Plant pathology Dr. Ahed A H Matloob- plant diseases - patho.pdf · Dr.Ahed A.Hadi Plant Diseases from plant diseases is about $220 ... He wrote a book named ... In other way, disease

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Plant Diseases Dr.Ahed A.Hadi

Republic of Iraq

Ministry of higher education & scientific researches

Al Furat Al Awsat tech. University

Al-Musaib Tech. College-Biological control Dep.

Plant Diseases

Dr.Ahed A.Hadi Matloob Assistance Professor in Biological

Control Department 2016-2017

E-mail:[email protected]

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Plant Diseases lec.1 Dr.Ahed A.Hadi

Plant pathology: is a science that studies plant diseases and

attempts to improve the chances for survival of plants when

they are faced with unfavorable environmental conditions

and parasitic micro-organisms that cause disease.

by their presence, prevent the cultivation and growth of food

plants in some areas; or food plants may be cultivated and

grown but plant diseases may attack them, destroy parts or all of

the plants, and reduce much of their produce before they can be

harvested or consumed. plant pathology is joined by the sciences

of entomology and weed science.

Importance of the Plant Diseases

Globally, enormous losses of the crops are caused by the plant

diseases. Important historical evidences of plant disease

epidemics are Irish Famine due to late blight of potato (Ireland,

1845), Bengal famine due to brown spot of rice (India, 1942)

and Coffee rust (Sri Lanka, 1967). Such epidemics had left their

effect on the economy of the affected countries.

The losses are usually lower in the more developed countries

and higher in the developing countries, i.e., countries that need

food the most. It has been estimated that of the 36.5% average

of total losses, 14.1% are caused by diseases, 10.2% by insects,

and 12.2% by weeds. Considering that 14.1% of the crops are

lost to plant diseases alone, the total annual worldwide crop loss

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from plant diseases is about $220 billion. To these should be

added 6–12% losses of crops after harvest, which are

particularly high in developing tropical countries where training

and resources such as refrigeration are generally lacking. Also,

these losses do not include losses caused by environmental

factors such as freezes, droughts, air pollutants, nutrient

deficiencies, and toxicities.

The Concept of Disease in Plants

Because it is not known whether plants feel pain or discomfort

and because, in any case, plants do not speak or otherwise

communicate with us, it is difficult to pinpoint exactly when a

plant is diseased.

It is accepted that a plant is healthy, or normal, when it can

carry out its physiological functions to the best of its genetic

potential. When the ability of the cells of a plant or plant part to

carry out one or more of these essential functions is interfered

with by either a pathogenic organism or an adverse

environmental factor, the activities of the cells are disrupted,

altered, or inhibited, the cells malfunction or die, and the plant

becomes diseased.

Disease in plants, can be defined as the series of invisible and

visible responses of plant cells and tissues to a pathogenic

organism or environmental factor that result in adverse changes

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in the form, function, or integrity of the plant and may lead to

partial impairment or death of plant parts or of the entire plant.

HISTORY OF PLANT PATHOLOGY

In many of the early references, plant diseases were considered

to be a curse and a punishment of the people by God for wrongs

and sins they had committed. Greek philosopher Theophrastus

(about 286 BC) recorded some plant diseases about 2400 years

ago. yet Theophrastus and his contemporaries, being unable to

explain plant diseases, believed that God controlled the weather

that “brought about” the disease. therefore, that avoidance or

control of the disease depended on people doing things that

would please that same superpower. In the fourth century b.c.;

the Romans suffered so much from hunger caused by the

repeated destruction of cereal crops by rusts and other diseases

that they created a separate god, whom they named Robigus. To

please Robigus, the Romans offered prayers and sacrifices in the

belief that he would protect them from the dreaded rusts. The

Romans even established a special holiday for Robigus, the

Robigalia, in an attempt to please and pacify Robigus so he

would not send the rusts to destroy their crops. some ancient

writers, e.g., Homer (c. 1000 b.c.), mention the therapeutic

properties of sulfur on plant diseases, and Democritus (c. 470

b.c.) recommended controlling plant blights by sprinkling plants

with the olive grounds left after extraction of the olive oil. This

branch of science could maintain a proper record on the plant

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disease and their causal organisms only after development of

compound microscope by the Dutch worker Antony von

Leeuwenhoek in 1675. He first visualized bacteria in 1683 under

his microscope. Some of the most famous phytopathologists that

contributed to development the plant diseases science were:

1) ANTON De BARY (Germany): He was the father and

founder of modern Mycology. He was the founder of modern

experimental plant pathologyIn 1863, he studied the epidemics

of late blight and renamed the casual organism as Phytophthora

infestans. He discovered heteroecious nature of rust fungi

(1865). He gave detailed account on life cycles of downy

mildew genera. He studied about vegetable rotting fungi and

damping off fungi. He wrote a book named “Morphology and

Physiology of fungi, lichens and Myxomycetes” (1866). He

reported the role of enzymes and toxins in tissue disintegration

caused by Sclerotinia sclerotiorum

2) T. J. BURRUILL (USA)(1878): He proved for the first time

that fire blight of apple and pear was caused by a bacterium

(now known as Erwinia amylovora)

3) Needham (1743): made The first report of nematodes

associated with a plant disease which was Anguina tritici caused

Wheat seed galls.

4) DOI (JAPANESE)(1967)

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he found that mycoplasma like organisms (MLO) could be

responsible for the disease of the yellows type. Doi observed

that MLO's are constantly present in phloem.

5)BEIJERINCK (Dutch)(1898) Founder of virology He

proved that the virus inciting tobacco mosaic is not a living

microorganism. He believed it to be contagium vivum fluidum

(infectious living fluid) then named it virus

6)DIENER (1971) discovered the potato spindle tuber was

caused by small naked RNA which he called viroid

FIGURE 1 A. Theophrastus, the“father of botany.” B. Antonius van Leeuwenhoek C. Anton DeBary D. Diener.

Causes of Plant Diseases

Plant diseases are caused by pathogens. Hence a pathogen is

always associated with a disease. In other way, disease is a

symptom caused by the invasion of a pathogen that is able to

survive, perpetuate and spread. Further, the word “pathogen”

can be broadly defined as any agent or factor that incites pathos

A B C D

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or disease in an organism or host. In strict sense, the causes of

plant diseases are grouped under following categories:

1. Animate or biotic causes: Pathogens of living nature are

categorized into the following groups.

(i) Fungi (vi) plant parasite

(ii) Bacteria (vii) Protozoa

(iii) Phytoplasma (viii) Nematodes

(iv) Rickettsia-like organisms (ix) Viruses

(v) Algae (X) Viroids

2. Inanimate or abiotic causes: In true sense these factors cause

damages (any reduction in the quality or quantity of yield or loss

of revenue) to the plants rather than causing disease. The causes

are:

(i) Deficiencies or excess of nutrients.

(ii) Light

(iii) Moisture

(iv) Temperature

(v) Air pollutants (e.g. black tip of mango)

(vi) Lack of oxygen (e.g. hollow and black heart of potato)

(vii) Toxicity of pesticides

(viii) Improper cultural practices

(ix) Abnormality in soil conditions (acidity, alkalinity).

********************

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components of disease

Specific conditions must be present for biotic disease to

develop. There must be a susceptible host plant, the pathogen

(fungi, bacteria, viruses, etc.) or when it is affected by an abiotic

agent, and environmental conditions conducive to disease

development; this called Disease Triangle, these must come

together in a given point in time. These conditions make up

what is called the Plant Disease Pyramid (Epidemic-lec. 4).

Biotic disease cannot occur if one of these pieces is missing.

[Figure 2] We can see that human putting in the top of Plant

Disease Pyramid because his role in disease development and

dissemination of pathogens by some wrong processes.

Human

A B

Figure 2, A . Disease Triangle B. Plant Disease Pyramid

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SOME TERMS AND CONCEPTS USED IN PLANT

PATHOLOGY

- Infection: The initiation and establishment of a parasite within

a host plant.

