Plant Disease management

Pl. Path. 111 (Cr. Hrs. 3+1)

P.N. Sharma

Department of Plant Pathology,

CSK HPKV, Palampur (H.P.)

Disease Management All plant diseases result from a

three-way interaction between

the host,

the pathogen and

the environment.

An epidemic develops if all three

of these factors are favourable to

disease development.

Therefore, disease can be

controlled by manipulating one or

more of these factors so that

conditions are unsuitable for

replication, survival or infection by

the pathogen.

Principles of Disease Management

The history of controlling the diseases is very old,

Man discovered empirical cultural and physical

control practices for crop protection even before

the causal nature of plant diseases.

General acceptance of the idea that fungi can

induce diseases of plant opened the way for

scientific study of means to control diseases and

The principle search was for use of chemical for

disease control. Bovdeaux mixture in 1882 by

Millardet.

Plants resistance to the diseases were

recognized in the nineteenth century (Biffen

1905) by breeding for disease resistant

varieties after the discovery of Mendals’ law

of heredity in 1900.

Success in controlling the diseases with

chemical and host resistance in early 20th

century shifted plant pathologists from

cultural control (Stevens, 1960).

The approach to the management of a

given disease was often based on type

of the disease, economic importance

of the crop and quality or demand of

the market.

Host resistance and cultural practices

were the principal methods used

against pathogens of agronomic crops

because of the low unit value of most

agronomic crops could not justify the

use of chemicals.

Since the main objective of plant

pathology is the economic control of

plant disease. However, the term

control evokes the notion of finality or

completeness, which is not true in

nature. Whereas the word

“Management” conveys the concept of

a continuous process and

management is based on the principle

of maintaining the damage or less

below an “economic injury level”.

The concept of IDM

The philosophy and approach of crop protection have been influenced significantly during last two decade by the evolution of the concept of “IPM”.

This concept has evolved into holistic, multi disciplinary, management system that integrates control methods on the basis of ecological and economic principles for pests of all classes (pathogen, insects induced that exits in an agroecosystem (Glass 1975).

Knowledge of different aspects of disease

development is essential for effective and

economic control.

Cause of the disease.

Mode of survival and spread of the

pathogen.

Host pathogen relationship and mode of

secondary spread.

Effect of environment a pathogeneses

and spread of disease i.e. Epidemiology

knowledge.

TIME-LINE OF INFECTION

Fungal pathogen:

Host:

Preinfection: Germination Germ tube search Appressorium formation Penetration peg

Postinfection Haustorium formation (biotroph) Toxin formation (necrotroph) Detoxification of phytoalexins Reproduction

General induced Papillae Cork & lignin Systemic defenses: formation layers acquired resistance Specific recognition: Hypersensitivity Phytoalexins

structural CONSTITUTIVE DEFENSES chemical

Outside of host Inside of host

Basis of

Disease management principles

Immunization-prophylaxis system: the cure

of diseased plant is not possible because

the disease become visible only after injury

to plant has taken place, thus preventive

measures are most important.

These include preventive measure through

induction of resistance in the host

(immunization) and protection of plant by

prophylactic measures.

Plant disease control Immunization Prophylaxis

Legislation

Cultural

Resistance

Protection

Eradication

Chemotherapy

Destruction

of alternate

hosts

Central State

Rotation Chemicals Sanitation

chemical

Choice of Control Measure

The efficient and economic control of a plant disease is dependent on crop value, effectiveness of control measure, risk involved and cost/benefit ratio. Fruits and vegetables diseases are best controlled

by fungicides whereas

for field crop disease, host resistance is the preferable method.

Management of environment and changes in the cultural practices are desirable in several cases e.g. soil borne disease.

Disease management can be brought about by manipulating the components of the disease pyramid i.e host, pathogen, environment and time.

Management of Disease Triangle

Environment

Pathogen Host

The host The host can be manipulated by increasing its

resistance to disease, which is the most effective and least expensive method of controlling diseases. However, frequent breakdown of resistance necessitate the Management of the resistance in nature and this can be done by: diverse sources of resistance (e.g. Sonalika and

Girija),

pyramiding of genes for resistance,

deploying genes for resistance in space and time,

use of multlines and the use of horizontally resistant or tolerant cvs.

or by applying chemicals, such as fungicides, that

prevent or halt infections in the plant.

