How to control Plant Diseases

1. Control of plant diseases
Amit Kumar Sahoo
II MSc Biosciences
15151
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2. Breeding resistant varieties,
Control through protection(chemicals and environmental manipulation),
Legislation (Quarantine and regulatory measures),
Eradication.
What is a Plant disease?
Plant disease, an impairment of the normal state of a plant
that interrupts or modifies its vital functions.
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3. The Big Picture
 Main purpose behind understanding pathogens and the diseases
they cause is so diseases can be controlled.
 For most crops, the goal is to save most of the plant population,
not selected individuals.
 Purpose of disease control is to prevent disease from exceeding
some level where profit or yield is significantly diminished.
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4. Plant Disease Triangle
4Host
Total Of all properties that affect susceptibility
Pathogen
Total of all properties of pathogen
(virulence, abundance, etc.)
Environment
Total of all conditions
That affect disease
 All three factors are necessary components of disease
 Area of triangle represents amount of disease
Amit Kumar Sahoo

5. Vanderplank’s Equivalence
Theorem
 Effects of host, pathogen and environment can be translated into
terms of the rate parameter of an epidemic.
 Changes in any component have an equivalent effect on disease.
 More-less susceptible host
 More-less aggressive pathogen
 More-less favorable environment
All affect amount of disease
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6. Disease Cycles 6
Survival Inoculation
Penetration
Infection
Disease
Pathogen
reduction
Dispersal
• Pathogens all go through a cycle with similar events.
• Knowing how particular pathogen go through their cycle is important
in developing management strategies.
Amit Kumar Sahoo

7. Basic principles of disease control:
Control strategies can be divided into two groups based on their effect on the
development of resistance to the control measure by the pathogen:
1. Eradicative control measures — designed to eliminate the entire pathogen
population - examples: pesticides, vertical or complete resistance - These tend
to select for resistant variants of the pathogen. Why? All individuals are
affected, so the pathogen must adapt or die.
2. Management control measures — designed to reduce the pathogen
population by destroying a portion of the population - examples: horizontal
or partial resistance, antagonism, cultural practices, quarantine - These do not
apply heavy selection pressure to the pathogen. Why? Portions of the
pathogen population remain unaffected, no pressure to adapt.
* Of the two, we prefer to use management strategies.
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8. Disease control
There are four basic types of control measures:
a. Biological control
b. Cultural control (includes physical control)
c. Legislative and regulatory control
d. Chemical control
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9. Biological control-I
Manipulation of biotic entities; host and antagonistic
microorganisms
1. Host resistance - control based on the genes and the
resistance mechanisms they control
a. Van der Plank described two types of resistance
(1960s; these are the "classics‘’):
i. Vertical resistance — resistance that is effective
against some, but not all, races of a pathogen; decreases the
effective amount of incoming inoculum (avirulent races can't
infect), but does not reduce the rate of disease development
(virulent races are not affected)
ii.Horizontal resistance — resistance that is effective against
all races of the pathogen; decreases the rate of disease
development for all races
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Biological control-II
b. Resistance has been defined in many other ways since Van der
Plank, including systems based on: epidemiologic effects,
number of genes involved, how long the resistance lasts
under field conditions; additional terms you should be
familiar with are:
i. tolerance — plants are diseased, but they do not yield less
than healthy plants.
ii. induced resistance — a normally susceptible plant treated
with an avirulent strain of a pathogen gives a resistant
reaction when challenged later with a strain that is virulent.
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Biological control-III
2. Antagonists — control using microorganisms that inhibit the growth,
development, or reproduction of pathogens
Four types of activity:
1. Antibiosis — inhibition of pathogen through antibiotics produced by the
antagonist - examples: streptomycin (antibacterial, from actinomycete),
penicillin (antibacterial, from fungus)
2. Competition — two organisms attempt to utilize the same limiting
factors (nutrients, oxygen); supply not large enough to support both
antagonist and pathogen
3. Amensalism — antagonist makes the environment unsuitable for the
pathogen (modifies pH, temperature, moisture)
4. Parasitism & predation — antagonist directly attacks the pathogen
example: nematode-trapping fungi
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14. 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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Cultural control-I
Cultural (physical) control — manipulation of the environment
There are many types of cultural control. Here are few selected examples:
1. Crop rotation — rotate crops and varieties over seasons to reduce
pathogen inoculum levels * This is probably the most widely employed
control measure in agriculture! example: rotate soybean with corn to
control soybean cyst nematode
2. Selection of planting date or planting location — choose a time/place
favorable for the host, rather than the pathogen: avoid pathogen or its
vector example: (time) plant cotton late to control damping-off caused
by Pythium (warm soil)
3. Seeding rate and canopy density — adjust within-row and between-row
spacing to open the canopy and reduce diseases that spread in the
humid, protected canopy environment
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Cultural control-II
Cultural (physical) control — manipulation of the environment
4. Irrigation
a. Pathogens can be spread in irrigation water or favored by wet soils-
example: late blight (Phytophthora)
b. Pathogens can be controlled by flooding - example: Fusarium wilt on
banana
5. Control insects and weeds — insects vector viruses and other
pathogens; weeds serve as alternate hosts for pathogens or vectors and
increase canopy density
6. Sanitation ~ keep area free of diseased plant material by pruning
diseased branches (fireblight), plowing under or burning debris,
washing and sterilizing harvesting and processing equipment (Rhizopus
soft rot); poor sanitation contributed to the late blight outbreak that
caused the Irish famine
7. Heat or refrigeration -- hot air, hot water, or steam treatments are used
to kill pathogens in seed or propagation materials; harvested fruits and
vegetables are kept refrigerated
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How are the tomatoes is in this photo
being watered? How does this help to reduce
disease?
Drip irrigation is used here. The water moves out into
the field in the blue pipes which, in turn, feed water
to small emitter lines that deliver water under the
plastic mulch right at the base of plants.
Because the foliage doesn't get wet, development
and spread of fungi and bacteria are much reduced.
What is the obvious
cultural control in this picture?
Mulching is the obvious answer. Of
course, there are many benefits to
be gained from mulching, including
weed control, soil moisture
optimization, and soil temperature
moderation. But mulches can serve
as a barrier between above-ground
plant parts and pathogens in the
soil. Also, by reducing weeds and
alternate hosts for pathogens, such
as several viruses, mulches help in
the battle against diseases.