- Pathogen: is any agent that causes Disease

- Parasite: Organisms which derive the materials they need for

growth from living plants

- Pathogenicity is the ability of the pathogen to cause disease.

- Symptom: The external or internal reactions or alterations of a

plant as a result of a disease. such as wilt, yellow, spot,

rot,…etc.

- Sign: The pathogen or its parts or products seen on a host

plant. Such as hyphae, spores…etc.

Inoculum potential: It is the inoculum needed for successful

infection. It is a function of inoculum density and their

capacity.

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HOST RANGE OF PATHOGENS

Pathogens differ with respect to the kinds of plants that they can

attack, organs and tissues that they can infect, and with respect

to the age of the organ or tissue of the plant.

- Some pathogens are restricted to a single species, others to one

genus of plants, and still others have a wide range of hosts.

- Some pathogens grow especially on roots, others on stems,

leaves or on fleshy fruits or vegetables.

- Some pathogens, e.g., vascular parasites, attack specifically

certain kinds of tissues, such as phloem or xylem.

- Some pathogens attack seedlings or the young parts of plants,

whereas others attack only mature tissues.

STAGES IN THE DEVELOPMENT OF DISEASE:

THE DISEASE CYCLE

In every infectious disease a series of events occurs in

succession and leads to the development of the disease and the

pathogen. This chain of events is called a disease cycle. The

events in a disease cycle are inoculation, penetration,

establishment of infection, colonization (invasion), growth and

reproduction of the pathogen, dissemination of the pathogen,

and survival of the pathogen in the absence of the host, i.e.,

overwintering or oversummering (overseasoning) of the

pathogen. Usually there is one cycle of disease which called

Monocyclic disease, In some diseases there may be several

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infection cycles within one disease cycle, called polycyclic

disease (figure 3)

figure 3. Mono and Polycyclic disease.

Steps of Disease Development

The steps of the disease cycle are inoculation, penetration,

infection, invasion, reproduction, and dissemination. Then

overseasoning.

1. Inoculation is the placement of the pathogen’s infectious unit

or propagule on or in close proximity to the host cell wall.

-The inoculum is any part of the pathogen that can initiate

infection.

Types of Inoculum An inoculum that survives dormant in the

winter or summer and causes the original infections in the spring

or in the autumn is called a primary inoculum, and the

infections it causes are called primary infections. An inoculum

produced from primary infections is called a secondary

inoculum and causes secondary infections.

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2. Penetration The propagule then penetrates the cell wall of

the host. In fungi, the propagule may germinate and the germ

tube may penetrate the wall directly or indirectly through a

wound or natural opening. Bacteria enter plants mostly through

wounds and less frequently through natural openings. Viruses,

viroids, mollicutes, fastidious bacteria enter through wounds

made by vectors.

Fungi, nematodes and parasitic higher plants enter through

direct penetration and less frequently through natural openings

and wounds.

3. INFECTION pathogens grow and multiply within the plant

tissues. Invasion of plant tissues by the pathogen, and growth

and reproduction of the pathogen (colonization) are two

concurrent stages of disease development.

4- dissemination After infection takes place, the pathogen may

grow and invade other parts of the host. The pathogen will

continue to reproduce and the new propagules will be dispersed

or disseminated by a variety of means such as wind, rain, within

or on vectors, seed, or on contaminated debris or equipment.

5- overseasoning in the end of season pathogen survive in plant

debris , in the soil, or formed survival bodies.

Incubation period: The period between penetration of a host by

a pathogen and the first appearance of symptoms on the host. It

varies with pathogens, hosts and environmental conditions.

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Levels of Parasitism

Pathogens can be classified into several groups.

- obligate parasites. (Biotrophs) are pathogens that require

living host tissue to complete their life cycle. e. g. fungi,

such as powdery mildews , rusts and some members of

the Oomycota, such as downy mildews and white

rusts ,viruses and viroids; phytoparasitic nematodes. and

they are called obligate parasites.

- facultative saprophyte : is a pathogen that often behaves

as a parasite but under certain conditions behaves as a

saprophyte on organic matter,

- facultative parasite: A pathogen that often behaves as a

saprophyte but under some conditions becomes a parasite.

Necrotrophs: Non biotrophic organisms kill before

feeding on the cells or cellular contents. These organisms

that live on dead tissues.

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HOW PATHOGENS ATTACK PLANTS

Healthy plant is a community of cells built in a fortress-like

fashion. Therefore, for a pathogen to infect a plant it must be

able to make its way into and through the plant, obtain nutrients

from the plant, and neutralize the defense reactions of the plant.

Pathogens accomplish these activities mostly through secretions

of chemical substances that affect certain components or

metabolic mechanisms of their hosts. Penetration and invasion,

however, some cases be entirely the result of the mechanical

force exerted by certain pathogens on the cell walls of the plant.

MECHANICAL FORCES EXERTED BY PATHOGENS

ON HOST TISSUES

• Only some fungi, parasitic higher plants, and nematodes appear

to apply mechanical pressure to the plant surface they are about

to penetrate.

• cutinase and cellulase enzymes released from the spore surface

help the spore adhere to the plant surface. After contact is

established, the diameter of the tip of the hypha or radicle in

contact with the host increases and forms the flattened, bulb-like

structure called the appressorium (Figs. 4). This increases the

area of adherence between the two organisms and securely

fastens the pathogen to the plant. From the appressorium, a fine

growing point, called the penetration peg arises and advances

into and through the cuticle and cell wall.

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• After penetration of the cuticle, the hyphal tube diameter often

increases considerably. The penetration tube attains the diameter

normal for the hyphae of the particular fungus only after it has

passed through the cell wall .

• Nematodes penetrate plant surfaces by means of the stylet,

which is thrust back and forth and exerts mechanical pressure on

the cell wall.

figure 4 germination and penetration of fungal spore and invasion

CHEMICAL WEAPONS OF PATHOGENS

The main groups of substances secreted by pathogens in plants

enzymes, toxins, growth regulators, and polysaccharides. and

their relative importance may be different from one disease to

another. Thus, in some diseases, such as soft rots ( enzymes ),

crown gall ( growth regulators) , the Bipolaris blight of Victoria

oats ( toxin ) and some of wilt types (Polysaccharides)).

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1-Enzymes: Most pathogens derive energy principally from

enzymatic break down of food materials from host tissue.The

epidermis of plants is covered by cuticle, whose major chemical

substance is cutin in addition to cuticular wax.

- Cuticular wax: Plant waxes are found as granular or rod like

projections or as a continuous layer outside / within the cuticle..

Most of the fungi and parasitic higher plants penetrate wax

layers by means of mechanical force alone (no enzyme).

- Cutin: Cutin is admixed with waxes on upper side and with

pectin and cellulose on the lower side. Cutinases break cutin

molecules , Cutinases reaches its highest concentration at

penetrating point of the germ tube and at infection peg of

appressorium forming fungi.

- Pectic substances: These are major components of middle

lamella (intercellular cement that holds in place the cells of plant

tissues). They also make up a large portion of primary cell wall

in which they form an amorphous gel filling the spaces between

cellulose microfibrils, The enzymes that degrade pectic

substances are known as pectinases.

- Cellulose: Cellulose occurs in all higher plants as the skeletal

substance of cell walls in the form of microfibrils. These

microfibrils are like bundles of iron bars in a reinforced concrete

building. Cellulose is degraded by cellulases

- Lignin: Lignin is found in the middle lamella, as well as in

the secondary cell wall of xylem vessels and the fibres that

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strengthen plants. White rot fungi (Basidiomycetes) secrete one

or more ligninases which enable them to utilize lignin.

- proteins: proteins are degraded by enzymes, proteases or

proteinases.

- Lipids: Various types of lipids occur in all plant cells. The

most important ones are phospholipids and glycolipids. These

lipids contain fatty acids. Lipolytic enzymes, called lipases

(phospholipases, glycolipases) hydrolyze lipids and release fatty

acids.

- Starch: It is a glucose polymer and exists in two forms:

amylose, a linear molecule, and amylopectin, a highly branched

molecule. Starch is degraded by enzyme, amylases.

