The document discusses plant diseases, their importance, and factors that affect disease development. It notes that crop losses due to diseases are estimated at 30-50% annually in less developed countries. Key environmental factors like temperature, humidity, soil moisture, pH, type, and fertility influence disease development. Control methods aim to exclude, eradicate, or protect against pathogens using principles like sanitation, crop rotation, and pesticide application.

1. PLANT DISEASE AND ITS
IMPORTANCE
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2. Plant disease, an impairment of the normal state of a plant that
interrupts or modifies its vital functions.
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3. Plant diseases are known from times preceding the earliest
writings.
Plant disease outbreaks with similar far-reaching effects in more
recent times include
Late blight of potato in Ireland (1845–60)
Powdery and downy mildew of grape - France (1851 and 1878)
coffee rust - Ceylon (starting in the 1870s);
Sigatoka leaf spot and panama disease of banana - central America
(1900–65)
Black stem rust of wheat - (1916, 1935, 1953–54)
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4. Loss of crops from plant diseases may result in hunger and
starvation, especially in less developed countries where access to
disease-control methods is limited and annual losses of 30 to 50
percent are common for major crops.
In some years, losses are much greater, producing catastrophic
results for those who depend on the crop for food.
Major disease outbreaks among food crops have led to famines
and mass migrations throughout history.
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5. The devastating outbreak of late blight of potato (
Phytophthora infestans) that began in Europe in 1845 and brought
about the Irish famine caused starvation, death, and mass migration
of the Irish population.
Diseases—a normal part of nature:
Plant diseases are a normal part of nature and one of many
ecological factors that help keep the hundreds of thousands of living
plants and animals in balance with one another
.
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6. Plant cells contain special signaling pathways that enhance their
defenses against insects, animals, and pathogens.
One such example involves a plant hormone called jasmonate
(jasmonic acid).
In the absence of harmful stimuli, jasmonate binds to special
proteins, called JAZ proteins, to regulate plant growth, Pollen
production, and other.
it also increase the defense mechanism of plants.
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7. Disease development and transmission
Pathogenesis and saprogenesis
Pathogenesis is the stage of disease in which the pathogen is in
intimate association with living host tissue.
Three fairly distinct stages are involved:
Inoculation: transfer of the pathogen to the infection court, or area
in which invasion of the plant occurs (the infection court may be the
unbroken plant surface, a variety of wounds, or natural openings.
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8. Incubation: the period of time between the arrival of the
pathogen in the infection court and the appearance of
symptoms
Infection: the appearance of disease symptoms accompanied
by the establishment and spread of the pathogen.
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9. Temperature:
Each pathogen has an optimum temperature for growth. In addition,
different growth stages of the fungus, such as the production of
spores, their germination, and the growth of the mycelium, may have
slightly different optimum temperatures.
Relative humidity :
Relative humidity is very critical in fungal spore germination and the
development of storage rots.
Environmental factors affecting disease development
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10. High humidity favours development of the great majority of leaf
and fruit diseases caused by fungi and bacteria. Moisture is
generally needed for fungal spore germination, the multiplication
and penetration of bacteria, and the initiation of infection.
Soil moisture:
High or low soil moisture may be a limiting factor in the
development of certain root rot diseases.
High soil-moisture levels favour development of destructive water
mold fungi, such as species of Aphanomyces, Pythium, and
Phytophthora.
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11. Soil pH
Soil pH, a measure of acidity or alkalinity, markedly influences a few
diseases, such as common scab of potato and club root of crucifers
(Plasmodiophora brassicae).
Growth of the potato scab organism is suppressed at a pH of 5.2 or
slightly below (pH 7 is neutral; numbers below 7 indicate acidity,
and those above 7 indicate alkalinity).
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12. Scab is not normally a problem when the natural soil pH is about
5.2.
Soil type
Certain pathogens are favoured by loam soils and others by clay
soils.
Phymatotrichum root rot attacks cotton and some 2,000 other plants
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13. Phymatotrichum fungus is serious only in black alkaline soils—pH
7.3 or above—that are low in organic matter.
Soil fertility:
Greenhouse and field experiments have shown that raising or
lowering the levels of certain nutrient elements required by plants
frequently influences the development of some infectious diseases.
