be removed from plants with a strong stream of water. To prevent slime molds from spreading, remove them while they are still moist before they form spores.

Fairy rings are found in grasslands, forests, and lawns after rains. Sixty different mushrooms can form fairy rings. Some fairy rings in lawns were as-sociated with trees that were cleared. The above-ground mushrooms are the fruiting bodies producing spores. The bulk of the fungus body (mycelium) is underground, expanding outward from the center of the ring as the fungus consumes organic matter in the soil. It has been estimated that large fairy rings

can be over 400 years old.

Common Abiotic (Noninfectious) Diseases

Abiotic or noninfectious diseases are caused by nonliving entities. Noninfectious diseases are very common where plants are grown outside of their normal habitat, outside of their normal range, or in habitats that are disrupted by human activity. Any nonliving thing that disrupts a plant’s basic func-tions may cause disease.

To be healthy, plants require the following con-ditions:

1. Plants must have a balance between moisture and air around roots. Root cells need oxygen to respire or they die. All the cells of the plant must have moisture.

2. Soil conditions must allow sufficient root growth for water and nutrient uptake.

3. Nutrients in the soil must be sufficient for plant needs but not be so excessive that they become toxic.

4. A balance of required nutrients is necessary since an excess of one nutrient can induce defi-ciency of another.

5. Light must be sufficient for the plant to carry out photosynthesis, but not so excessive that chlorophyll is destroyed.

6. Conditions at the site in winter must be such that the plant can adapt and survive while maintaining vigor.

If one or more of these requirements are not met, the plant can be expected to exhibit symptoms of disease including:

• general slowing of growth• decline in vigor• yellowing of older leaves• branch dieback• wilt• marginal leaf burn or needle tip death• premature fall leaf coloration• smaller than normal leaves• heavy seed production.

Diagnosing an abiotic disease is difficult because many different things can cause similar symptoms. Accurate diagnosis requires that you observe all symptoms and note when they first appear; obtain a history of growing conditions; examine the site care-fully; and examine neighboring plants of the same species and different species for similar symptoms (best done by the person maintaining the plants).


Excessive Moisture and Poor Soil AerationExcessive rainfall or irrigation can cause symp-

toms typical of excessive moisture. However, if the soil is poorly drained, even a small amount of mois-ture can cause symptoms. Soils with high clay con-tent are poorly drained even on a slope.

Symptoms of excessive moisture include yellow-ing of older leaves or needles, defoliation, wilting although the soil is moist, and limp roots. Signs in-clude algae or moss growing on the soil surface, soggy soil, and soil that is dark olive green to olive brown in color 6 to 8 inches below the surface.

Insufficient MoistureDisease caused by insufficient moisture can result

from not watering deeply or insufficient rain, from weather that dries the plant faster than the root system can supply moisture, and from plants not rooting well after transplanting. If the planting site is restricted in size (shallow to bedrock or paving all around the root system) or if the drainage pattern is changed, sufficient moisture may not be available to the plant. If the root system size is reduced by exca-vation, root rot, nematode feeding, or soil compac-tion, the plant will not be able to take up sufficient moisture. In addition, if the soil is frozen while air temperatures are mild, broad-leaved and needled evergreens will continue to lose moisture and be un-able to replenish their supply.

Symptoms include yellowing, wilting, premature fall leaf coloration, and marginal leaf burn. Signs include soil that is very dry, dusty, hard, or cracked. The potting mix in containers may be pulling away from the edge of the pot, or the mix may not be moist entirely through.

Plants in very windy, exposed locations or in containers are very susceptible to problems caused by insufficient moisture.

Nutrient Deficiencies and ImbalancesPlants require certain nutrients in varying

amounts. Major elements, those required in rela-tively large amounts, include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Minor elements, those required in lesser amounts, include iron, boron, manganese, copper, zinc, and others.

Symptoms of deficiencies vary with host species. In trying to diagnose nutrient problems, it is best to find a reference book that deals with a specific plant. Soil and tissue tests should also be run.

Symptoms of nutrient toxicities, deficiencies, and imbalances are related to the function the ele-ment has in the plant and the element’s mobility in the plant. Mobile elements can be moved from older to younger tissue within the plant. Mobile elements include N, phosphorus, potassium, and magnesium. When a deficiency of one of these elements develops, the older tissues show the de-ficiency because those elements are transported to the younger, developing tissues. The younger tissues appear normal.