The pathogen The aim of pathogen management is to reduce

inoculum to such a level that economic losses are minimum

The pathogen can be manipulated by excluding or reducing inoculum available to infect plants.

If a particular pathogen is not present in an area then attempts can be made to prevent its introduction.

Quarantine regulations govern the introduction of plant material from high-risk areas. Seed certification schemes also reduce the risk of introducing a new pathogen, and propagating material can be treated to kill any inoculum that is present, before it is introduced to a new area.

The pathogen

The aim of pathogen management is to reduce

inoculum to such a level that economic losses are

minimum

This can be achieved in addition to host resistance, through

the use of chemicals (used only when other methods prove

inadequate).

Chemicals are generally used to crops of high cash values,

against very destructive diseases and in seasons favoring

epidemic development

main aim of managing the pathogen is:

Reducing initial inoculum

Reducing infection rate

Elimination of an existing pathogen is achieved by

chemical and physical treatment and trapping,

crop rotation,

i) Reduction in the initial amount of inoculum can

be brought about by:

Seed treatment (e.g. Vitavax for loose smut of

wheat, barley, bunts,)

Soil treatment e.g. formaldehyde (4%)

Dip treatment of fruits (Thiabendazoles for

storage rots)

quarantine practices and preventative treatments

The pathogen

ii) Reduction in the infection rate can be

achieved by:

Spray chemicals either protectants or systemic.

However, the no. of sprays is dependent on the degree of

freedom from disease desired (which may be high in case of

fruits and vegetables where quality is of paramount

importance)

For field crop, more than one spray may be uneconomic and therefore in such cases Chemicals can be used only if they are inherently more efficient (eg.

Indar (RH124) against leaf rust of wheat

or only against Very destructive diseases e.g. rice blast or

On varieties having a fair degree fo reisistance to disease. Or

In environment not particularly favourable for pathogen development. Or

If forecasting system for judicious use of spray is available (potato blight).

The pathogen

The environment

The environment can be modified so that

it is suitable for plant growth but not for

disease development.

For example, improving soil

drainage, changing the time of sowing,

reducing the density of plants in a crop or

changing irrigation practices can all produce

conditions unfavourable to particular

diseases, while maintaining conditions

suitable for the growth of the plant.

Management of environment

Temperature management by adjusting sowing time to avoid bunt and flag smut of wheat, use of solar heat to eradicate internally seed borne loose smut infection of wheat and barley hot water treatment to eradicate black rot bacterium from cauliflower seeds are typical examples.

Moisture regulation

Irrigation is effective against potato scab, whereas simple water spray under pressure can eradicate powdery mildew.

Flooding of soil can be effective in eradicating Fusarial wilt organism.

Pathogens which are serious in alkaline soil (e.g. Sclerotinia on beans) can be effective controlled by low34ering soil pH to <5.2 by applying sulphur.

Nutritionl status of soil

Affects several soil borne and some airborne diseases. Diseases caused by Rhizoctonia are reduced by nitrate N application. Inorganic N also suppresses S. rolfsii. Adequate N fertilization considerably reduces Helminthosporium on rice and maize. Fusarial wilt can be checked by adequate K nutrition.

Management of cultural practices Proper following of sanitation

Sanitation is effective in greatly reducing incidence of late blight, ring rot and leaf roll of potato.

Use of clean seed or disease free seed is effective in checking different diseases (e.g. potato viruses and bean anthracnose). Harvesting of potato tuber only after defoliation of foliage is effective in preventing tuber rot due to late blight.

Tillage help in soil loosening, thus provide aeration which is effective in preventing/ against Fusarium root rot of beans, Sclerotium rolfsii.

Deep ploughing during dry season followed by a fellow period for nine months destroy the bacterium causing wilt of solanacaeous plants.

Spacing:Disease which spread very fast under crowdy conditions (e.g. Cercospora on pulses) are checked considerably by wide spacing.