2- ROLE OF TOXINS IN PLANT PATHOGENESIS

Toxin can be defined as a microbial metabolite excreted or

released by cells which in very low concentration is directly

toxic to the cells of the host.

- Classification based on specificity of toxins

1. Host specific / Host selective toxins: These are the metabolic

products of the pathogens which are selectively toxic only to the

susceptible host of the pathogen Ex: Victorin, T-toxin and

Phyto-alternarin.

2. Non-specific / Non-selective toxins

These are the metabolic products of the pathogen, but do not

have host specificity and affect the protoplasm of many

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unrelated plant species that are normally not infected by the

pathogen Ex: Tab-toxin, Fusaric acid, Piricularin,

Lycomarasmin.

Effect of toxins on host tissues

A) Changes in cell permeability: Toxins kill plant cells by

altering the permeability of plasma membrane

B) Disruption of normal metabolic processes, Increase in

respiration due to disturbed salt balance. Malfunctioning of

enzyme system.

C) Interfere with the growth regulatory system of host plant.

3- ROLE OF GROWTH REGULATORS IN PLANT

PATHOGENESIS

Growth regulators Growth regulators are of two types:

1. Growth promoting substances and 2. Growth inhibiting

substances

Auxins, gibberellins and cytokinins are growth promoting

substances, whereas, dormin, ethylene and abscissic acid are

growth inhibiting substances. The imbalance in growth

promoting and growth inhibiting substances causes

hypertrophy (excessive increase in cell size) and atrophy

(decrease in cell size). Symptoms may appear as tumors, galls,

knots, witches broom, stunting, excessive root branching,

defoliation and suppression of bud growth.

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4- ROLE OF POLYSACCHARIDES IN PATHOGENESIS

Polysaccharides: Fungi, bacteria and nematodes release

varying amounts of mucilaginous substances that coat their

bodies and provide interface between the outer surface of the

micro-organism and its environment. In the vascular wilts, large

polysaccharide molecules released by the pathogen in the xylem

causes mechanical blockage of vascular bundles and initiate

wilting. Ex: Ralstonia solanacearum (Bacterial wilt of

Solanaceous plants)

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- Epidemic diseases

An epidemic has been defined as any increase of disease in a

population. The study of epidemics and of the factors that

influence them is called epidemiology. Epidemiology is

concerned simultaneously with populations of pathogens and

host plants as they occur in an evolving environment

THE ELEMENTS OF AN EPIDEMIC

Plant disease epidemics develop as a result of the timely

combination of the same elements that result in plant disease:

susceptible host plants, a virulent pathogen, and favorable

environmental conditions over a relatively long period of time.

Humans may unwittingly help initiate and develop epidemics

through some of their activities, (the disease triangle,

tetrahedron, or pyramid, (discussed in Chapter 1) (Fig.-2).

HOST FACTORS THAT AFFECT THE DEVELOPMENT

OF EPIDEMICS

Several internal and external factors of particular host plants

play an important role in the development of epidemics

involving those hosts.

* Levels of Genetic Resistance or Susceptibility of the Host.

* Degree of Genetic Uniformity of Host Plants a new

pathogen race will appear that can attack their genome and

result in an epidemic.

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* Type of Crop- In diseases of annual crops, such as corn,

vegetables, rice, and cotton, epidemics generally develop much

more rapidly (usually in a few weeks) than they do in diseases

of branches and stems of perennial woody crops such as fruit

and forest trees.

Age of Host Plants the age of the host plant at the time of

arrival of the pathogen may affect considerably the development

of infection and of an epidemic.

PATHOGEN FACTORS THAT AFFECT

DEVELOPMENT OF EPIDEMICS

*Levels of Virulence Virulent pathogens capable of infecting

the host rapidly ensure a faster production of larger amounts of

inoculum, and, thereby, disease, than pathogens of lesser

virulence.

*Quantity of Inoculum near Hosts The greater the number of

pathogen propagules (bacteria, fungal spores and sclerotia,

nematode eggs, virus infected plants, etc.) within or near fields

of host plants, the more inoculum reaches the hosts and at an

earlier time, thereby increasing the chances of an epidemic

greatly.

*Type of Reproduction of the Pathogen

All pathogens produce many offspring, Some plant pathogenic

fungi, bacteria, and viruses have short reproduction cycles and

therefore are polycyclic, i.e., they can produce many generations

in a single growing season. Polycyclic pathogens include fungi

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that cause rusts, mildews, and leaf spots and are responsible for

most of the sudden, catastrophic plant disease epidemics in the

world.

*Ecology of the Pathogen Some pathogens, such as most fungi

and all parasitic higher plants, produce their inoculum (spores

and seeds, respectively) on the surface of the aerial parts of the

host. From there, spores and seeds can be dispersed with ease

over a range of distances and can cause widespread epidemics.

Other pathogens, such as vascular fungi and bacteria,

mollicutes, viruses, and protozoa, reproduce inside the plant. In

this case, spread of the pathogen is rare or impossible without

the help of vectors.

*Mode of Spread of the Pathogen The spores of many plant

pathogenic fungi, such as those causing rusts, mildews, and leaf

spots, are released into the air and can be dispersed by air

breezes or strong winds over distances varying from a few

centimeters up to several kilometers. These kinds of fungi are

responsible for the most frequent and most widespread

epidemics.

ENVIRONMENTAL FACTORS THAT AFFECT

DEVELOPMENT OF EPIDEMICS

The environment may affect the availability, growth stage,

succulence, and genetic susceptibility of the host plants. It may

also affect the survival, vigor, rate of multiplication, sporulation,

and ease, direction, and distance of dispersal of the pathogen, as

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well as the rate of spore germination and penetration. In

addition, the environment may affect the number and activity of

the vectors of the pathogen. The most important environmental

factors that affect the development of plant disease epidemics

are moisture, temperature, and the activities of humans in terms

of cultural practices and control measures.

EFFECT OF HUMAN CULTURAL PRACTICES AND

CONTROL MEASURES

Many activities of humans have a direct or indirect effect on

plant disease epidemics, some of them favoring and some

reducing the frequency and the rate of epidemics.

*Site Selection and Preparation

*Selection of Propagative Material The use of seed, nursery

stock, and other propagative material that carries various

pathogens increases the amount of initial inoculum within the

crop and favors the development of epidemics greatly. The use

of pathogen-free or treated propagative material can reduce the

chance of epidemics greatly.

*Cultural Practices Continuous monoculture, large acreages

planted to the same variety of crop, high levels of nitrogen

fertilization, no-till culture, dense plantings, overhead irrigation,

injury by herbicide application, and poor sanitation all increase

the possibility and severity of epidemics.

*Disease Control Measures

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*Introduction of New Pathogens The ease and frequency of

worldwide travel have also increased the movement of seeds,

tubers, nursery stock, and other agricultural goods. These events

increase the possibility of introducing pathogens into areas

where the hosts have not had a chance to evolve resistance to

these pathogens.

**************************************

Sources of Inoculum

- In some fungal and bacterial diseases of perennial plants,

such as shrubs and trees, the inoculum is produced on the

branches, trunks, or roots of the plants.

- The inoculum sometimes is present in the plant debris or

soil;

- Other times it comes into the field with the seed,

transplants, tubers, or other propagative organs.

- the inoculum survives in perennial weeds or alternate

hosts.

- Fungi, bacteria, parasitic higher plants, and nematodes

either produce their inoculum on the surface of infected

plants.

- Viruses, viroids, mollicutes, fastidious bacteria, and

protozoa produce their inoculum within the plants

transmitted from one plant to another by some kind of

vector, such as an insect.

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ENVIRONMENTAL EFFECTS ON THE DEVELOPMENT

OF INFECTIOUS PLANT DISEASE

1- EFFECT OF TEMPERATURE

Pathogens differ in their preference for higher or lower

temperatures. some species of the fungi Typhula and Fusarium,

which cause snow mold of cereals and turf grasses, thrive only

in cold regions. Also, the late blight pathogen Phytophthora

infestans is most serious in the winter. Many diseases, such as

the brown rot of stone fruits caused by Monilinia fructicola, are

favored by relatively high temperatures. Several diseases, such

as the fusarial wilts, many anthracnoses caused by

Colletotrichum, and the bacterial wilts of solanaceous plants

caused by Ralstonia solanacearum, are favored by high

temperatures and are limited to hot areas.