Immobile elements (calcium, iron, and man-ganese) cannot be moved within the plant. While older tissue may appear normal, new developing tis-sues will lack nutrients and exhibit symptoms.

Nutrient deficiencies can be caused by: (1) a lack of nutrients; (2) a soil pH that makes the ele-ment unavailable to the plant; and (3) an imbalance between two elements, disrupting intake. As an example of the last factor, a delicate balance ex-ists among calcium, magnesium, and boron. Excess calcium leads to magnesium deficiency. Nitrogen deficiency can induce Mg deficiency. Potassium de-ficiency can cause excessive uptake of molybdenum.

Symptoms of major element deficiencies can be found in Chapter 1, p. 12.

Excessively High Temperatures or Light Condi-tions

High temperature damage can occur when: (1) a plant is not acclimated to its location because the plant was recently moved to that new location; (2) it was moved from greenhouse or shade house to full sun; or (3) it was recently exposed to full sun by the removal of overstory trees. In some cases, a building or other structure reflects heat and light onto the plant, or the plant is exposed to heat exhausts from air conditioners, clothes driers, or steam lines.

To diagnose the problem, compare the date of a sudden change in environmental conditions to the date symptoms began. Note whether symptoms are more severe on one side of the plant than another. Symptoms include a light- green coloration of leaves, bleaching of leaves or bark, mottling or un-even green coloration on leaves, death of leaf tissue or roots, and dead tissue at the soil line of seedlings.

Protect plants while they are being acclimated to a new location. Use shading or frequently mist the plant with water during the heat of day. Posi-tion a lathe or cloth between the plant and the heat-reflecting structures until the plant grows large


enough to shade a portion of that structure. Mulch the base of seedlings in hot locations.

Excessively Low TemperaturesDamage depends on when the low temperature

occurs. Continuous cold around hardy, acclimatized plants in the winter causes little problem. However, widely fluctuating temperatures do cause problems. Symptoms include the blackening of young tissues, uniform browning of evergreen tips, marginal leaf burn of broad-leaved evergreens, reduced flowering, and death of a plant that was healthy the previous autumn.

Allow plants to harden in the fall by not apply-ing excessive nitrogen in autumn.

Use plant hardiness zone maps to select plants for particular locations. Even within a given hardi-ness zone, if a site is particularly harsh, consider us-ing only plants adapted to a lower number hardiness zone.

Water plants in late summer and autumn to prevent them from entering winter under drought stress. Drought predisposes plants to winter injury and cankers. Pack potted plants close together and cover them with a translucent plastic sheet that does not touch plants. Mulch or mound soil around pots, and ball and burlap plants to insulate roots. Wrap burlap or build a lathe structure around plants in exposed locations.

Environmental Stress and Plant DiseaseMany organisms, especially facultative parasites

(those that do not have to be parasites in order to live), are not extremely aggressive and attack plants only under certain conditions. Diseases caused by these organisms often only develop when a plant is not vigorous or when conditions are not optimum for the plant to react to attack. In the landscape, these environmental stress-related diseases are the most common of those encountered.

The living organisms most often involved are fungi. Some environmental stress is required for them to be able to cause disease. The stress alone does not kill the plant but weakens it, making it more susceptible to disease.

For example, canker-causing fungi may invade tissue during a plant’s dormant season, when the plant’s reaction to invasion is slow and the plant is low in vigor. Most canker-causing fungi are not able to cause disease unless the plant is under an envi-ronmental stress such as drought. Root pruning that

occurs during transplanting causes a drought-like stress and predisposes woody plants to cankers.

Attack by insects can stress plants and predis-pose them to fungal attack. For example, repeated defoliation of oaks by gypsy moth larvae and leaf rollers weakens trees and predisposes them to oak root rot.

DeclinesA decline is the gradual reduction in growth,

dieback, and finally death of a population of plants. Declines: (1) occur well within a plant’s normal geographical range and not just at its fringes; (2) are occurring when 5% or more of the population is lost per year over several years; and (3) occur in 10- to 25-year cycles.

One theory proposes that three factors are in-volved in declines:

1. Long-term factors that change very slowly im-pose a stress. These include site characteristics, soil type, soil pH, climate, and host genetics.

2. Short-term factors that change greatly from year to year add to the stress. These include drought, flooding, and cold injury (Figure 7.13).

3. Contributing factors actually kill the plants. These include diseases and insects, especially weak parasites, and secondary fungi. Contrib-uting factors are readily found and receive the greatest emphasis, but without the long- and short-term stresses, no decline would occur.