Use of organicamendments

4. Time dimensions

Although time dimension is important in

the epidemic development of most

diseases, not much can be done to

manage thus component except the use

of early maturing varieties (e.g. wheat to

escape rust or fast emerging vars. To

escape bunt and flag smut of wheat.

Principle of disease management

The principles of disease

management are generally

based on i) identity of the

disease to be mange

Whetzel (1929) was the first

to classify methods for the

control of disease as :

Exclusion, Eradication,

protection and Resistance,

and later two more methods

were included, avoidance and

Therapy (National Sci.

Academy, 1968).

Six principles

Avoidance

Exclusion

Eradication

protection

Resistance

Therapy

Principles of Plant Disease Management 1. Avoidance of the pathogen

This involves those methods/ practices which avoid the contact of the host with pathogen o susceptible stage of the plant and conditions favorable for the pathogen should not coincide.

Most of diseases can be prevented by selecting suitable site or time of sowing of crop, varietal selection, seed and planting material and by modifying the cultural practices.

Selection of geographical areas

Selection of field

Choice of time of sowing

Disease escaping varieties

Selection of seed and planting material

Modification of cultural practices.

a) Selection of geographical areas

It depends upon suitability of prevailing environmental condition for

particular cop e.g. temp and RH e.g. Certain diseases (fungal and

bacterial) are more serious in wet areas than in dry areas. Thus growing

of such crops in dry areas with irrigation facilities can be avoided from

those diseases e.g. Smut of bajra (Tolyposporium penicillariae) and

ergot of bajra (Cleviceps microsphala) are serious in wet areas.

b) Selection of field

Soil borne diseases can be avoided e.g. Red rot of sugar cane,

late blight, wilt diseases of arhar, root knot nematodes, bacterial

wilt of solnaceous plants.

Selection of site in case fruit crops e.g. Apple trees should not

be planted in areas where oak trees were grown previous to

avoid attack from collar rot (Rosellinia sp.).

c) Choice of time of sowing

In this coincidence of susceptible stage of crop and environment fabourable for

pathogen is taken care off., e.g. Pea and Gram plnted soon after rain, when soil

temp and moisture level are high, shows high incidence of root rot, blight and

wilt. So late sowing (Nov.-Dec.)

d) Disease escaping varieties

The disease escaping quality of varieties due to characteristics of

growth and time of maturity e.g. var. of Pea which matures early (by

January) usually escape damage from powdery mildew and rust.

e) Selection of seeds and planting material

Smuts, red rot of sugarcane, virus free potato tubers etc.

Avoidance of the pathogen

Exclusion It can be defined as a legal restriction of the movement

of agricultural commodities for the purpose of

exclusion, prevention or delay in the spread of plant

pests and diseases in uninfected areas.

Objective of the exclusion is to prevent spread of the

diseaseand this can be achieved by:

Seed treatment

Inspection and certification

Eradication of insect vectors.

Quarantines

Exclusive quarantine or Embargo

Regulatory quarantine

Domestic quarantine

Inspection &Seed Certification

• Setting Certification Standards

• General Seed Certification Standards

– Common for all crops put under certification

• Specific Seed Certification Standards

–Applicable for individual crop or group

of crops.

0.10 0.10

3. Eradication of the pathogen

Biological control of the pathogens

Crop rotation

Removal and destruction of diseased plants and organs. Rouging

Eradication of alternate and colleteral hosts

Sanitation

Polythene traps and mulches

Control through trap plants

Heat and chemical treatment of diseased plants

Soil treatment

SANITATION

Sanitation involves all procedures that prevent the spread of disease to new plants, plant products, and new areas, or to reduce the amount of inoculum in an already affected area.

It includes thorough washing or chemical treatment of machinery, tools, potting equipment, shoes and hands that will come in contact with multiple plants, washing produce and its containers and storage areas, protecting soil used for propagation from contamination.

Removing and disposing of infected leaves and other plant material is also crucial for reducing the amount of inoculum available to cause new infections.

