ANTHRACNOSE OF COTTON

Causal organism: Colletotrichum capsici
Sub-division: Deuteromycotina

Symptoms

• In seedlings: Lesions develop on the collar region, causing girdling, wilting, and death.
• In bolls: Small, circular, reddish-brown, sunken spots appear.
• Lint becomes yellow or brown and turns into a brittle mass of fiber.
• Infected bolls stop growing, burst, and dry prematurely.

Etiology

• Primary inoculum: Dormant mycelium in seeds or conidia on seed surfaces.
• Secondary spread: Air-borne conidia.

Disease cycle

1. Primary infection occurs through contaminated seeds carrying dormant mycelium or conidia.
2. Secondary spread occurs via air-borne conidia, infecting neighboring plants.
3. The fungus survives on rotten bolls and plant debris in the soil.

Favourable conditions

• Prolonged rainfall during boll formation.
• Close planting leading to high humidity.

Management

• Treat delinted seeds with Carboxin + Thiram @ 2 g/kg.
• Spray Carbendazim + Mancozeb @ 0.2% at boll formation.
• Remove and burn infected plant debris.
• Rogue out reservoir weed hosts.

VASCULAR WILT OF COTTON

Causal organism: Fusarium oxysporum f.sp. vasinfectum
Sub-division: Deuteromycotina

Symptoms

• In seedlings: Cotyledons turn yellow, then brown, followed by wilting and drying.
• In mature plants: Yellowing of leaf edges, marginal chlorosis spreading to the midrib.
• Leaves lose turgidity, turn brown, droop, and drop off.
• Browning or blackening of vascular tissues.
• Black streaks extending to branches and lateral roots.
• Infected plants are stunted with fewer, smaller bolls that open prematurely.

Etiology

• Primary inoculum: Dormant hyphae and chlamydospores in soil.
• Secondary spread: Conidia and chlamydospores through irrigation water.

Disease cycle

1. Primary infection through dormant hyphae or chlamydospores in the soil.
2. Secondary spread occurs by conidia and chlamydospores disseminated by irrigation water.
3. The fungus invades roots, colonizes vascular tissues, and blocks water and nutrient flow.

Favourable conditions

• Soil temperature of 20-30°C.
• Hot, dry periods followed by rains.
• Heavy black soils with alkaline reaction.

Management

• Treat acid-delinted seeds with Carboxin + Thiram @ 2 g/kg of seeds.
• Remove and burn infected plant debris after deep summer ploughing.
• Crop rotation with non-host crops.
• Soil solarization to reduce soil-borne inoculum.

BLACK ARM OF COTTON (BACTERIAL BLIGHT / ANGULAR LEAF SPOT)

Causal organism: Xanthomonas campestris pv. malvacearum

Symptoms

• Angular Leaf Spot: Small, dark green, water-soaked spots on lower leaf surfaces.
  • Spots enlarge, becoming angular when restricted by veins.
  • Spots are visible on both leaf surfaces.
• Black Arm: Dark brown to black lesions on stems and fruiting branches.
  • Lesions girdle branches, causing premature leaf drop.
  • Cracking of stems with gummosis leading to “black arm” appearance.

Etiology

• Primary inoculum: Seed-borne bacteria in slimy mass on seed coat fuzz.
• Secondary spread: Through wind, rain splash, irrigation water, insects, and tools.

Disease cycle

1. Primary infection occurs from seed-borne bacteria.
2. Secondary spread via wind, rain splash, irrigation water, and contaminated tools.
3. The bacterium multiplies rapidly under favorable conditions, causing systemic infection.

Favourable conditions

• Optimum soil temperature of 28°C.
• High atmospheric temperature (30-40°C).
• Relative humidity of 85%.
• Early sowing, delayed thinning, poor tillage, late irrigation, and potassium deficiency.

Management

• Remove and destroy infected plant debris.
• Spray with Streptomycin sulphate (Agrimycin 100) @ 500 ppm with Copper oxychloride @ 0.3%.
• Use disease-free seeds and resistant varieties.
• Crop rotation and good field hygiene practices.