Plant PathologyAN INTRODUCTION TO

PLANTS AND THEIR DISEASES FOR MASTER GARDENERS

MAUREEN THIESSENAREA COMMERCIAL

ORNAMENTAL SPECIALIST

PathologyPathos – diseaselogos – study of

Plant pathology is the study of diseases affecting plants

What is disease?“The series of invisible and visible responses of plant cells and tissues to a pathogenic organism or environmental factor that result in adverse changes in the form, function or integrity of the plant and may lead to a partial impairment or death of plant parts or entire plant.”

-Dr. Datnoff, Plant Pathologist, LSU AgCenter

Disease triangleENVIRONMENT

PATHOGEN SUSCEPTIBLE HOST

Disease

Host

Envi

ronm

ent

Path

ogen

Diseases can be caused by abiotic and biotic factors…

ABIOTIC Non-living causes of disease – disorders

◦ Air/Water pollution◦ Nutrient or water imbalance◦ Temperature stress◦ Light stress◦ Chemical or mechanical injury

Can have effects on biotic disease factors

BIOTIC Diseases are caused by other living organisms

◦ Fungus◦ Bacteria◦ Virus◦ Nematodes◦ Parasitic plants◦ Algae and protozoa

Abiotic Factors

Sunscald Phosphorus deficiency

Drought Stress

Sulfur damage from atmosphere

Boron Toxicity

Abiotic Factors

Calcium deficiency-induced blossom end rot

Iron Deficiency

Magnesium Deficiency

2,4-D Damage

Winterkill on turfgrass

Biotic Factors

Signs vs. symptoms A sign is any visible part of the actual pathogen◦ Spores or fungal hyphae◦ Bacterial streaming◦ Parasitic plant parts◦ Nematode Eggs

A symptom is a manifestation of the damage caused by the pathogen◦ Chlorosis, discoloration◦ Wilting, stunting◦ Distortion, loss of vigor

How do we confirm biological pathogens?

Robert Koch – a German medical doctor and bacteriologist who discovered causal agents of anthrax, tuberculosis, and cholera.

Developed steps for identifying and confirming causal disease agents, known today as Koch’s postulates:

1. 1. The suspected causal agent must be present in every diseased organism

2. 2. The suspected causal agent must be isolated from the diseased host organism and grown in pure culture.

3. When a pure culture of the suspected causal agent is inoculated into a healthy susceptible host, the host must reproduce the specific disease.

4. The same causal agent must be recovered again from the experimentally inoculated and infected host, and have the same characteristics as the organism in step 2.

Koch’s Postulates for Dogwood Anthracnose

Basic Disease Cycle

Primaryinoculum

Secondary inoculum

Infection

Where do diseases come from? Disease cycle begin when pathogen inoculum contacts an infectable site on a susceptible host.◦ Can include fungal spores, bacterial cells, viruses, seeds◦ Carried by wind, water, insects, humans, equipment

Inoculum Types and Locations of Survival of Plant Pathogens Between Crops

Dissemination of Plant Pathogens

Attachment, Penetration, Infection•Fungal inoculum lands on plant surface, and adheres either by present moisture or excretion of enzymes and mucilaginous substances•Bacterial and viral cells can be washed in by water droplets or insects

Attachment, Penetration, Infection•Bacterial and viral cells can be washed in by water droplets or insects

•Nematodes enter directly or through root stoma

Effects of disease on vital processesInfection is the establishment and growth of pathogenic organisms and their parasitic effects in the host

As pathogens obtain food and resources for themselves, they interfere with vital physiological functions

Symptoms appear due to destruction of certain plant parts

Photosynthesis and Respiration Photosynthesis - process by which plants turn sunlight into energy

◦ Ability to create energy through photosynthesis is limited by:◦ Leaf necrosis◦ Spotting◦ Defoliation◦ Chlorosis (loss of green pigment)◦ Destruction of enzymes involved in photosynthesis.