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14. Plant disease
symptoms
description and causes examples
Water-soaking A water-soaked, translucent condition
of tissues caused by water moving
from host cells into intercellular
spaces
late blight lesions on potato
and tomato leaves; bacterial
soft rot of fleshy vegetables
Wilting Temporary or permanent drooping of
leaves, shoots, or entire plants from
lack of water
bacterial wilt of cucumber;
Fusarium wilt of tomato
Abnormal
coloration
Yellowing, reddening, bronzing, or
purpling in localized areas of leaves
where chlorophyll has been
destroyed; may be due to a variety of
causes
cabbage and aster yellows;
halo blight of beans;
potassium or phosphorus
deficiency
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15. Necrotic Localized or general death of cells or
disintegration of tissues
Leaf spot
Blast Sudden blighting or death of young buds,
flowers, or young fruit; failure to produce
fruit or seeds
Botrytis blight of peony buds; oat
blast
Blight Sudden or total discoloration and killing of
large numbers of blossoms, leaves,
shoots, or limbs or the entire plant; usually
young tissues are attacked; the disease
name is often coupled with the name of
the host and the part attacked—blossom
blight, twig blight, tip blight
Leaf blight
Plant disease
symptoms
description and causes examples
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16. Canker A definite, dead, often sunken or swollen and
cracked area on a stem, limb, trunk, tuber, or root
surrounded by living tissues
Nectria canker of hardwoods
Damping-off Decay of seed in soil, rapid death of germinating
seedlings before emergence, or emerged seedlings
suddenly wilting, toppling over, and dying from
rot at or near the soil line
Nursery
Dieback Progressive browning and death of shoots,
branches, and roots starting at the tips
Diebacks in rose
Firing Drying and dying of leaves
Plant disease
symptoms
description and causes examples
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17. Fleck A small, white to translucent spot or lesion
visible through a leaf
ozone injury
Mummification Final stage in certain fruit rots, in which the
dried, shriveled, and wrinkled fruit is called a
"mummy"
Downy mildew in grapes
Net necrosis An irregular crisscrossing of dark brown to black
lines giving a netted appearance
in potato tubers of plants with virus
leaf roll
Pitting Small dead areas within fleshy or woody tissue
that appears healthy externally; definite sunken
grooves or pits are formed
virus stem-pitting in apple
Plant disease
symptoms
description and causes examples
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18. Rot Decomposition and putrefaction of cells, later
of tissues and organs; the rot may be dry, firm,
watery, or mushy and is characterized by such
names as hard rot, soft rot, dry rot, black rot,
and white rot
bacterial soft rot; berry rot; bud rot;
bulb rot
Scald Blanching of young fruit, foliage, and shoot
tissue; generally superficial
sunscald; apple and pear scald
Scorch Sudden death and "burning" of large, indefinite
areas in leaves and fruit
toxicity from pesticides and air
pollutants; drought; wind; lack or
excess of some nutrient
Spot A definite, localized, round to regular lesion,
often with a border of a different colour,
characterized as to location (leaf spot, fruit
spot) and colour (brown spot, black spot); if
numerous or if spots enlarge and merge, a large
irregular blotch or blight may develop
gray leaf spot of tomato; black spot of
rose; tar spot of maple
Plant disease
symptoms
description and causes examples
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19. Exclusion and avoidance
The principle of exclusion and avoidance is to keep the pathogen
away from the growing host plant.
This practice commonly excludes pathogens by disinfection of
plants, seeds, or other parts, using chemicals or heat.
Inspection and certification of seed and other planting stock help
ensure freedom from disease.
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20. Eradication
Eradication is concerned with elimination of the disease agent after
it has become established in the area of the growing host or has
penetrated the host.
Such measures include crop rotation, destruction of the diseased
plants, elimination of alternate host plants, pruning, disinfection, and
heat treatments.
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21. Protection
The principle of protection involves placing a barrier between the
pathogen and the susceptible part of the host to shield the host from
the pathogen.
This can be accomplished by regulation of the environment, cultural
and handling practices, control of insect carriers, and application of
chemical pesticides.
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22. Regulation of the environment
Selection of outdoor growing areas where weather is
unfavourable for disease is a method of controlling disease by
regulating the environment.
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23. Cultural practices
Selection of the best time and depth of seeding and planting is an
effective cultural practice that reduces disease impact.
adjustment of soil moisture is another cultural practice of
widerspread usefulness. Adjustment of soil pH also leads to control
of some diseases.
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24. Regulation of fertility level and nutrient balance:
Potash and nitrogen, and the balance between the two, may affect
the incidence of certain bacterial, fungal, and viral diseases of corn,
cotton, tobacco, and sugar beet.
Adjusting the soil pH, adding chelated or soluble salts to the
soil, or spraying the foliage with these or similar salts is a corrective
measure.
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25. Control of insect vectors
There are many examples in which losses by bacteria, viruses, and
mycoplasma-like disease agents can be reduced by controlling
aphids, leafhoppers, thrips, beetles, and other carriers of these
agents.
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26. Biological control:
Biological control of plant diseases involves the use of micro
organisms other than humans to reduce or prevent infection by a
pathogen.
These organisms are called antagonists; they may occur naturally
within the host’s environment, or they may be purposefully applied
to those parts of the potential host plant where they can act directly
or indirectly on the pathogen.
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27. To sum up
The crop loss due to diseases is estimated to be approximately 30-50%.
Cultivated plants are often more susceptible to diseases than are their wild
relatives. Important environmental factors that may affect development of plant
diseases are temperature, relative humidity, soil moisture, soil pH, soil type,
and soil fertility. Each pathogen has an optimum temperature for growth .
High soil-moisture levels favour development of destructive water mold fungi,
such as species of Aphanomyces, Pythium, and Phytophthora. High humidity
favours development of the great majority of leaf and fruit diseases caused by
fungi and bacteria. Soil pH, a measure of acidity or alkalinity, markedly
influences a few diseases, such as common scab of potato and clubroot
(Plasmodiophora brassicae) of crucifers. Raising or lowering the levels of
certain nutrients also influences the development of some infectious diseases.
Most control measures are directed against inoculums of the pathogen and
involve the principles of exclusion and avoidance, eradication, protection, host
resistance and selection, and therapy.
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