18. Legislative and regulatory control
DISEASE MANAGEMENT: QUARANTINE
The prevention of pathogens entering new regions is known as exclusion, and
is achieved by quarantine or by treating propagating material (elimination)
before its introduction. If an outbreak of a disease occurs in a new area,
efforts are made to eradicate the pathogen from that area.
 Risk management aims to reduce the risk of introduction and
establishment of pest species, for example by fumigating biological
material when it enters the country.
Ways to do it-
 Sanitation
 Disease-Free Propagating Material
 Eradication
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19. 19Chemical Control-I
Application of pesticides
Pesticide — chemical that kills a pest (fungicide, bactericide, nematicide,
etc.); fungicides as examples, since fungi are the largest group of plant
pathogens
Types of fungicides and selected examples:
1. Inorganic
a. Sulfur -- oldest known fungicide
b. Copper — oldest formulated fungicide is the Bordeaux mixture (downy
mildew of grape); still the most widely used copper fungicide in the world
2. Organic
a. Protective fungicides -- protect infection court
i. thiram (Thiram, Tersan) — seed and bulb treatment of vegetables
ii. dichloran (Botran) ~ used against Botrytis on vegetables and flowers
iii. azoxystrobin (Quadris) -- used against leaf spots and blights, fruit rots
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20. 20Chemical Control-II
Types of fungicides and selected examples:
2. Organic
b. Systemic fungicides — are absorbed through foliage
or roots and are translocated upward through the
xylem; control already established pathogens and
protect against new infections
i. metalaxyl (Ridomil, Apron) -- controls oomycetes
ii. benomyl (Benlate) — broad-spectrum fungicide
iii. propiconazole (Tilt) — broad-spectrum fungicide
iv. aldicarb (Temik) – broad spectrum – bacteria, nematodes,
etc.
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21. 21Chemical Control-III
Types of fungicides and selected examples:
1. Inorganic
2. Organic
3. Fumigant — highly volatile, small molecular weight
compounds with activity against a wide variety of
pathogens (not limited to fungi); dangerous to humans
example: methyl bromide; currently being pulled from market
due to danger to non target organisms, including humans
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22. Chemical Control-IV
 Antibiotics are chemical produced by microorganisms,
which destroy or injure living organisms, in particular,
bacteria.
 Streptomycin is effective against a few fruit pathogens,
such as blights and cankers, and cyclohexamine can be
used to control some fungal pathogens of crops,
particularly powdery mildews and rusts.
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24. ISSUES RELATED TO CHEMICAL
DISEASE MANAGEMENT
 The main concerns are the risk of poisoning humans or animals,
contamination of livestock products, harm to beneficial insects, and the
contamination of food products, waterways and soil.
 The main risk to humans is during the preparation or application of these
chemicals, when they can be inhaled, ingested or absorbed through the
skin, and upon consumption of plants or their products.
 For example, Copper and Sulphur sprays have the potential to affect a
broad range of organisms if they are washed off the leaves and
accumulate in the soil or are washed into the waterways.
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25. To sum up
 The control of diseases in crops is still largely dominated by the use of
fungicides, but with the increasing incidence of fungicide resistance, plus
mounting concern for the environment resulting from excessive
agrochemical use, the search for alternative, reliable methods of disease
control is gaining momentum.
 The purpose is to examine the development and exploitation (or potential
for exploitation) of a range of non-chemical approaches to disease control,
with a focus on the need for a greater understanding of crop ecology as
the basis for effective disease control in the field.
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26. References
 Martinelli, F., Scalenghe, R., Davino, S., Panno, S., Scuderi, G., Ruisi, P., Villa, P., Stroppiana, D., Boschetti, M.,
Goulart, L.R., Davis, C.E., Dandekar, A.M. (2014). "Advanced methods of plant disease detection. A
review". Agronomy for Sustainable Development. 35: 1–25. doi:10.1007/s13593-014-0246-1.
 "Plant Diseases - History Of Plant Pathology". Retrieved 5 February 2015.
 Agrios, George N. Plant Pathology. 3rd ed. New York: Academic Press, 1972. print.
 Jackson RW (editor). (2009). Plant Pathogenic Bacteria: Genomics and Molecular Biology. Caister Academic
Press. ISBN 978-1-904455-37-0.
 erec.ifas.ufl.edu/plant_pathology_guidelines/module_07.shtml
 www.apsnet.org › APS › Education › Introductory › Topics in Plant Pathology
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27. Thank You
Sairam
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