Disease management – Principles Exclusion of the Pathogen/avoidance,/eradication/protective of pathogens

 Disease management – Principles Exclusion of the Pathogen:

The aim is to prevent entry of a pathogen in a field or area (state, country) suppose 
free from that pathogen.

1. Quarantine:

Plant quarantine aims at preventing entry of the pathogens from infested areas 
into non-infested area.

2. Inspection and Certification:

The crops grown exclusively for seed are periodically inspected for presence of 
diseases that are disseminated by seed. Necessary precautions are taken to remove 
the diseased plants. The method is supposed to prevent regional and inter-regional 
spread of the pathogen.

Seed Treatment

Seeds, tubers, grafts, bulbs and other propagative materials can be given heat, gas 
or chemical treatments to exclude the pathogen present in or over them. Seed 
treatment is mandatory for seed agencies supplying certified seed.

4. Eradication of Insect vectors:

Since the flight of insects cannot be checked so the crop should be given preventive sprays at the time of increase in insect vector population.

II. Avoidance of the Pathogen

It involves the tactics that prevent contact between the host and the pathogen, 
presuming that pathogen has crossed the barriers placed by exclusion or it is already 
present in the area and can attack the host.

It includes following strategies:

1. Choice of Geographic Area: 
Selection of geographic area for any crop is made on the basis of suitability of 
climate for the crop. Many fungal and bacterial diseases are common in wet areas.

2. Selection of field: 

Management of drainage is one of the important aspect of disease management. 
Fields free from soil borne diseases such as wilt, ear cockle and other fungal 
bacterial and nematode diseases should be taken for cropping.

3. Choice of time of Planting: 

The disease is severe or devastating when the susceptible stage of the plant growth coincides with the favorable condition of the pathogen.

4. Disease escaping varieties: 

Pea varieties that mature early (January) generally escape much damage from 
powdery mildew pathogen which becomes serious in January or later.

5. Selection of seed and planting material

A disease which is carried by seed or planting material and spread the infection in the field requires proper selection of healthy seeds to avoid multiplication of the pathogen in field and contaminate the healthy crops.

III. Eradication of the pathogen:

The principle involves the removal of inoculum already present in the field or the crop.

1. Biological control of Pathogens: 

The biological control aims at eradication and reduction of inoculum and protection of plant surfaces through the activity of microorganisms.

2. Crop rotation: 

It is one of the oldest method of fighting soil sickness and root diseases. The method is restricted against the pathogens which have limited host range and restricted survival ability in the soil.

3. Removal or destruction of diseased plants or plant organs: 

The presence of diseased plants in fields or in orchards is a source of continuous release of inoculum. Removal of alternate and collateral hosts is also recommended.

4. Heat and chemical treatment of diseased plants: 

The pathogen present in plants or its special organs can be inactivated or killed by heat or chemical treatments. Root dip in fungicides is a method of sanitizing the seedlings before transplanting.

5. Soil treatments: 

To inactivate or eradicate the pathogens present in the soil. It involves use of chemicals, heat and cultural practices such as flooding or fallowing.

6. Soil solarization: 

It is a novel method of soil treatment to destroy most fungal, bacterial and nematode propagules on as well as weed seeds. It is a system of raising temperature of wet soil kept covered with polyethylene sheets which traps the solar heat. The system is highly useful in sanitizing the nursery soils and small fields.

IV. Protective measures:

These measures include use of chemical sprays and dusts to create a toxic barrier between the host surface and the propagules of the pathogen and necessary modification of the environment to make it unfavorable for development of the disease.

1. Chemical treatment: 

The aim of most chemical sprays, dusts and seed treatment is to form a protective toxic layer on the host surface so that when the pathogen comes in contact with the surface it is killed or prevented from growth.

2. Control of Insect vectors: 

Those chemicals which kill the insect within few seconds are most effective in control of insect transmitted diseases.

3. Modification of the environment: 

Mixed cultivation of crops, intercropping, trap crops, antagonistic crops, root diseases favored by high temperature are often controlled by irrigation. Post harvest decay of fruits and vegetables is reduced or prevented by cold storage.

Modification of host nutrition:

High nitrogen dose increases susceptibility of the plants. High potassium renders plants resistance to many diseases such as rice blast. High calcium increases the resistance towards wilt and soft rot diseases.

V. Development of resistance in the host:

1. Selection and hybridization: 

Selection of resistant plant and hybridize them with commercial high yielding but susceptible cultivars, to produce new resistant plants with high commercial yield.

