1Control of plant diseases

Amit Kumar SahooII MSc Biosciences15151

2Breeding 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.

3The 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.

Amit Kumar Sahoo

4Plant Disease Triangle

HostTotal Of all properties that affect susceptibility

PathogenTotal of all properties of pathogen(virulence, abundance, etc.)

EnvironmentTotal of all conditionsThat affect disease

All three factors are necessary components of disease Area of triangle represents amount of disease

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5Vanderplank’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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6Disease CyclesSurvival 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.

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7Basic 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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8Disease control

There are four basic types of control measures:

a. Biological controlb. Cultural control (includes physical

control) c. Legislative and regulatory control d. Chemical control

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9Biological 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-IIb. 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-III2. 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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14Control 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-ICultural (physical) control — manipulation of the environmentThere 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-IICultural (physical) control — manipulation of the environment4. 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.

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Legislative and regulatory control DISEASE MANAGEMENT: QUARANTINEThe 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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Chemical Control-IApplication of pesticidesPesticide — 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 fungicideb. Copper — oldest formulated fungicide is the Bordeaux mixture

(downy mildew of grape); still the most widely used copper fungicide in the world

2. Organica. Protective fungicides -- protect infection court

i. thiram (Thiram, Tersan) — seed and bulb treatment of vegetablesii. dichloran (Botran) ~ used against Botrytis on vegetables and flowersiii. azoxystrobin (Quadris) -- used against leaf spots and blights, fruit rots

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Chemical Control-IITypes 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 infectionsi. metalaxyl (Ridomil, Apron) -- controls oomycetesii. benomyl (Benlate) — broad-spectrum fungicideiii. propiconazole (Tilt) — broad-spectrum fungicideiv. aldicarb (Temik) – broad spectrum – bacteria,

nematodes, etc.Amit Kumar Sahoo

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Chemical Control-IIITypes of fungicides and selected examples: 1. Inorganic2. Organic3. 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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22Chemical 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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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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25To 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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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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Thank You

Sairam

Amit Kumar Sahoo