4. Protection

(For destruction and inactivation of inoculum; involves physical and chemical control methods)

Chemical treatments: (Sprays and seed treatment

Chemical control of insect vectors.

Modification of environment: includes management of physical environment like temp. moisture, light, aeration, pH, nutrition.

Modification of host nutrition

Host Resistance

What is resistance?

It is the ability of a host to hinder/ resist the harmful effects of a

pathogen.

It is a relative term. Resistance can be expressed at any stage of

development of disease.

The resistance is expressed as

Morphological resistance

Biochemical resistance

Classification of resistance

Genetic basis

Monogenic resistance (single gene controlled resistance)

Oligogenic resistance (controlled by two genes)

Polygenic resistance (controlled by more than two genes)

Extra chromosomal resistance (Cytoplasmic resistance e.g.

TMS lines in maize resistant to couthern corn leaf blight

Epidemiological basis of resistance

Van der Plank, (1963) introduced the terms,

“Vertical” and “Horizontal” for resistance.

Vertical resistance vertical resistance is the resistance that is controlled by oe or

two genes and is effective against one or some races of the

pathogen and not against others . VR acts epidemiologically

by decreasing the initial inoculum (X0) but not the rate of

increase (r) of virulent races or checks the epidemic.

Horizontal resistance horizontal resistance is controlled by manu genes

and is effective against all races of the pathogen.

HR reduces the rate of spread of disease (r) i.e.

slows down the epidemic.

Vertical resistance

Is monogenic or oligogenic.

VR is also called as

differential resistance, field

immunity, hypersensitive

resistance, major gene

resistance, qualitative

resistance, R-gene resistance,

race -specific resistance, and

specific résistance.

VR is a temporary resistance

but provides a complete

control of disease against

specific race

Checks the epidemic

Horizontal resistance

Controlled by many

genes

is also called Race non

specific resistance, minor

gene resistance,

quantitative resistance,

general resistance,

durable resistance,

partial resistance

It is permanent

resistance

Slows down the

epidemic

Apparent resistance It is generally a result of disease escape or

tolerance to disease

Disease escape: it occurs when genetically

susceptible plant do not become infected because

of non coincidence of susptble stage with

favouorable condiditions or duw to fast growth rate

of host as compared to pathogen or due to mixed

cropping or age of the host plant

Disease tolerance: it’s the ability of the plant to

produce a good crop even when its attacked by the

pathogen.

Development of resistant varieties The resistance varieties can be developed by using following

methods

Selection oh host for resistance

Hybridization using resistant (R) and agronomically superior

susceptibe (S) varieties

Resistance from various sources

Crossing between R and S parents

Mutation

Physical mutagens X-rays; UV rays etc

Chemical mutagens: EMS

Resistance through host nutrition

Cross protection

Induced resistance

SAR

CP mediated resistance

Use of genetic engineering: resistance can be transferred to a plant

species from alien sources e.g. BT gene in cotton crom bacteria

Bacillus thuringiensis

Management of Resistance

The following strategies of R-gene management are

available.

Pyramiding of vertical genes

Regional deployments of vertical genes

Rotation of vertical genes or gene cycling.

Intra-specific within field diversity by genotype mixtures

Cultivar mixtures

Multilines

6. Therapy

Chemotherapy

Chemical seed treatment

Thermotherapy

Hot water or air treatment

Tree surgery

Removal of disease part and treatment with

chemicals

Integrated disease management

Integrated disease management uses all suitable techniques that complement each other with the aim of keeping the disease below the threshold at which economic damage occurs.

This approach also aims to avoid the problem of developing resistance in pathogens to widely used fungicides or antibiotics.

In IDM, various control measures are combined for effective and economic management of the disease e.g. Resistant crop varieties might be used in conjunction with chemical treatment, crop rotation and manipulation of the environment.

Multiple approaches to disease management are particularly useful when the disease cycle is not fully understood.

IDM of Potato late

blight

Figure: Effect of plant disease

resistance and fungicide

treatment on defoliation induced

by potato early and late blights

during two different years.

• Variety Norchip is

susceptible and

• Elba and Allegany are

moderately resistant to

both diseases.