Symptoms of decline are:• Growth is reduced. Shoots are shorter, es-

pecially on evergreens, and annual rings are smaller (reduced radial growth).

• Roots and mycorrhizae degenerate.• Food reserves are depleted, compared to those

of non-declining plants.• Leaves color prematurely in the fall.• Winter twig diebacks develop and are invaded

by weak parasites.• Portions of the canopy die.• Water sprouts develop on main stem and

branches of hardwoods.• Root rot and decay fungi attack.• Secondary insects attack, including borers and

bark beetles.Several declines, including declines of ash, oak,

maple, and dogwood, have occurred and continue to occur periodically in the Southeast.


To manage a declining plant, alleviate the short-term stress factors if possible. Fertilize, irri-gate, drain, or mulch as appropriate. Protect weak-ened plants from contributing factors by controlling leaf diseases, especially those causing defoliation and twig death, and by controlling defoliating and bor-ing insects.

Managing Plant DiseaseSuccessful plant disease management entails a

three-pronged approach:1. Monitoring. The objective of monitoring or

examining plants is to avoid introducing a pathogen into an area, to detect and eradicate any disease found, and to detect diseases early so that other appropriate actions can be taken.

2. Prevention. Preventive techniques include purchasing and planting pathogen-free or -re-sistant material, treating soil before planting to eliminate disease-causing organisms, and, if necessary, applying chemicals before diseases normally appear.

3. Management. When a disease is detected, take actions to manage the disease by reducing ei-ther its severity or its spread.

Many different control measures can be used in managing diseases. For most diseases, more than one type is required. Integrating several types of controls usually is more successful than relying on only one method.

Regulatory MethodsRegulatory methods of disease control attempt

to exclude disease-causing organisms, highly sus-ceptible host plants, or alternate host plants from a growing area. Inspection and quarantine of plant materials are tools for enforcing rules and regula-tions that prevent the introduction of a disease-causing organism or a disease-carrying plant into an area. A list of plant materials that cannot be brought into the United States is available from the U.S. Customs Service.

Inspections can be done at the point of origin before the material is packed and shipped or at the port of entry or receiving site upon arrival. Trained inspectors are required to watch for known pests on particular plants, for indications of problems due to some unknown cause, or for unwanted plants.

Quarantine is the holding of plants, potting materials, or packing materials for a period of time to allow a disease to develop or a disease-causing organism to grow so that it can be detected if it is present.

Cultural Methods

Closed Seasons and Dry FallowA closed season or a host-free period is a time

in the greenhouse or field when no host plants are present. Having a closed season is especially effec-tive when obligate parasites are of concern.

Dry fallow involves soil tillage to prevent weed growth and to keep the upper layers of the soil dry. Nematodes such as the lesion nematode and some bacteria cannot survive those conditions.

Closed seasons and dry fallow are not effective against soilborne pathogens such as Pythium, Phy-tophthora, Verticillium, or Fusarium. These organ-isms produce spores or sclerotia that are resistant to adverse environmental conditions. Resistant spores allow the organisms to remain in a living but dor-mant state for long periods of time.

Closed seasons and dry fallow are also inef-fective against foliar pathogens, such as powdery mildew or gray mold. These fungi produce wind-disseminated spores that readily recontaminate the area.

Crop RotationCrop rotation is a modification of closed season.

Plants may be present, but they are not host plants.

Stunted hollies caused by compacted, poorly drained soil and inadequate sunlight.

Figure 7.13



Thus, the land is still in use, but the crop being grown is not threatened by a certain pathogen.

An example of a poor rotation is tomato fol-lowed by eggplant followed by potato since all are susceptible to similar pathogens. A better rotation is tomato followed by corn followed by beans.

When selecting plant replacements for land-scape sites, the gardener should consider previous rotations, because the organisms that killed the original plant may still be present in the soil or on surrounding host plants. For example, if Japanese hollies (Ilex crenata) are removed because they have black root rot (caused by the fungus Thielaviopsis basicola), do not follow with pansies, which also are susceptible to this disease. The same example ap-plies to root-knot nematodes.

SanitationSanitation is a cultural method of disease man-

agement that involves reducing the amounts of inoculum present, destroying sites where pathogens overwinter, removing noncrop plants that harbor pathogens, and disinfesting equipment. Raking fall-en leaves, removing infected plants, burning debris, pruning infected twigs and branches, and cleaning soil and plant sap from tools are all forms of sanita-tion. Sanitation is useful for managing cankers, bacterial diseases, vascular wilt diseases, and some foliar diseases.

