Principles and methods of plant disease control

 Principles and methods of plant disease control

A. Principles

Principles of plant disease control include

1. Prophylaxis

·         Exclusion (Quarantine and other legislative measures)

·         Eradication (Cultural eradication and Chemical eradication)

·         Protection (Cultural manipulation, Physical and chemical protections)

2. Immunization
• Genetic resistance
• Therapy (Physical therapy and chemical therapy)

1. Avoidance of pathogen

Avoiding disease by planting at times when or where inoculum is absent or ineffective due to unfavorable environment conditions.

• Choice of geographic area
• Selection of field
• Choice of time of sowing
• Disease escaping varieties
• Selection of seed and planting stock
• Modification of cultural practices e.g.
1. Exclusion of inoculum

Preventing the inoculum from entering or establishing in the field or area where it does not exist.

·         Seed treatment

·         Inspection and certification

·         Quarantine

·         Eradication of insect vectors

3. Eradication of pathogens

Reducing, inactivating, eliminating or destroying inoculum at the source, either from a region or from an individual plant in which it is already established.

·         Biological control of plant pathogen

·         Crop rotation

·         Removal or destruction of diseased plant organs

·          Rouging

·         Eradication of alternate and collateral hosts

·         Sanitation

·         Heat and chemical treatment of disease plants

·         Soil treatments

4. Immunization

It involves the modification of certain physical or physiological character(s) of the host such that it can repel infection or can reduce disease development or can minimize damage caused by the pathogen. The methods used are

·         Use of resistant varieties

·         Cross-protection

      Cross protection refers to the protection of a plant by use of a mild strain of a pathogen against a virulent strain of the same pathogen that can cause more severe symptoms and damage. The method is generally used for viral disease management.

Disease resistance: Altering the effectiveness of the pathogen by selection or introduction of resistance genes in the plant.

• Selection and hybridization for disease resistance
• Resistance through chemotherapy
• Resistance through host nutrition

5.      Protection measures

Preventing infection by creating a chemical toxic barrier between the plant and the pathogen is comes under protection measures.

·         Chemical treatment

·         Chemical control of insect vectors

·         Modification of environments

6.      Therapy

Reducing severity of disease in an infected individual

·         Chemotherapy

·         Heat therapy

·         Tree surgery

B. Methods

1. Cultural methods

Crop rotation: Crop rotation with non-host will help to control some of the pathogens. E.g. Ahrar wilt Fusarium oxysporum f. udum can be controlled by rotation of crop rice, groundnut etc.

Summer ploughing exposes the pathogen to direct sun and kill them. Meloidogyne can be reduced by summer ploughing.

Flooding will create anaerobic condition and is helpful to reduce the diseases of angular leaf spot of cotton- Xanthomonas malvacearum and banana wilt- Fusarium oxysporium f. cubense

Sanitation – collection and destruction of infected parts of the plant after harvest of the crop is useful to reduce many plant diseases.

Time of sowing- is important for checking soil borne pathogen.

Depth of sowing- seed sown too deep in the soil will get infection and cause pre-emergence damping off.

High seed rate creates suitable environment for post emergence damping off.

Use of fertilizers- high nitrogen favors rice blast diseases but suppress brown spot and vice versa.

Addition of organic amendments is the easy and cheap method of managing the plant diseases.

Green manuring will reduce the nematode problem.

Mixed cropping is sometime useful to control the diseases e.g. Ahrar and Jawar when grown together. Jawar secretes HCN that kills the ahrar wilt pathogen.

2. Physical Methods

·         Dry heat treatment

·         Hot water treatment

·         Firing

These things will kill indiscriminately the microorganisms living in the soil. They may be harmful or useful. It is practiced to controlled Sclerotium rolfsii causing root rot in many crops.

3. Regulatory methods

Plant quarantine is the legal restriction on the movement or interchange of plant or plant products between countries, states and communities for the purpose of preventing or the introduction and establishment of plant pathogens where they do not exist. There are domestic quarantine and international quarantine established.

Plant quarantine actions are grouped in to 5 major groups.

1. Embargo– Absolute prohibition on the movement of infected materials from a quarantined area to a protected area.
2. Detention- A delays in the release from a period when the plant material are held under careful observation until freedom from the parasite have been assured or organism has been eliminated.
3. Inspection: Plant material examined either on the source of produce or at the point of entrance.
4. Disinfection: If the material is found diseased, than either seed treatment or hot air or sorting is done.
5. Unrestricted entry: When it is considered that there is no harmful disease it is allowed to enter everywhere.

4. Biological methods

Introduction or activation of less harmful or saprophytic microorganisms to check more harmful or pathogenic organisms is known as biological control method. The fungi Trichoderma and Gliocladium are used to control soil borne pathogens like Rhizoctonia, Sclerotium, Sclerotinia and Fusarium. Similarly, saprophytic bacteria are used to control the fungal or bacterial disease.