Respiration – process by which living organisms burn energy to perform physiological processes.◦ Rate is increase in diseased plants, reserves depleted more quickly as plants

mobilize defense mechanisms

Photo: Alan Windham

Effects on Translocation•Translocation is the movement of water and nutrients through the plant vascular system.•Pathogens interfere with translocation by:

Rotting roots and stems, inhibiting their ability to absorb and transport water and nutrients

Producing cankers, galls, and root knots that block transport

Effects on Translocation

Clogging vascular tissue with growth (ex. Fusarium, Verticillium wilts)

Damaging protective cuticle and integrity of cells, thereby increasing water loss

Effects on Growth and Development•Several viruses and some fungal pathogens affect plants by changing genetic material• Curling, mottling, stunting

•Some pathogens destroy reproductive structures and seeds

Plant Defenses Plants encounter thousands of pathogens every day Infection fails to take place for most encounters due to:

◦ Lack of compatibility◦ Structural Barriers◦ Formation of toxins and other chemical defenses

This is what causes plants to be resistant

Structural Defenses Many defenses keep pathogens from landing or sticking to plant tissue Preexisting

◦ Waxy cuticle – water film resistant◦ Thick cell walls◦ Leaf hairs◦ Thorns

Structural Defenses Induced

◦ Plant cell receptors that recognize certain pathogens or their compounds can cause formation of defense structures to limit infection

◦ Cytoplasm changes◦ Cell wall thickening and

surrounding◦ Formation of corky barriers or

abscission layers◦ Tyloses

Chemical Defenses•Plants produce substances that kill or repel pathogens or neutralize pathogen toxins

•Phenolics and acids – “fungitoxic” and/or prevent spore germination

•Phytoanticipins (antimicrobials)

•Inhibit digestive pathogen enzymes

•Tomatine and avenacin in tomatoes and oats have membraneolytic hydrolytic activity – break down pathogen

•Can also be induced – plants have receptors for some pathogens that can elicit cascade of responses to wall off or kill infection

Fungal Symptoms

Types of Disease PathogensFungal

Bacterial

Viral

Parasitic plants

Nematodes

Fungal Diseases•Kingdom Fungi – Eukaryotic (nucleus containing) organisms that are heterotrophic, absorbing carbon fixed by other organisms (unlike plants)•Once considered to be plants without chlorophyll•Made up of hyphae – strands/branches that make up the body of the fungus – mycelium•Some are easily seen by their fruiting bodies, others remain hidden in the soil, some are unicellular (yeast)

Fungal Diseases•Nutrient uptake – Fungi secrete enzymes that digest material to be absorbed by the fungal hyphae.•Cause disease by maceration, cell destruction, and disruption of cell and whole-plant processes•Reproduce using spores housed in many different types of fruiting bodies•Fungal diseases spread by spores or transferal of hyphae

Some fungal life cycle examples P 437 vert basic rhizopus

447 plum pocket – diff kinds of spores may be involved

P 462 septoria leaf spot like on tomato

Rose powdery mildew p 451

Stem canker p 481 and pictures 477

Apple scap p 506 may be good replacement of one of the above

Pp 606 wood rotting mushroom fungi

Add your own pics of ganoderma

Apple ScabVenturia inaequalis

Powdery Mildew of Rose Sphaerotheca pannosa f. sp. rosae

Stem Cankers

Wood- Rotting fungiex. Gannoderma

Fungal-like organisms Different kingdoms, but grow, reproduce, and cause disease much like Kingdom Fungus

May have differences in cell wall structure, but still produce spores

Myxo and Plasmoidium are more like masses of cytoplasm, not def cell wall.

Oomycetes more like tru fungi ◦ Phytophthora, pythium, downy

Control of Fungal Diseases Plant diseases are nearly impossible to “cure”

Control is mostly prevention-based

Holistic Approach - Follow best management practices

Use techniques that preserve plant health and minimize inoculation◦ Resistant varieties◦ Only accepting disease free plant material and seeds◦ Minimize moisture , increase root aeration◦ Destroy diseased material◦ Sanitation of tools and environment

Chemicals (synthetic and natural) can also be used, but should be in combination with above methods, and used preventatively as well

If have time, chemical control How some synthetics (chlorothalonil, etc. work_

How neem oil and other oils work specifically for fungi

Consult ch 9

Bacterial diseases•Prokaryotic single-celled organisms – do not form structures

•Are transported through the vascular tissue and intercellularly

•Cause disease by multiplying and secreting toxins or clogging vascular tissues, maceration, cell destruction, toxins, and disruption of cell and whole-plant processes

