Verticillium Wilt of Cotton
Causal Organism
Name: Verticillium dahliae
Classification:
Symptoms
Verticillium wilt of cotton presents several characteristic symptoms:
- Leaf symptoms: Initial symptoms appear as chlorosis (yellowing) and necrosis (browning) on the margins and between veins of older leaves. These symptoms often form a "V" shape.
- Wilting: Affected plants exhibit wilting, especially during hot, dry conditions. The wilting can be temporary in the early stages but becomes permanent as the disease progresses.
- Stunting: Infected plants may be stunted due to impaired vascular function.
- Defoliation: Severe infections can lead to premature leaf drop.
- Vascular discoloration: Brown to black streaks can be observed in the vascular tissue (xylem) when the stem is cut open. This discoloration is a diagnostic feature of Verticillium wilt.
Etiology
Verticillium dahliae is a soil-borne fungus that infects cotton plants through their roots. It survives in the soil as microsclerotia, which are small, dark, and resilient fungal structures that can persist in the soil for many years. The fungus can also be introduced to new areas through contaminated soil, water, or infected plant material.
Disease Cycle
1. Survival: V. dahliae survives in the soil as microsclerotia.
2. Germination: When conditions are favorable (warm and moist), microsclerotia germinate and produce hyphae.
3. Root infection: The hyphae penetrate the root tissues of susceptible cotton plants, entering the vascular system.
4. Colonization: The fungus colonizes the xylem vessels, spreading upwards through the plant.
5. Symptom expression: As the fungus proliferates, it produces toxins and enzymes that disrupt water transport, leading to wilting and other symptoms.
6. Reproduction: The fungus produces conidia (asexual spores) within the plant tissues, which can be spread by water, wind, or equipment, further disseminating the disease.
7. Persistence: At the end of the growing season, the fungus forms microsclerotia in the decaying plant tissues, which return to the soil, completing the cycle.
Management
Effective management of Verticillium wilt of cotton requires an integrated approach:
1. Remove and burn infected plant debris: After deep summer ploughing, remove and burn the infected plant debris to reduce inoculum in the soil.
2. Organic manures: Apply farmyard manure or other organic manures at a rate of 4 tons per acre to improve soil health and potentially suppress the pathogen.
3. Mixed cropping: Follow mixed cropping with non-host plants to provide shade and lower soil temperatures below 20°C, which can reduce the survival of V. dahliae.
4. Crop rotation: Rotate crops by growing paddy, lucerne, or chrysanthemum for 2-3 years to break the disease cycle and reduce soil pathogen levels.
5. Seed treatment with Chlorothalonil: Treat acid-delinted seeds with Chlorothalonil at 4 grams per kilogram of seed to protect seedlings from early infection.
6. Biological seed treatment: Treat seeds with Bacillus subtilis (10 grams per kilogram) or Trichoderma asperellum (4 grams per kilogram) to enhance resistance against V. dahliae.
7. Soil application of Trichoderma asperellum: Apply Trichoderma asperellum at 1 kilogram per acre, twice—once during sowing and once at 90 days after sowing (DAS). Multiply 1 kilogram of T. asperellum in 100 kilograms of farmyard manure for 15 days before application to enhance its effectiveness.
Bacterial Blight of Cotton
Causal Organism:
- Name: Xanthomonas citri pv. malvacearum (previously known as *Xanthomonas campestris pv. malvacearum*)
- Classification:
- Kingdom: Bacteria
- Phylum: Proteobacteria
- Class: Gammaproteobacteria
- Order: Xanthomonadales
- Family: Xanthomonadaceae
- Genus: Xanthomonas
- Species: *Xanthomonas citri*
- Pathovar: malvacearum
Symptoms and Stages:
1. Seedling Blight:
- Symptoms: Seedlings exhibit water-soaked lesions that turn dark brown to black. These lesions often lead to damping off, where the seedling collapses and dies.
2. Angular Leaf Spot:
- Symptoms: Water-soaked, angular lesions appear on the leaves, confined by leaf veins. These lesions initially are translucent but later become necrotic and dark brown. Severe infection can cause premature leaf drop.
3. Vein Necrosis or Vein Bending:
- Symptoms: The veins and veinlets of leaves turn dark brown to black, resulting in vein necrosis. This leads to bending and twisting of the veins, giving a distorted appearance to the leaves.
4. Blackarm:
- Symptoms: Black, elongated lesions appear on the stems, petioles, and branches, causing girdling and blackening of affected areas. This stage can lead to the breaking off of branches and severe defoliation.
5. Square Rot or Boll Rot:
- Symptoms: Water-soaked lesions develop on squares (flower buds) and bolls, which later turn black and rot. This leads to shedding of the squares and bolls or the production of poor-quality cotton.
Etiology:
- Pathogen: The bacterium Xanthomonas citri pv. malvacearum.
- Inoculum Source: The primary source of inoculum is infected plant debris and seed. Secondary spread occurs through water splashes, wind-driven rain, and mechanical means (e.g., cultivation equipment, human handling).
- Infection Process: The bacteria enter the plant through natural openings like stomata or wounds caused by insect feeding, cultivation, or mechanical damage.
Disease Cycle:
1. Survival: The bacterium survives in infected plant debris, soil, and seeds.
2. Primary Infection: With favorable conditions, the bacterium infects seedlings and young plants from the contaminated seed or plant debris.
3. Secondary Spread: During the growing season, the pathogen spreads from plant to plant through splashing rain, wind, and contaminated tools.
4. Symptoms Development: As the bacteria proliferate, they cause various symptoms on different parts of the plant.
5. Overwintering: The pathogen survives in plant debris and seeds left in the field, ready to initiate the cycle in the next growing season.
Management:
1. Seed Treatment:
- Delint the cotton seeds with concentrated sulphuric acid at 100 ml/kg of seed.
- Treat the delinted seeds with Carboxin or Oxycarboxin at 2 g/kg or Carboxin 37.5% + Thiram 37.5% WS at 2.5 g/kg.
2. Cultural Practices:
- Remove and destroy the infected plant debris.
- Rogue out the volunteer cotton plants and weed hosts.
- Follow crop rotation with non-host crops.
- Perform early thinning and early earthing up with potash.
3. Chemical Control:
- Spray Streptomycin sulphate at 100 g + Copper oxychloride at 500 g per acre as a preventive measure.