5. Chemical methods

Chemical methods of plant protection bring about the desired results quickly and for this reason alone they are so popular in practices. Among these methods, the use of various forms of insecticides against insect pests and different kinds of fungicides to safeguard crop against pathogenic fungi are very much in practice. Generally these chemicals are used to treat seeds, soil, propagated plant materials other than seeds and standing crops.

Classification and formulation of chemicals used in disease control

Fungicides mean a chemical, which are able to kill the fungi. However all the fungicides do not kill the fungi, they may check the growth of fungi temporarily or they may check the germination of fungal spores. Therefore they can be classified as i) Protectants and ii) Therapeutants

Protectants are those fungicides, which are used to protect the plants from the attack of pathogens. They should be applied before infection e.g. Zineb, Maneb, Sulfur etc.

Therapeutants are those chemicals which are capable of eradicating the pathogen even after causing the infection e.g. Vitavax, Hinosan, antibiotics etc.

Based on mode of action they can also be classified as systemic and non-systemic.

Systemic fungicides are those, which act by entering the system of the plants. They are the therapeutants.

Non-Systemic fungicides are those, which cannot enter the system of the plants but act on the surface of the plants. They are generally protectants.

Based on general use the fungicides are classified as;

1. Seed protectants- oraganomercurials, captan, thiram
2. foliage fungicides- zineb, maneb, captan, ferbam
3. soil fungicides- PCNB, thiram, captan, vitavex
4. fruit protectants- benomyl, captan

Based on chemical nature the fungicides are classified as

1. Sulfur fungicides
• Inorganic – Elemental sulfur, Lime sulfur
• Organic – Dithiocarbamates e.g. zineb, maneb, ziram, nabam, vapam, thiram
2. Copper fungicides
• Copper sulfate – Bordeaux mixture, Burgundy mixture, Chestnut compound
• Copper carbonate- Chaubattia paste, Masuadia paste
• Copper oxychloride- Blitox-50, Fytolan, Cupramar
• Cuprous oxide- Perenox, fungimar
3. Mercurial fungicides
• Inorganic- Mercuric chloride, Mercurous chloride
• Organic- Agrosan, Ceresan, Herbasan, Lunasan
4. Quinone- Chloranil, Diclone
5. Heterocyclic nitrogenous-Captan, Folpet
6. Oxathiin- Vitavax, Plantavax
7. Bendimidazole- Benomyl, Thiobandazole, Bavistin, Derosal
8. Pentachloronitrobenzene- Brassicol
9. Antibiotics- Aureofungin, Agrimycin, streptomycin, streptocycline

Formulation of fungicides

1. Water dispersible or wettable powders- This is the most commonly available formulation. They have the capacity to wet or disperse in the water. Generally wetting agent is present in each WP. Frequent agitation is necessary if spreader or dispersing agent is not added while spraying the fungicides.
2. Dusts- They have generally 4-10% active ingredient and the rest is inert material. They are applied as dry or dust form. The particle size should be 200-300 mesh size.
3. Suspension or slurries- Very limited fungicides are available in this form. Some of the systemic fungicides are found in the form of suspension. 

6. Integrated disease management

It is a new conceptual approach, which is based on ecological principles and integrates multidisciplinary methodologies in agroecosystem management strategies that are practical, effective, economical and protective of both public health and environment. Various control measures that can be incorporated in IPM includes, judicious use of chemicals preferably botanical pesticides, biological control, plant quarantine, seed certification, resistant cultivars, genetic engineering.

7. Host resistance or use of resistant varieties

It is a wonder of nature that some plants posses the inherent character of producing an optimum yield of good quality despite association with economic insect pests. The variety of crop possessing this sort of character is known as a resistant variety. The ways this property manifests in plants are as follows:

• Tolerance: Surviving the heavy insect pest infestation.
• Nonpreferance: being less attractive to insect pests
• Antibiosis: having an adverse physiological effect on insect pests.

The most practical means of disease control is using resistant varieties. In some cases this is the only feasible means of controlling plant disease such a cabbage yellows, verticillium and bacterial wilt of tomatoes, black rot of cauliflower, bacterial blight of rice and bacterial stalk rot of maize.

1. Advantage of resistant varieties

-          Resistance is usually specific to a pest or a complex of pest organisms.

-          They check the present pest population and work for the successive generations of pests too.

-          Quality persists for a longer period.

-          There is harmony with the environment

-          Adoption is easy

-          There is compatibility with other tactics of plant protection methods.

2. Limitations of resistant varieties

-          Much time is involved in searching for resistant entities.

-          New pests’ races evolve to which the variety is not resistant.

-          There is increasing susceptibility to other pests due to increase in resistance to the target pests.

-          There is the possibility of decomposition of resistant characters due to natural forces.