•Can also have DNA-altering activity

Infection•Bacterial cells can enter through wounds, stomata, hydathodes

•Can be vectored by insects and nematodes as well•Cause spots, soft rots, wilts, cankers, galls,

Vascular Wilts◦ Bacterial cells colonize vascular cells, destroying their integrity and limiting

water and nutrient movement

Affected xylem

Affected phloem

Stem Cankers•Bacteria often enter through nodes or buds•Colonize and damage tissues•Causes girdling

Fireblight on PearErwinia amylovora

Galls◦Bacteria parasitize the genetic and metabolic machinery of host

◦Bacterial cells enter through wounds and introduce own genetic material to host cell through a Ti plasmid.

◦These newly integrated genes cause host cells to synthesize materials useful only to bacteria, and to divide rapidly, forming galls, or tumors.

◦Similar to the establishment of cancer

Crown GallAgrobacterium

Control Even more difficult to control than fungal diseases Many disease-causing bacteria survive in the soil - source of inoculum

◦ Sterilize media and soil, clean up and destroy diseased crops

Holistic approach Resistant varieties Practice good sanitation:

◦ Frequently clean or change tools, especially in grafting and pruning◦ Minimize production of wounds

Copper compounds and antibiotics have had limited success

Viral Diseases•Viruses are considered non-living, obligate parasites

•Much smaller than bacteria and fungi

•Consist of a nucleic acid (genome) and protein covering, sometimes its own enzymes

•Parasitizes the genetic and metabolic machinery of its host

•Cause disease by disruption of normal cell functioning• Up- or down- regulate production of normal enzymes, compounds, and hormones, thus

altering growth and appearance

•Symptoms most often will consist of abnormal growth – mottling, mosaic discolorations, leaf stem and root malformations, stunting, streaking, distortion, etc.

Viral Symptoms

Infection and Spread•Viral particles enter through tiny wounds in cell walls. Nucleic acids are copied by viral or plant enzymes

•Viral nucleic acid also codes for new viral “parts” – new protein coats, enzymes, and copies of genome. New viral particles are made

•New particles are spread between cells (they are tiny enough) and through vascular tissues

Infection and Spread Can be spread by humans, animals, insects, fungi, mites, nematodes. Can remain in and spread through pollen and seed. Put figure 14-17 here

Eriophyid Mites and Rose Rosette

Thrips and Tomato Spotted Wilt Virus

Control of Viruses•Hardest to control•Resistant varieties•Limit insects and other disease-spreading organisms•Transgenic plants (TYLCV, tomato mottle begomovirus)•Virus free culture of apical meristem tips•No viricides exist

Other Plant PathogensParasitic Plants

Examples include Dodder and Witchweed◦ Produce flowers and seed but little to no

chlorophyll◦ Seeds exist in soil and germinate much like

plants◦ Strands wrap around stems and send fungus-

like roots into plant tissues to harvest nutrients

◦ Usually controlled with herbicide

Nematodes Microscopic worms living freely in the soil

Feed on roots with piercing stylet, inject saliva with enzymes, suck nutrients

Enzymes in saliva cause degradation of plant tissues

Causes root knots and lesions, root destruction and therefore typical ill-health symptoms above ground

Root Knot Nematode (Meloidogyne)

•Cause extensive damage to many plant types (tomato, potato, carrot, peanut, dogwood•Causes galls, root stunting, and root necrosis•Control by rotation with resistant varieties, soil solarization or fumigation, nematicides.

Diagnosing1. What is the host plant?

2. How many of the host plants are affected?

3. Is the pattern of damage random or uniform?

4. What plant parts are affected?

5. What are the signs and symptoms?

6. What recent activity has happened near the plants? Weather, temps, chemicals…

7. Cultural history (irrigation, fertilization). Has the soil been excessively wet?

8. Are there any wounds or mechanical damage?

Resources Extension

◦ County Agent◦ Soil Plant, Pest Center◦ search.extension.org

Resources Plant Pathology by George Agrios. 5th Edition, 2005

What’s Wrong With My Plant? And How Do I Fix It? by David Deardorff and Kathyrn Wadsworth. 2009