When tools are used to handle infected plants or to prune out infected tissues, they likely are con-taminated with the pathogen. As such, they may spread the pathogen to the next plant they contact. To prevent the bacterial disease fire blight and other pathogens, disinfest tools by soaking them in a solu-tion of 1 part bleach (sodium hypochlorite 6.0%) to 4 parts water for at least 1 min. Before storing the tools, rinse them in water and then lubricate with oil to prevent rusting.

Manipulating the EnvironmentManipulating the environment is also a cultural

disease management practice. Environment is one of three factors influencing disease development. If environmental conditions can be manipulated to favor plant vigor and to be unfavorable for disease development, certain diseases can be completely controlled and, for many diseases, the effectiveness of additional management techniques is enhanced.

Controlling humidity to reduce the duration of leaf wetness greatly influences foliar disease devel-

opment. Weed control and plant spacing influence air circulation in and around the planting site. The irrigation method used influences the duration of leaf wetness. Drip or trickle irrigation keeps plant sur-faces dry and therefore unfavorable for most disease-causing bacteria and fungi. Sprinkle irrigation wets the foliage, providing moisture necessary for fungal and bacterial activity, and also splashes these patho-gens from place to place.

Controlling soil moisture strongly influences the development of Pythium and Phytophthora root rots. Both fungi require wet soil to develop. By draining excessive moisture away from plants, soil conditions become less favorable for these fungi.

Improving Plant VigorWatering, fertilizing, preventing soil compac-

tion, preventing damage from excavation or pedes-trian traffic, and other practices that promote plant vigor strengthen the plant’s ability to resist disease. Maintaining plant vigor is extremely important in declines, diebacks, and canker diseases. For example, steps should be taken to prevent winter injury, to retain moisture by mulching, to drain excessively wet sites, and to avoid root injury by reducing tillage.

Resistant VarietiesOne of the most effective ways to prevent disease

is to plant varieties not susceptible to the pathogen. Plants may be susceptible, tolerant, or resistant to various pathogens. The term susceptible indicates that the plant readily becomes diseased if the factors of environment, time, and pathogen are favorable. The term tolerant implies that the plant may become diseased but is not severely affected.

Resistant plants do not readily become diseased unless environmental factors are extremely favorable to the pathogen. Plants that never get a particular disease are said not to be hosts of that particular pathogen.

Resistance, tolerance, and susceptibility are rela-tive terms. There is a continuum among species and cultivars from extremely susceptible to extremely resistant.

Plant resistant cultivars when: (1) minimum maintenance is desired (home grounds, rights of way, low budgets), (2) inoculum is always present, (3) en-vironmental conditions usually favor disease at some time of year, (4) chemical controls are not available, are not likely to be used in a timely fashion, or are difficult to apply, or (5) plants become severely disfig-ured by disease.


Using susceptible cultivars is permissible when inoculum is rarely present, environmental condi-tions rarely favor disease development, or intensive management practices are being used. Nonhost species should be used when no resistant cultivars of another species are available and when the patho-gen has a very wide host range and cannot be elimi-nated from a site.

Biological ControlsIn biological control, one organism is used to at-

tack or inhibit the activity of a pathogen. Examples are:

• Bacteriophages are viruses that infect bacteria. Some tomato growers spray commercial prepa-rations of bacteriophages to control bacterial spot of tomato.

• Antibiotics are chemicals produced by one organism that kill another organism. The bio-logical fungicide Serenade® contains a bacte-rium that produces three antibiotics.

• Parasitism occurs when a fungus directly attacks the body of another fungus or a nematode.

• Cross-protection occurs when a plant infected with one strain of fungus or virus is made less susceptible to another fungus or virus.

In biological control, a certain level of disease must be tolerated, since (1) the control agent must have a nutrient source, (2) the control agent may have to be frequently reintroduced if it is so efficient that it eliminates its food source and dies out itself, and (3) most control agents only work well under very specific environmental conditions. These dis-advantages must be weighed against the advantage of reducing or eliminating the use of some chemical pesticides.

Physical ControlsHeat treatments can be used to reduce the num-

bers of pathogens in soil or on plants. Seeds, bulbs, and rootstocks can be heated to a point that kills the pathogen but not the plant tissue.

Refrigeration is used to chill plants and retard the development of pathogens. This technique is most often used when plants or plant parts are being held in storage.