2. Genetic manipulation through biotechnology: 

Creation of transgenic plants in which resistance genes from sources other than the particular plant species is done, through genetic engineering.

3. Induction of acquired resistance: 

There are numerous methods through which plants acquire localized or systemic resistance during their life time via use of chemicals or microorganisms. PGPR induces systemic resistance. Salicylic acid induces systemic acquired resistance.

4. Resistance through chemotherapy: 

Systemic fungicides or antibiotics when applied to plants in low concentration results in development of resistance in plants

5. Resistance through host nutrition: 

By making available major and micronutrients through sprays, seed treatment or soil application is reported to strengthen the tissues that can defend the invasion of the pathogen.

VI. Therapy of diseased plants:

In many crops and particularly fruit trees chemical and physical therapy has been applied to cure diseases.

1. Chemotherapy: 

Chemical treatments are applied to eradicate pathogens from diseased to avoid its spread to other healthy parts of plants. Use of systemic fungicides, antibiotics, Bordeaux pastes etc.

2. Thermotherapy: 

These treatments are usually used for seed, tuber, bulb, grafts etc. Ratoon stunting disease of sugarcane and many viruses of sugarcane are eradicated by hot water, air or moist heat treatment of seeds.

3. Tree surgery: 

Large sized fruit trees are cleaned of infection by cutting or scrapping of the diseased part and covering the wound with a fungicidal paste.Disease management – Practices or Methods

I. Cultural methods:

Adjustment of crop management procedures has been an age old practice with the farmers for prevention of losses of crops due to diseases and other causes.

1. Production and use of disease free propagating material:

i) Dry Climate for seed production: Bean anthracnose, bacterial blight of legumes, black rot of cabbage etc.

(ii) isolation distance for seed plots: Separation of seed plots from source of inoculum of neighboring field.

iii) Inspection of seed plots: Negative selection

iv) Drying and ageing of seeds: Long term storage inactivates many seed borne diseases.

v) Cleaning of seeds: For removal of spores and sclerotia.

vi) Thermal and chemical treatment of seeds: Wheat seeds with carboxin (Smuts) and root dip treatments.

vii) Site and treatments of nursery beds: Soil solarization, removal of crop debris etc.

viii) Adjustment of harvesting time of crop: Delayed harvesting provides more time for pathogen to attack and contaminate seeds.

2. Adjustment of crop culture to minimize disease incidence: 

i) Crop rotation

ii) Avoid Monoculture

iii) Mixed cropping

iv) Green manuring

v) Adjustment of date of sowing:

 Early planting saves crop from white rust in mustard, blast in rice, karnal bunt and stem rust in wheat, bacterial blight of cotton etc.

vi) Adjustment of depth of seedlings: 

deep placement of seed gives pathogen more time to invade the seed and seedlings.

vii) Plant spacing, rate of sowing and density of stand:- 

Fungi and bacteria are generally more severe in dense planting due to development of congenial micro-climate while viral diseases are reduced due reduction in vector movement.

viii) Management of irrigation: -

disadvantages of excess irrigation and prolonged stay of water around the root zone are many. It makes plants susceptible to damping off and other root diseases.

ix) Management of host nutrition

x) Management of soil acidity and alkalinity:

 Common scab of potato (Sterptomyces scabies) is sensitive to acidity. The disease is reduced when pH is brought down below 5.2. The disease increases from pH 5.2 to 8. Lowering the pH by use of sulphur is effective method. Club root of crucifer can be controlled by increasing soil pH by adding lime and gypsum.

xi) Organic amendments of soil: Compost. FYM, Green manure etc. Fusarium oxysporium fsp. cubense: Sugarcane bagasse

xii) Management of top soil:

 Mulching – covering of top soil with organic residues often help in reduction of diseases, Soil solarization.

xiii) Minimizing influx of inoculum from neighboring crops

xiv) Choice of crop variety

3. Field and plant sanitation:

i) Management of crop debris: Deep ploughing, summer ploughing etc.

ii) Management of diseased plants: roguing

iii) Management of irrigation water

iv) Crop free period or crop free zone: narrow host range pathogens

v) Creating barrier by non hosts

vi) Decoy crops, trap crops and antagonistic crops: Decoy crops are non host or cover crops, trap crops are highly susceptible host crops antagonistic crops produce toxic chemicals that directly destroy the pathogen in soil, Marigold, Crotalaria, Asparagus etc. 

These are non hosts of pathogen.

vii) Management of weed, collateral and volunteer host

viii) Management of insect vectors

ix) Harvesting time and practices: Improper harvesting spreads: cysts of nematodes, spores of fungus (smut), seeds of phaenerogamic parasites. 