2- EFFECT OF MOISTURE

Moisture, may exist as rain or irrigation water on the plant

surface or around the roots, as relative humidity in the air, and

as dew.

- Moisture is indispensable for the germination of fungal

spores and penetration of the host by the germ tube.

- It is also indispensable for the activation of bacterial,

fungal, and nematode pathogens before they can infect the

plant.

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- Moisture, in such forms as splashing rain and running

water, also plays an important role in the distribution and

spread of many of pathogens on the same plant and on

their spread from one plant to another.

- Finally, moisture increases the succulence of host plants

and thus their susceptibility to certain pathogens, which

affects the extent and severity of disease.

late blight of potato, apple scab, downy mildew of grapes, and

fire blight are found or are severe only in areas with high rainfall

or high relative humidity during the growing season.

In many diseases affecting underground parts of plants, such as

roots, tubers, and young seedlings, e.g., in the Pythium damping

off of seedlings and seed decays, the severity of the disease is

proportional to the amount of soil moisture and is greatest near

the saturation point.

Several other fungi, e.g., Fusarium solani, which is the cause of

dry root rot of beans, and Macrophomina phaseoli, the cause of

charcoal rot of sorghum and of root rot of cotton, grow fairly

well in rather dry environments.

EFFECT OF WIND

Wind influences infectious plant diseases by:

- increasing the spread of plant pathogens.

- increasing the number of wounds on host plants.

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- accelerating the drying of wet surfaces of plants.

EFFECT OF LIGHT

Several diseases are depended on the intensity and the duration

of light which may increase or decrease the susceptibility of

plants to infection and also the severity of the disease. ex.

Reduced light intensity generally increases the susceptibility of

plants to virus infections.

EFFECT OF SOIL PH AND SOIL STRUCTURE

The pH of the soil is important in the occurrence and severity of

plant diseases caused by certain soilborne pathogens. For

example, the clubroot of crucifers caused by Plasmodiophora

brassicae is most prevalent and severe at about pH 5.7, the

common scab of potato caused by Striptomyces scabies can be

severe from pH 5.2 to 8.0 or above.

EFFECT OF HOST–PLANT NUTRITION

Nutrition affects the rate of growth and the state of readiness of

plants to defend themselves against pathogenic attack.

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PLANT DISEASES CAUSED BY FUNGI Fungi are small, generally microscopic, eukaryotic, usually

filamentous, branched, that lack chlorophyll. More than 10,000

species of fungi can cause disease in plants.

CHARACTERISTICS OF PLANT PATHOGENIC FUNGI

Morphology

• Most fungi have a filamentous vegetative body called a

mycelium. The individual branches of the mycelium are

called hyphae. may or may not be partitioned by cross

walls (septa).

• Reproduction Fungi reproduce chiefly by means of

spores. Spores are reproductive bodies consisting of one

or a few cells. Spores may be formed asexually, like buds

produced on a twig, or as the result of sexual fertilization.

In some fungi, asexual spores are produced inside a sac

called a sporangium. Some of these spores can swim by

means of flagella and are called zoospores. Other fungi

produce asexual spores called conidia by the cutting off of

terminal or lateral cells from special hyphae called

conidiophores. In some fungi, terminal cells of a hypha

enlarge, round up, form a thick wall, and separate to form

chlamydospores.

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• Sexual reproduction occurs in most groups of fungi. In

Zygomycetes, produce a zygospore. In Ascomycetes,

ascospores. In Basidiomycetes, sexual spores are

produced called the basidiospores. In the Oomycetes,

oospores. Some fungi (formerly known as fungi imperfect

or deuteromycetes), no sexual reproduction is known or it

has not yet been discovered.

Diseases Caused by class: Plasmodiophoromycetes

Their body is a plasmodium, an amoeboid mass of protoplasm

that has many nuclei and no definite cell wall. Three

Plasmodiophoromycetes cause the following common diseases

of plants which were The pathogens are obligate parasites:

Plasmodiophora, causing clubroot of crucifers

Polymyxa, causing a root disease of cereals and grasses

Spongospora, causing the powdery scab of potato

1-CLUBROOT OF CRUCIFERS

The clubroot disease of cruciferous plants, such as cabbage and

cauliflower, is widely distributed all over the world. Clubroot

can cause serious losses to susceptible varieties.

30


Symptoms

Infected plants at first have pale green to yellowish leaves.

Later, infected plants show wilting in the middle of hot, sunny

days, recovering during the night. plants stunted and fail to

produce marketable heads. The most characteristic symptoms of

the disease appear on the roots (Fig. 6) as spindle-like, spherical,

knobby, or club-shaped swellings.

The Pathogen: Plasmodiophora brassicae

Its body is a plasmodium. The plasmodium gives rise to

zoosporangia or to resting spores which on germination,

produce zoospores.

Development of Disease

Wintering\ as resting spores in the soil and debris.

Inoculation\ The single zoospore produced from resting spores

Penetration and infection\ zoospore penetrates (directly or from

the wounds) root hairs, and there develops into a plasmodium.

After a few days, the plasmodium cleaves into multinucleate

portions and each develops into a zoosporangium containing

four to eight secondary zoospores.

Dissemination\ The zoospores are discharged outside the host

through pores dissolved in the host cell wall (FIG 5).

31


FIGURE 5. Life cycle of Plasmodiophora brassicae

Resting spore

Zoospores are released Through pores

Fungus-filled cells of

cabbage root

Multinucleate plasmodium

releasing resting spores

Zoospores formed in Zoosporangia

still in host

Figure 5. club-root on crucifers

32


(The plasmodium-infected clubs not only utilize much of the

food required for the normal growth of the plant, they also

interfere with the absorption and translocation of mineral

nutrients and water through the root system. This results

stunting and wilting of the aboveground parts of the plant).

Control

1- Addition hydrated lime to raise the soil to pH 7.2. At that pH,

spores of the clubroot organism germinate poorly or not at all.

2- treating Seed bed areas with appropriate soil fumigants

approximately two weeks before planting. can be kept free of

clubroot .

3- using the clean, clubroot-free seedlings, and watered with a

solution of an effective fungicide.

4- Some varieties of cruciferous hosts are resistant to certain

races of the clubroot organism and can be grown in areas

infested with these races.

33



Diseases Caused by Oomycetes

Oomycetes produce oospores (sexual spore) and zoospores

(asexual spores). The most important plant pathogenic pathogen

belong the order called Peronosporales. This order includes

several of the most important genera of plant pathogens known

such as:

-Pythium sp., one of the most common and most important

causes of seed rot, seedling damping-off, and root rot of all

types of plants, and also of soft rots of fleshy fruits in contact

with the soil (Figure 7).

-Phytophthora sp., late blight of potato and several others

causing root rots, fruit rots, and blights of many other annual

and perennial plants, and root and stem rots, cankers and

diebacks of trees.

-Downy Mildews

The Pathogen: There are many genera in the family

Peronosporaceae The genera are distinguished by the highly

differentiated branching patterns of sporangiophores that emerge

through stomata on lower leaf surfaces, Some of the most

common or most serious downy mildew oomycetes and the

diseases they cause are listed below.

Bremia, sporangiophore with binary branch and terminal

swellings like hand, causing downy mildew of lettuce(B.

lactucae).

34


Peronospora, sporangiophore binary branches curved with ends

like claws, causing downy mildew of onion (P. destructor).

Plasmopara, sporangiophore branches 2-8 with right angles

causing downy mildew of grape (P. viticola).

Pseudoperonospora, sporangiophore like Plasmopara and

Peronospora right angles wit claw ends, causing downy mildew

of cucurbits (P. cubensis) .