Chemical ControlsThe objective of chemical disease control is to

eliminate or inhibit pathogen germination, growth, invasion, colonization, reproduction, or spread.

Chemicals may kill the pathogen directly, or may only inhibit pathogen activity to allow plants to grow to a stage of lessened susceptibility. Some chemicals may induce a resistant reaction in an otherwise susceptible plant and not be toxic to or directly inhibit the organism.

Most chemicals are protectants and must be applied before or very soon after disease begins. To be effective, chemicals must come in direct contact with the pathogen. This requires that pesticides be applied uniformly on the plant surface and fre-quently enough to protect new tissue. Contact pesticides are generally applied as sprays, dusts, seed treatments, fogs, or mists to above-ground portions of plants, or as granulars or drenches mixed into soil.

Systemics are chemicals that enter the plant and are redistributed within the plant from the initial point of contact. Uniform coverage is not as important as with contacts, although it is help-ful. Reapplication is usually less frequent since the chemical is inside the plant; is not weathered away by sun, wind, and rain; and is redistributed to newly forming plant parts. Systemics must have a high level of compatibility with the host because they are absorbed and moved within the plant.

The most important aspect of using chemicals effectively is to apply the appropriate material at the proper time. No single chemical is effective against all pathogens. Therefore, the cause of the disease must be known so that the proper chemical can be selected. Most pathogens have certain stages of development that are extremely resistant to adverse environmental conditions, including the presence of inhibitory chemicals. The biology of the pathogen must be known so that the chemical can be applied when the pathogen is in a stage of development sen-sitive to the chemical. If the wrong chemical is ap-plied or if the appropriate chemical is applied at the wrong time, chemical control will fail. ❦


References and Further ReadingDiseases and Pests of Omamental Plants. 5th ed. P. P. Pirone, 1978. John Wiley & Sons, NY.

Diseases of Trees and Shrubs. 1989. W. A. Sinclair, H. H. Lyon, and W. T. Johnson. Cornell University Press, Ithaca, NY.

Westcott’s Plant Disease Handbook. 1978. R. K. Horst. Van Nostrand Reinhold Co., NY.

Essential Plant Pathology. 2006. Gail Schumann and Cleo D’Arcy. American Phytopathological Society Press, Minneapolis, MN.

Plant Pathology: Past to Present. 1998. Frank H. Tainter. American Phytopathological Society Foundation, Minneapolis, MN (http://www.apsnet.org/members/opae/storybook/top.asp (Illustrated storybook/coloring book about the history and science of plant pathology available to download).

Learning Biology with Plant Pathology .1994. Juliet E. Carroll. National Assoc. of Biology Teachers, Reston, VA. (Although written as a guide for science projects for high school teachers, this book has useful sections on terms and history of plant pathology).

Magical Mushrooms, Mischievous Molds .1998. George W. Hudler. Princeton University Press, Princeton, NJ.

Plant Diseases: Their Biology and Social Impact. 1991. Gail Schumann. American Phytopathological Society Press, Minneapolis, MN. (This textbook teaches basic plant pathology while explaining its cultural relevance with many historical perspectives.)

Internet Resources

American Phytopathological Society (www.apsnet.org/)

Clemson Extenson Home & Garden Information Center (hgic.clemson.edu)

Extenson Plant Pathology @ University of Georgia (http://www.plant.uga.edu/Extension/home.htm)

Plant Disease Information Notes, NC State (http://www.ces.ncsu.edu/depts/ent/clinic/indexa.html)

Texas Plant Disease Handbook, Texas A&M (http://plantpathology.tamu.edu/Texlab/default.asp)


1. Indicate the situations that are likely to lead to development of disease in plants.2. Name the group of organisms (biotic) responsible for 70% of plant disease.3. Describe the best protection against plant disease.4. Complete the following chart, indicating the best, current approach to the disease.

Organism/Disease Impact Likely plant Symptoms Control Other hosts

Azalea die back

Azalea leaf gall

Bacterial wilt

Dieback

Dutch Elm Disease (=vascular wilt)

Leaf Spot

Nematodes (microscopic roundworms)

Rose stem canker

Nonpersistent viruses

Basic Plant Pathology Review

Documents

Chapter 7 Basic Plant Pathology - Clemson Universitymedia.clemson.edu/.../ch_7_plant_pathology.pdf · Basic Plant Pathology 152 Basic Plant Pathology Introduction “What is wrong