II. Biological methods:

The reduction of inoculum density or disease producing activities of a pathogen or parasite in its active or dormant state by one or more organisms except man, accomplished naturally or through manipulation of the environment, host or antagonists or by mass introduction of one more antagonists.

1. Destruction of surviving propagules: eg. Trichoderma viridae against Rhizoctonia solani while Pseudomonas fluoroscenes against Plasmodiophora brassicae.

Trichoderma is one of the successful biocontrol agent.

2. Prevention of inoculum formation: Ampelomyces quisqualis against powdery mildew fungi.

3. Reduction of virulence of pathogen (Hypovirulence):Mycoviruses (dsRNA) are present in fungi which reduce their surviving ability as well as virulence. Eg. Rhizoctonia solani, Magnaporthe oryzae.

4. Cross protection: When a mild or avirulent strain of virus is introduced into the host may induce resistance to a virulent strain of the same or serologically related viruses. 

III.Host resistance for disease management

Resistant varieties can be the most simple, practical, effective and economical method 

of plant disease management.

1. True resistance

i) Vertical resistance

ii) Horizontal resistance

2. Apparent resistance 

i) Disease escape

ii) Tolerance

3. Development of resistant varieties:

i) Selection

ii) Mutation

iii) Hybridization

4. Breeding for resistance using biotechnology:

i) Transgenic plants

ii) Recombinant DNA technology

IV.Physical Methods

Depends on physical factors like heat, cold, light wavelengths and radiation to reduce or eradicate pathogen inoculum.

Heat Treatment: Generally used for soil sterilization, disinfection of propagative materials, freeing plants from viruses and treating storage organs with hot air.

2. Refrigeration: Used to control post harvest diseases. The principle behind it is that low temperatures at or slightly above the freezing point inhibit or greatly reduce the growth activities of pathogens.

3. Radiation: Such as UV light, X-rays etc. have used to manage post harvest diseases of many fruits and vegetables.

Physical Methods – Heat

1. Hot Water Treatment

2. Hot Air Treatment

3. Aerated Steam Therapy

4. Moist Hot Air Treatment

5. Solar Heat Treatment

6. Soil Solarisation

7. Steam Sterilization

8. Hot Air Sterilization

9. Hot Water Treatment

10. Refrigeration

V. Disease management through chemicals:

1. Aim of use of chemicals in plant disease management:

i) To create a toxic barrier between the host surface or tissue and the pathogen

ii) To eradicate the pathogen present at a particular site on or in the host including seed, foliage and roots.

2. Fungicides are classified on the basis of functions:

i) Protectants: contact fungicides as spray, dust or paste.

ii) Eradicant: Destroy the dormant or active pathogens from host.

iii) Chemo-therapeuants: Specifically remove pathogen from host after 

infection. Systemic fungicides.

3. Contact fungicides:

a. Sulphur fungicides:

lime sulphur: Effective against Powdery mildew fungi.

Dithiocarbamates: Organic compounds of sulphur. Thiram (Commonly 

used for seed treatments), Mancozeb (Dithane M-45).

b. Copper fungicides: 

Bordeaux mixture: Copper sulphate + slaked lime + water. Effective 

against downy mildew fungi.

Copper oxychloride: Blitox 50. False smut of rice

4. Systemic fungicides:

a. Oxathiins:

The first systemic fungicide developed in 1966 by Von Schemuling and Kulka was Carboxin (Vitavax) – Smut. Oxycarboxin – Rust.

b. Benzimidazoles: 

Carbendazim (Bavistin), effective against wide range of fungi except oomycetes and dark colored fungi such as Alternaria, Helminthosporium etc.

c. Acylalanine: effective against oomycetes.

Metalaxyl (Ridomil and Apron) – Pythium, Phytophthora and downy mildew fungi.

d. Sterol biosynthesis inhibitor:

i)Triazoles: Propiconazole (Tilt) – Rust, smut, blastTricyclazole (Beam) – Rice blast

Antibiotics:

Metabolites of microorganisms which, in very dilute concentration have the capacity to inhibit the growth of, or even destroy other microorganisms.

a. Streptomycin – First antibiotic used in plant disease fire blight of apple and pear in 1953.

b. Tertracycline (Acromycin), Oxytetracycline (Terramycin) and Chlorotetracycline (Aureomycin) – against fastidious bacteria, phytoplasma, MLO’s.

Methods of application of fungicides

1. Seed treatmentS
2. treatment
3. Foliar application
4. Postharvest application
5. Special Method of application