Sclerospora, sporangiophore compound binary branch, causing

downy mildew of grasses and millets (S. graminicola) .(Fig 8)

Basidiophore, sporangiophore without branches and has

basidium shape.

Development of Disease (life cycle).

- overwinters as oospores in dead leaf lesions and shoots.

- During rainy periods in the spring the oospores germinate

to produce a sporangium.

- The sporangium or its zoospores are transported by wind

or water to the wet leaves near the ground, which they

infect through stomata of the lower surface, The mycelium

then spreads into the intercellular spaces of the leaf

- sporangiophores arise and emerge through the stoma.

- The sporangia may be carried by wind or rain to nearby

healthy plants, germinate quickly, and produce many

zoospores that cause secondary infections and thus spread

the disease rapidly.

35


- At the end of the growing season fungus form Oospore

(Fig 9)

FIGURE 7. (A) Pythium seed rot., One healthy bean (B) Pythium, barley seedlings (C) Soft rots of squash (A) and potato (B) caused by Pythium

A

C B

36


Figure 8. Some genera of fungi causing Downy mildew

37


Fig 9. life cycle of Downy mildew pathogen

38



Diseases Caused by Zygomycetes

Zygomycetes have well-developed mycelia without cross walls

and produce non-motile spores in sporangia; their sexual spore

is a thick-walled Zygospore. Three genera of Zygomycetes are

known to cause disease in plants or plant products:

(1) Choanephora, which attacks the withering floral parts of

many plants after fertilization and from there invades the fruit

and causes a soft rot of primarily summer squash but also of

pumpkin, pepper, and okra.

(2) Rhizopus and (3) Mucor, both common bread mold fungi,

which in addition cause soft rot of many fleshy fruits,

vegetables, flowers. Other genera are fungi that become

associated with roots of plants (Mycorrhizae) e.g., Endogone,

Glomus, that are beneficial to plants.

Rhizopus SOFT ROT OF FRUITS AND VEGETABLES

Rhizopus soft rot of fruits and vegetables occurs throughout the

world on harvested fleshy organs of vegetable, fruit, and flower

crops during storage, transit, and marketing of these products.

The Pathogen: Rhizopus spp.

The mycelium of the fungus produces long, aerial

sporangiophores at the tips of which black spherical sporangia

develop. the sexually produced spore is called a zygospore

(zygospore produced by the union of two morphologically

39


similar gametes) and is the overwintering of the fungus. When it

germinates it produces a sporangiophore bearing a sporangium

full of sporangiospores.(Figure 10).

Figure 10. life cycle of Rhizopus


-Throughout the year, sporangiospores float about and if they

land on wounds of fleshy fruits, roots, corms, or bulbs.

- sporangiospores germinate. The resulting hyphae secrete

pectinolytic and cellulolytic enzymes, which break down and

dissolve the pectic substances of the middle lamella “soft rot.”.

- The fungus continues to grow inside the tissues. When the

epidermis breaks, the fungus emerges through the wounds and

produces aerial sporangiophores, sporangia, the mycelium can

penetrate even healthy fruit. When the food supply in the

infected tissues begins to diminish and compatible strains are

40


present together, zygospores are produced. Zygospores help the

fungus survive (Figure 11).

Figure 11. A. Zygospore B. Soft rot on squash

A B

41



DISEASES CAUSED BY ASCOMYCOTA

The Ascomycota are true fungi with a well-developed, septate

mycelium The sexual structure of the ascomycetes is an ascus,

which is shaped like a sac and contains the products called

ascospores

The Powdery Mildews

•Powdery mildews are obligate parasites. have specialized

feeding cells called haustoria that absorb nutrients from their

hosts.

• Pathogen: The powdery mildew diseases of the various crop or

other plants are caused by many species of fungi of the family

Erysiphaceae grouped onto several main genera. These genera

are distinguished from one another by the number (one versus

several) of asci per cleistothecium and by the morphology of

hyphal appendages growing out of the wall of the

cleistothecium. The main genera are illustrated in, and the most

important diseases they cause are listed here.

Blumeria, B. graminis causing powdery mildew on cereals and

grasses.

42


Erysiphe, E. cichoracearum causing powdery mildew of

begonia, chrysanthemum, cucurbits; E. polygoni of legumes,

beets, crucifers, and cucurbits; E. betae of beets; and E. orontii

of tomato

Leveillula, L. taurica causing powdery mildew of eggplant and

pepper.

Microsphaera, M. alni causing powdery mildew of many shade

trees and woody ornamentals.

Phyllactinia spp., causing powdery mildew of shade and forest

trees.

Podosphaera, P. leucotricha causing powdery mildew of apple,

pear, and quince; P. oxyacanthae, of apricot, cherry, peach, and

plum; and P. xanthii, of cucurbits.

Sphaerotheca, , S. pannosa of peach and rose.

Uncinula necator, causing powdery mildew of grape.

Development of Disease.

- The fungus overwinters mostly as mycelium in the buds.

Cleistothecia form occasionally toward the end of the

season. (Fig. 13).

- Ascospores or conidia are carried by wind to young green

tissues.

- the spores germinate and infect these tissues. The germ

tube grows directly into the epidermal cells and forms a

haustorium by which the fungus obtains its nutrients.

43


- The germ tube, however, continues to grow and branch on

the surface of the plant tissue, producing a network of

mycelium that sends haustoria into the epidermal cells.

The absorption of nutrients from the cells depletes their

food supply, weakens them, and may sometimes lead to

their death. Photosynthesis in the affected areas is reduced

greatly.

- The aerial mycelium produces numerous conidia, which

cause new infections on the expanding leaves and shoots.

In the end season sexual spore (ascospore) be formed in

cleistothecia to surviving.

FIGURE 13 Life cycle of a powdery mildew (a) Conidium. (b) Condium germinating and forming an appressorium on the leaf surface. (c) Conideophores and conidia. (d) Sexual reproduction between compatible mating types. (e) Ascocarps, ascus, and ascospores. Note germinating

44



DISEASES CAUSED BY BASIDIOMYCETES

Basidiomycetes are fungi that produce their sexual spores,

called Basidiospores, on a club-shaped spore producing

structure called a Basidium. Most Basidiomycetes are fleshy

fungi, such as the common mushrooms, the puffballs, also

include two very common and very destructive groups of plant

pathogenic fungi that cause the rust and the smut diseases of

plants.

SMUTS

Plant smuts, caused by Basidiomycetes of the order

Ustilaginales, occur throughout the world. smuts attack the grain

kernels themselves and replace the kernel contents with the

black, dusty spore masses that resemble soot or smut. Most smut

fungi attack the ovaries of grains and grasses and develop in

them and in the fruit, the kernels of grain crops, which they

destroy completely. Most smut fungi produce only two kinds of

spores: Teliospores and Basidiospores. The most common smut

fungi and the diseases they cause are the following:

Ustilago, causing corn smut [U. zeae (maydis)], loose smut of

cereals ( U. nuda, and U. tritici),

Tilletia, causing covered smut or bunt of wheat [T. caries and T.

foetida)]

Sphacelotheca, causing the sorghum smuts (S. sorghi, and S.

reiliana)

45


Urocystis, causing onion smut (U. cepulae)

1-COVERED SMUT, OR BUNT, OF WHEAT

Covered smut, or bunt, or stinking smut of wheat occurs in all

wheat-growing areas of the world. Bunt destroys the contents

(except cover) of infected kernels and replaces them with the

spores of the fungus.

Pathogen. Tilletia caries and T. foetida


-The pathogens of common bunt overwinter as teliospores on

contaminated wheat kernels and less frequently in the soil.

- Teliospore on the kernel or near the seedling germinates

through the production of the basidium, the mycelium they

produce infects the young seedling and penetrate the wheat

seedlings after seedling emergence.

-After penetration, the mycelium grows intercellularly and

invades the developing leaves and the meristematic tissue at the

growing point of the plant. When the plant forms the head of the

grain, the mycelium invades all parts of it and consume the

contents of the kernel cells. kernels are usually break and release

their spores on harvest or threshing. The liberated spores

contaminate the healthy kernels and are also blown away by air

currents, thus contaminating the soil.

