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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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Plant Diseases lec.2 Dr.Ahed A.Hadi
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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Plant Diseases lec.3 Dr.Ahed A.Hadi
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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Plant Diseases lec.4 Dr.Ahed A.Hadi
- 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.
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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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Plant Diseases lec.5 Dr.Ahed A.Hadi
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 lec.6 Dr.Ahed A.Hadi
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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Plant Diseases Dr.Ahed A.Hadi
• 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.
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Plant Diseases Dr.Ahed A.Hadi
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).
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Plant Diseases Dr.Ahed A.Hadi
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
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Plant Diseases Dr.Ahed A.Hadi
(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.
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.7 Dr.Ahed A.Hadi
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).
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Plant Diseases Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
- 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
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Plant Diseases Dr.Ahed A.Hadi
Figure 8. Some genera of fungi causing Downy mildew
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Plant Diseases Dr.Ahed A.Hadi
Fig 9. life cycle of Downy mildew pathogen
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Plant Diseases lec.8 Dr.Ahed A.Hadi
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
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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
Development of Disease
-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
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Plant Diseases Dr.Ahed A.Hadi
present together, zygospores are produced. Zygospores help the
fungus survive (Figure 11).
Figure 11. A. Zygospore B. Soft rot on squash
A B
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.9 Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
- 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
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.10 Dr.Ahed A.Hadi
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)
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Plant Diseases Dr.Ahed A.Hadi
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
Development of Disease
-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.
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Plant Diseases Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
Figure 15. A. Loose smut on Barley, B. common smut on corn, C. covered smut on wheat
B
A
C
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.11 Dr.Ahed A.Hadi
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
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Plant Diseases Dr.Ahed A.Hadi
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
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Plant Diseases Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
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
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.12 Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
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 .
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Plant Diseases Dr.Ahed A.Hadi
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
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Plant Diseases Dr.Ahed A.Hadi
twigs the symptoms progress down to the branches, where
cankers are formed.
The Pathogen Erwinia amylovora.
Development of Disease
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,
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Plant Diseases Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.13 Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
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
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Plant Diseases Dr.Ahed A.Hadi
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
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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
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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.
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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
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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.
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shapes of viruses structure
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Plant Diseases Dr.Ahed A.Hadi
Plant Diseases lec.14 Dr.Ahed A.Hadi
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
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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
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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
Development of Disease
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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Plant Diseases Dr.Ahed A.Hadi
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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Plant Diseases Dr.Ahed A.Hadi
- 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.
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Plant Diseases lec.15 Dr.Ahed A.Hadi
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.
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Plant Diseases Dr.Ahed A.Hadi
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.
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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
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Plant Diseases Dr.Ahed A.Hadi
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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