46


2-LOOSE SMUT OF CEREALS

In an infected plant, usually all the heads and all the spike lets

and kernels of each head are smutted

The Pathogens: Ustilago nuda and Ustilago tritici

3- CORN SMUT (common smut)

Minute galls form on the leaves and stems, Infected areas are

permeated by the fungus mycelium, which stimulates the host

cells to divide and enlarge, thus forming galls. Galls are first

covered with a greenish white membrane. Later, as the galls

mature, they reach a size from 1 to 15 centimeters in diameter,

which contained the millions of sooty teliospores, which are

released into the air.

The Pathogen: Ustilago zeae , The fungus overwinters as

teliospores in crop debris and in the soil, where they can survive

for several years.

47


Figure 15. A. Loose smut on Barley, B. common smut on corn, C. covered smut on wheat

B

A

C

48



RUSTS

Plant rusts, caused by Basidiomycetes of the order Uredinales,

are among the most destructive plant diseases.

The most important rust fungi and the diseases they cause are

listed here

- Puccinia, causing severe and often catastrophic diseases on

numerous hosts such as the stem rust of wheat and all other

small grains (P. graminis); yellow or stripe rust of wheat,

barley, and rye (P. striiformis); leaf or brown rust of wheat and

leaf rust of barley (P. hordei); corn rust (P. sorghi);

- Gymnosporangium, causing cedar-apple rust (G. juniperi-

virginianae)

- Uromyces, causing the rusts of legumes (U. appendiculatus)

WHEAT RUSTS

Wheat rusts are caused by three related fungi:

- Stripe rust ( yellow rust ) is caused by Puccinia striiformis f.

sp. tritici.

- Leaf rust (orange rust) is caused by Puccinia triticina.

- Stem rust is caused by Puccinia graminis f. sp. tritici.

STEM RUST ON WHEAT

The symptoms on wheat appear as elliptical blisters or pustules,

known as uredia, that develop parallel with the long axis of the

49


stem, leaf, or leaf sheath powdery mass of brick red-colored

uredospores. Later in the season, as the plant approaches

maturity, the pustules turn black as the fungus produces

teliospores instead of uredospores and uredia are transformed

into black telia.

Development of Disease. The fungus overwinters as teliospores

on infected wheat debris. Teliospores germinate in the spring

and produce a basidium on which form four basidiospores. The

basidiospores are carried by air currents for a few hundred

meters. Basidiospores landing on young barberry leaves

germinate and penetrate the epidermal cells. After that, the

mycelium grows mostly intercellularly. Within 3 or 4 days the

mycelium develops into a spermagonium which ruptures the

epidermis, and its opening emerges on the surface of the plant

tissue. This called spermatial stage then form aecial stage.

mycelium grows intercellularly toward the lower side of the

leaf, where it forms thick mycelial mats that develop into aecia.

On the lower surface of the leaf The aecia form in groups and

protrude considerably beyond the surface of the barberry plant.

Aeciospores are released in late spring and are carried by wind

to nearby wheat plants on which they germinate and infect

wheat stems, leaves, or sheaths through stomata. After the

mycelium grows intercellularly for a while, uredospores are

produced that exert pressure on the epidermis, which is pushed

outward and forms a uredial pustule. Finally, the epidermis is

50


broken irregularly, revealing several hundred thousand rust-

colored uredospores, which give a powdery appearance to the

uredium The uredospores are easily blown away by air currents,

Uredospores cause new infections on wheat plants up to the time

the plant reaches maturity. the uredia produce teliospores

instead of uredospores or new telia may develop from recent

uredospore infections. Teliospores do not germinate

immediately and do not infect wheat; rather, they are the

overwintering stage of the fungus.

51


FIGURE 16. Disease cycle of stem rust of wheat caused by Puccinia graminis tritici.

Note\ this life cycle has two hosts with 5 stages thus called long life cycle diseases , Some pathogens complete only one, or even part of one, disease cycle in tow stage telial and basidail stages on one host and these are called short life cycle .

Spermagonia on barberry leaf

Receptive hypha

Spermatia

Clusters of aecia on under side of barberry leaf

Telia and uredia on

wheat stem or leaf

Spermatia fertilize compatible receptive hypha

Dikaryotic mycelium

Aecium

Fertilized receptive hypha

Aeciospores

Uredium on wheat Aeciospore infects wheat stem or leaf through stomata

Uredospore infects wheat through stomata

Telia on wheat at the end of

season

Overwintering teliospore

Karyogamy

Basidiospores infect barberry leaf directly

Germinating teliospore teliospore

Barberry stem and

leaves

Basidiospore

52



PLANT DISEASES CAUSED BY BACTERIA Bacteria and mollicutes are prokaryotes. These are generally

single-celled microorganisms whose genetic material (DNA) is

not bound by a membrane and therefore is not organized into a

nucleus.

Characteristics of Plant Pathogenic Bacteria

Most of the DNA in bacteria is present as a single circular

chromosome. Additional DNA is found in many bacteria as

independently reproducing plasmids composed of smaller

amounts of DNA. Most plant pathogenic bacteria are Gram

negative, with the exception of Clavibacter

(Corynebacterium).Phytopathological bacteria are either rod

(0.6 to 3.5 micrometers in diameter). or filamentous shaped,

may or may not be flagellate, The cell walls of bacteria of most

species are enveloped by a viscous, gummy material, which, if

thin and diffuse, is called a slime layer, but if thick, forming a

definitive mass around the cell, is called a capsule. and

reproduce by binary fission. Traditionally, bacteria were

classified based on Gram stain, cell shape, cultural Pathogenic

bacteria are known as wound pathogens because they usually do

not penetrate the host directly. They may also enter through

natural plant openings such as nectaries, hydathodes, and stoma.

53


They are disseminated by air currents, water, insects, plant

materials, and contaminated equipment.

Symptoms Caused by Bacteria

Plant pathogenic bacteria induce as many kinds of symptoms on

the plants they infect as do fungi. They cause leaf spots and

blights, soft rots of fruits, roots, and storage organs, wilts,

overgrowths, scabs, and cankers.

GENERA OF PLANT PATHOGENIC BACTERIA

Agrobacterium—Rods motile by 1 to 6 peritrichous flagella.

Agrobacterium species cause proliferations on many plants:

smooth and rough galls (A. tumefaciens), Tumorigenic

Agrobacterium species carry parasitic plasmids vectoring tumor

DNA (T-DNA).

Pseudomonas—Rods motile by one or more polar flagella.

Many plant pathogens produce water-soluble pigments that

fluoresce light blue to greenish-yellow with ultraviolet light.

Pseudomonas marginalis is a soft-rotting pathogen of many

plants.

Xanthomonas—Rods motile by one polar flagellum. Colonies

are yellow due to xanthomonadin pigments. Xanthomonas

campestris has over 140 pathovars, including important

pathogens of rice, field beans and cabbage.

Erwinia- Rods motile by peritrichous flagella. E. amylovora,

which causes fire blight of pear and apple, and the soft-rotting .

54


Bacillus—Large rods motile by peritrichous flagella with oval

central endospores. Strongly Gram-positive. Bacillus species

may cause rots of tobacco leaves, tomato seedlings, and soybean

and white stripe of wheat.

Streptomyces—Vegetative, extensively branched hyphae. Aerial

mycelium matures to form three or more spores in chains.

Streptomyces scabies and S. acidiscabies cause potato scab.

Some PROKARIOTE caused plant diseases are

Phytoplasma—Polymprphic. The aster yellows phytoplasma

causes proliferation or yellows in over 120 host plants.

• Sprioplasma—Spiral shape supported around a central protein

“rod” embedded in the membrane. Plant pathogenic

spiroplasmas cause stubborn disease of Citrus species (S.

citri) and corn stunt (S. kunkelii).

1- FIRE BLIGHT OF PEAR AND APPLE

.Fire blight causes damage to pear and apple orchards in many

parts of the world.

Symptoms: Infected flowers become water soaked, then shrivel,

turn brownish black, and fall or remain hanging in the tree. Soon

leaves on the same spur or on nearby twigs develop brown-black

blotches along the midrib and main veins or along the margins

and between the veins. The tip of the twig is hooked , and the

leaves turn black and cling to the twig. From fruit spurs and

55


twigs the symptoms progress down to the branches, where

cankers are formed.

The Pathogen Erwinia amylovora.


Bacteria overwinter at the margins of cankers and possibly in

buds and apparently healthy wood tissue. In the spring, bacteria

in the cankers become active again, multiply, and spread into the

adjoining healthy bark. During humid or wet weather, bacterial

masses exude through lenticels and cracks. The bacterial ooze

appears at about the time when the pear blossoms are opening.

Various insects, such as bees, flies, and ants, are attracted to the

sweet, sticky, bacteria-filled exudate, become smeared with it,

and spread it to the flowers they visit afterward. In some cases,

56


bacteria are also spread from oozing cankers to flowers by

splashing rain. When the ooze dries, it often forms aerial strands

that can be spread by wind and serve as inoculum. Bacteria

multiply rapidly in the nectar and, through

the nectar thodes, enter the tissues of the flower. Bees visiting

an infected flower carry bacteria from its nectar to all the

succeeding blossoms that they visit. Once inside the flower,

bacteria multiply quickly and cause death and collapse of nearby

cells. Bacteria move quickly through the intercellular spaces and

also through the macerated middle lamella and flower cells. In

some cases, fairly large cavities form that are filled with

bacteria. From the flower, bacteria move down thepedicel into

the fruit spur. Infection of the spur results in the death of all

flowers, leaves, and fruit on it Penetration and invasion of leaves

are similar to those of flowers. Bacteria may enter through

stomata and hydathodes, but usually they enter through wounds

made by insects, hail storms,

bacteria move rapidly from the vessels to other tissues, killing

cells, and causing blight and canker symptoms in the process.

Invasion of large twigs and branches is restricted primarily to

the cortex. Infection of succulent tissues is rapid under warm,

humid conditions. bacteria may progress from spurs or shoots

into the second-year, third-year, and older growth, killing the

bark all along the way.

57



PLANT DISEASES CAUSED BY

VIRUSES

Virus is a nucleoprotein that multiplies only in living cells and

has the ability to cause disease. All viruses parasitize cells and

cause a multitude of diseases in all forms of living organisms. A

plant may sometimes be infected by more than one kind of virus

at the same time.

Virus particles (known as virions) consist of two or three parts:

i) the genetic material made from either DNA or RNA, long

molecules that carry genetic information; ii) a protein coat that

protects these genes; and in some cases iii) an envelope of lipids

that surrounds the protein coat when they are outside a cell. The

shapes of viruses range from simple helical and icosahedral

forms to more complex structures.

CHARACTERISTICS OF PLANT VIRUSES

In general, there are three main morphological virus types:

1- Helical

These viruses are composed of a single type of capsomer

stacked around a central axis( nucliec acid) to form a helical

structure, This arrangement results in rod-shaped or filamentous

virions.

58


2- Icosahedral

icosahedral or near-spherical. A regular icosahedron is the

optimum way of forming a closed shell from identical sub-units.

Pentose (5) and hexons(6)

3-Complex

These viruses possess a capsid that is neither purely helical nor

purely icosahedral, and that may possess extra structures such as

protein tails or a complex outer wall. Some bacteriophages.

Structure of tobacco mosaic virus: RNA coiled in a helix of

repeating protein sub-units

Electron micrograph of icosahedral adenovirus

Herpes viruses have a lipid envelope

59


FIGURE Electron micrographs of the various shapes of plant viruses. (A) Rod-shaped virus (tobacco mosaic virus) (36,000×). (B) Flexuous thread virus (sugarcane mosaic virus) (80,000×). (C) Isometric virus (cowpea chlorotic

mottle virus) (100,000×). TRANSMISSION OF PLANT VIRUSES

Viruses can not moved there are tow type of Plant viruses transmission

1-Transmission between plant cells

Viral infections often develop into systemic infections as a

means of transmission. The virus often infects many tissues, if

not the whole plant, where it can continue to replicate. There are

a variety of methods the virus can use to spread throughout the

organism but the most common route utilise the vascular

60


system, otherwise known as the xylem and phloem, and the

plasmodesmata, which interconnect adjacent cells.

2- transmission between plant hosts

viruses are transmitted from plant to plant in a number of ways.

Modes of transmission include:

vegetative propagation,

mechanically through sap,

through seed,

pollen,

dodder,

and by specific insects, mites, nematodes, and fungi.( Vectors

that feed on plant sap).

FIGURE: Transmission of viruses, mollicutes, and other pathogens

through vegetative propagation, natural root grafts, and dodder.

By budding By grafting By cutting

By corms By bulbs

By stolons

By tubers By rhizomes

Through dodder Through natural

root grafts

61


Symptoms around the site of virus inoculation are denoted local

symptoms. When virus spreads from the site of inoculation and

to other parts of the plant, this is referred to as systemic

symptoms. Some symptoms leaf color.

In this case leaves can show more severe symptoms such as leaf

color and shape, Mottle(abnormal coloration), Mosaic( green,

yellow parts on leaf) leaf distortion (e.g. curling) and/or other

growth distortions (e.g. stunting of the whole plant,

abnormalities in flower or fruit formation).

Viruses cause many important plant diseases and are responsible

for huge losses in crop production and quality in all parts of the

world. Such as

- Tobacco Mosaic Virus (TMV)

- Cucumber Mosaic Virus (CMV)

- Grapevine fan leaf Virus (GFLV)

- Citrus tristeza virus (CTV)

- Potato leaf curly Virus

Yellow mosaic symptoms on lettuce caused by Lettuce mosaic virus.

62


Yellow vein-banding symptoms on grapevine caused by Grapevine fanleaf virus.

Fruit distortion on eggplant fruit caused by Tomato bushy stunt virus. A healthy fruit is shown on the left.

Infection does not always result in visible symptoms (as

witnessed by names such as Carnation latent virus.

virus infection can result in symptoms of ornamental value, such

as 'breaking' of tulips.

- Control

63


Plant viruses cannot be directly controlled by chemical

application. The major means of control (depending on the

disease) include:

• Chemical or biological control of the vector (the

organism transmitting the disease, often an insect).

• Growing resistant crop varieties

• Use of virus-free planting material: in vegetatively

propagated crops (e.g. potatoes, many fruit crops) and

where viruses are transmitted through seed major efforts

are made through breeding, certification schemes etc., to

ensure that the planting material is virus-free.

• Exclusion: the prevention of disease establishment in

areas where it does not yet occur. This is a major objective

of plant quarantine procedures throughout the world as

well as more local schemes.

64


shapes of viruses structure

65



PLANT DISEASES CAUSED BY

NEMATODA

Nematodes belong to the kingdom Animalia. Nematodes are

wormlike in appearance.

Life Cycles

The life histories of most plant parasitic nematodes are, in

general, quite similar. Eggs hatch into juveniles, whose

appearance and structure are usually similar to those of the adult

nematodes. Juveniles grow in size, and each juvenile stage is

terminated by a molt. All nematodes have four juvenile stages,

with the first molt usually occurring in the egg. After the final

molt the nematodes differentiate into males and females. The

female can then produce fertile eggs either after mating with a

male or, in the absence of males, parthenogenetically. A life

cycle from egg to egg may be completed within 2 to 4 weeks

under optimum environmental, especially temperature,

conditions but will take longer in cooler

HOW NEMATODES AFFECT PLANTS

The direct mechanical injury inflicted by nematodes while

feeding causes only slight damage to plants. Most of the

66


Juvenile and adult ectoparasitic ring nematodes feeding on root.

damage seems to be caused by a secretion of saliva injected into

the plants while the nematodes are feeding.

as the females of species that become established in or on roots

permanently,

The feeding process causes the affected plant cells to react,

resulting in dead or devitalized root tips and buds, lesion

formation and tissue breakdown, swellings and galls of various

kinds, and crinkled and distorted stems and foliage. Some of

these manifestations are caused by the dissolution of infected

67


tissues by nematode enzymes, which, with or without the help of

toxic metabolites, cause tissue disintegration and the death.

NEMATODA help other pathogen ( fungi, bacteria ,viruses) to

penetrate an infect plants

- PLANT NEMATODA have Stylet use in penetration and

feeding.

ROOT-KNOT NEMATODES: MELOIDOGYNE SPP. Symptoms

Aboveground symptoms are reduced growth and fewer, small,

pale green, or yellowish leaves that tend to wilt in warm

weather. Blossoms and fruits are few and of poor quality or

dead. Characteristic symptoms of the disease appear on the

underground parts of the plants. Infected roots develop the

typical root-knot galls that are two to several times as large in

diameter as the healthy root. Several infections along the root

give the root a rough, clubbed appearance. Roots infected by

certain species of the nematode also develop a bushy root.

The Pathogen: Meloidogyne spp.

The males are wormlike and about 1.2 to 1.5 millimeters long

by 30 to 36 micrometers in diameter. The females are pear

shaped and about 0.40 to 1.30 millimeters long by 0.27 to 0.75

millimeters wide.

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Each female lays approximately 500 eggs in a gelatinous

substance


Second-stage juveniles enter roots behind the root tip Cells near

the path of the juveniles begin to enlarge. Two or 3 days after

the juvenile has become established, some of the cells around its

head begin to enlarge. Their nuclei divide, but no cell walls are

laid down. The existing walls between some of the cells break

down and disappear, giving rise to giant cells, which are due to

substances contained in the saliva secreted by the nematode in

the giant cells during feeding. and produce their egg sacs, they

push outward, split the cortex, and may become exposed on the

surface of the root or remain completely covered, depending on

the position of the nematode in relation to the root surface.

In addition to the disturbance caused to plants by the nematode

galls themselves, damage to infected plants is frequently

increased by certain parasitic fungi, which can easily attack the

weakened root tissues and the hypertrophied, undifferentiated

cells of the galls. Moreover, some fungi, e.g., Fusarium,

Rhizoctonia, and the oomycete Pythium, grow and reproduce

much faster in the galls than in other areas of the root, thus

inducing an earlier breakdown of the root tissues.

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Disease cycle of root knot caused by nematodes of the genus Meloidogyne.

Control

- Root knot can be controlled effectively in the greenhouse

with steam sterilization of the soil or soil fumigation

- In the field the best control of root knot is obtained by

fumigating the soil with approved chemical nematicides.

- varieties resistant to root-knot nematodes are also available.

- Several cultural practices, such as crop rotation, fallow

soil, soil solarization, and certain soil amendments, are also

helpful in reducing root-knot losses.

II Stage juvenile free in soil

II Stage juvenile Attack rootlets

II Stage juveniles invade rootlet

and cause formation of giant cells

Adult nematodes. Male leaves root

I Stage juvenile

II Stage juvenile

egg

Female lays eggs into egg sac

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- Biological control of root knot has been obtained

experimentally by treating nematode infested soil with

endospores of the bacterium Pasteuria penetrans, which is an

obligate parasite of some plant parasitic nematodes, or with

preparations of the fungus Trichoderma harzianum; by treating

transplants or infested soils with spores of the fungus Dactylella

oviparasitica, which parasitizes the eggs of Meloidogyne

nematodes; and in some experiments by treating transplants or

infested soils with spores of the vesicular arbuscular

mycorrhizal fungi Gigaspora and Glomus. Fairly good

experimental control of root knot

- also been obtained by mixing essential oils from plant spices

into nematode-infested soil before planting and through an

increase in plants of their local and systemic-induced resistance

to root knot nematodes by mixing in the soil or spraying the

plants with amino-butyric acid and other amino acids.

71



CONTROL OF PLANT DISEASES

The various control methods can be classified as regulatory,

cultural, biological, physical, and chemical, depending on the

nature of the agents employed.

1- Quarantines and Inspections

When plant pathogens are introduced into an area in which host

plants have been growing in the absence of the pathogen, such

introduced pathogens may cause much more catastrophic

epidemics than the existing endemic pathogens. Similar

quarantine regulations govern the interstate, and even intrastate,

sale of nursery stock, tubers, bulbs, seeds, and other propagative

organs, especially of certain crops such as potatoes and fruit

trees. The movement and sale of such materials within and

between states are controlled by the regulatory agencies of each

state.

2- Use of Pathogen-Free Propagating Material

When a pathogen is excluded from the propagating material

(seed, tubers, bulbs, nursery stock) of a host. Here we can used

clean, uninfected or treated propagating material to prevent the

diseases appearance.

3- Cultural Methods That Eradicate or Reduce the Inoculum

- Crop Rotation

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Soilborne pathogens that infect plants of one or a few species or

even families of plants can sometimes be reduced in the soil by

planting, for 3 or 4 years, crops belonging to species or families

not attacked by the particular pathogen.

- Sanitation

Sanitation consists of all activities aimed at eliminating or

reducing the amount of inoculum present in a plant, a field, or a

warehouse and at preventing the spread of the pathogen to other

healthy plants and plant products. Thus, plowing under infected

plants after harvest, such as leftover infected fruit, stems, tubers,

or leaves, helps cover the inoculum with soil and speeds up its

disintegration (rotting) and concurrent destruction of most

pathogens carried in or on them. Similarly, removing infected

leaves of house or garden plants and Pruning infected plants

helps remove or reduce the inoculum.

- Creating Conditions Unfavorable to the Pathogen

Stored products should be aerated properly to hasten the drying

of their surfaces and inhibit germination and infection by any

fungal or bacterial pathogens present on them. Similarly,

spacing plants properly in the field or greenhouse prevents the

creation of high-humidity conditions on plant surfaces and

inhibits infection by certain pathogens, such as Botrytis and

Peronospora tabacina.

73


4- Biological Methods That Eradicate or

Reduce the Inoculum

Biological control of pathogens: the total or partial destruction

of pathogen populations by other organisms, occurs routinely in

nature.

The mycelium and resting spores (oospores) or sclerotia of

several phytopathogenic soil oomycetes and fungi such as

Pythium, Phytophthora, Rhizoctonia, Sclerotinia, and

Sclerotium are invaded and parasitized (mycoparasitism) or are

lysed (mycolysis) by several fungi, which as a rule are not

pathogenic to plants. Several nonplant pathogenic oomycetes

and fungi, including some chytridiomycetes and hyphomycetes,

and some pseudomonad and actinomycetous bacteria infect the

resting spores of several plant pathogenic fungi. Among the

most common mycoparasitic fungi are Trichoderma sp., mainly

T. harzianum.

74


Attachment of the yeast biocontrol agent Pichia guilliermondii on hyphae of the

plant pathogenic fungi Botrytis cinerea (A) and Penicillium expansum (B).

( C)Hypha of a nonpathogenic species of Pythium (P. nunn) penetrating (arrow)

a hypha of the pathogenic fungus Phytophthora (D) fungi parasite on pathogenic

nematoda

C D

75


5-Physical Methods That Eradicate or

Reduce the Inoculum

The physical agents used most commonly in controlling plant

diseases are temperature (high or low), dry air, unfavorable light

wavelengths, and various types of radiation. With some crops,

cultivation in glass or plastic greenhouses provides physical

barriers to pathogens and their vectors and in that way protects

the crop from some diseases. Similarly, plastic or net covering

of row crops may protect the crop from infection by preventing

pathogens or vectors from reaching the plants.

6-Chemical Methods that Eradicate or Reduce

the Inoculum

Chemical pesticides are generally used to protect plant surfaces

from infection or to eradicate a pathogen that has already

infected a plant. A few chemical treatments, however, are aimed

at eradicating or greatly reducing the inoculum before it comes

in contact with the plant. They include soil treatments (such as

fumigation), disinfestations of warehouses, sanitation of

handling equipment, and control of insect vectors of pathogens.

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