DISEASES OF COTTON – Agricultural study materials

1. Fusarium WILT –Fusarium oxysporium f.sp.vasinfectum

Symptoms

The disease affects the crop at all stages. The earliest symptom appears on the seedlings in the cotyledons which turn yellow and then brown. Defoliation starts from the older leaves at the base, followed by younger ones towards the top. Browning or blackening of vascular tissues can also be seen which spreads downwards to lateral roots. In severe cases, discolouration may extend throughout the plant starting from roots extending to stem, leaves and even bolls.

Mode of Spread 

Soil and Wind borne conidia (Microconidia 0-1 septate, Macroconidia 3-5 septate) 

Survival

The pathogen survives as externally and internally seed borne inoculum

Epidemiology

Soil moisture and soil temperature play important role in Fusarium wilt incidence. Hot and dry periods followed by rains and a soil temperature of 20 to 30⁰ C favours the disease. 

Management

• Remove and burn the infected plant debris in soil 
• Deep summer ploughing
• Treat the acid-delinted seeds with carboxin or chlorothalonil at 4 g/kg
• Application of FYM or compost @ 125 tons/ha
• Grow resistant varieties like Varalakshmi, Vijay, Pratap, Jayadhar and Verum

2. Verticillium WILT – Verticillium dahliae

 Symptoms

The disease attacks the crop at squares and boll formation stages. The first symptoms can be seen as bronzing of veins followed by interveinal chlorosis and yellowing of leaves. The characteristic diagnostic feature is the drying of the leaf margins and areas between veins, which gives a ‘’tiger stripe’’ appearance. In severe cases, when infected stem and roots are split open, shows a pinkish discolouration of the woody tissues. If an Affected leaf is plucked off and petiole is cut across and examined, brown spots marking the vascular elements are seen as characteristic symptom of the disease.  

Mode of Spread 

Conidia in soil spread through irrigation water and implements

Survival

Infected plant debris in soil helps in survival of the pathogen (The fungus exists as micro sclerotia in soil upto 14 years without the host plant) 

Epidemiology

Low temperature (15-20^o C), continuous rain, Heavy soils with alkaline reaction and heavy application of nitrogenous fertilizer favour the disease development.

Management

• Remove and burn the infected plant debris in soil after deep summer ploughing
• Treat the acid-delinted seeds with carboxin or chlorothalonil at 4 g/kg of seed
• Application of FYM or compost @ 12.5 tons/ha
• Grow resistant varieties like Sujatha, Suvin and CBS 156.

3. GREY/ AREOLATE MILDEW – Ramularia areola

Symptoms

Irregular to angular, pale translucent lesions (1-10 mm) develop on the lower surface, usually bound by vein lets. In severe cases, affected leaves dry up from margin, cup inward, turn yellowish brown and fall off prematurely.

Mode of Spread 

The pathogen spreads through wind borne conidia and water

Survival

The pathogen survives in the infected plant debris

Epidemiology

Wet humid conditions and intermittent rain encourage the disease

Management

• Remove and burn the infected plant debris in soil
• Spray the crop with carbendazim at 250 g/ha or wettable sulphur at 1.25 g/ha
• Grow resistant varieties like Sujatha, Varalakshmi and Savitri

4. ROOT ROT – Rhizoctonia bataticola

Symptoms

Germinating seedlings [1-2 weeks old] are attacked at the hypocotyls regions. The pathogen produce black lesions, girdling of stem and results in death of the seedling. The most prominent symptom is sudden and complete wilting of plants in patches. The affected plants when pulled, reveal the rotting of entire root system except tap root and few laterals. The bark of the affected plant shreds and even extends above ground level.

Mode of Spread 

Sclerotia are spread by irrigation water and cultural operations

Survival

The pathogen survives as Soil borne sclerotia

Epidemiology

Dry weather followed by heavy rain, high soil temperature and low soil moisture encourage the disease development.

Management

• Seed treatment with carboxin or thiram or carbendazim at 2 g/kg or T.viride at 4 g/kg of seed or  P.fluorescens at 10 g/kg of seed
• Application of FYM at 10 tons/ha or neem cack at 250 kg/ha
• Inter cropping with sorghum / moth bean (Vigna aconitifolius) 

5. ANTHRACNOSE – Colletotrichum capsici and Glomerella gossipii

Symptoms

Infected seedlings produce small, reddish, circular spots on the cotyledonsand primary leaves. Small water-soaked, circular reddish brown depressed spots appear on the bolls, grow in size and disease spreads to bracts and interior portions of the bolls.

Mode of Spread and Survival

The pathogen spreads through Wind borne conidia and survives in the seed

Epidemiology

Prolonged rain fall at boll formation stage helps in the disease development

Management

• Treat the delinted seeds with captan or thiram or carboxin at 2 g/kg of seed
• Spray the crop at boll formation stage with mancozeb @ 2 kg/ha or carbendazim @ 500 g/ha

6. LEAF BLIGHT – Alternaria macrospora

Symptoms

Small, pale to brown, irregular or round spots having a central necrotic lesion surrounded by concentric rings can be seen. In severe cases, the spots may appear on bracts and bolls eventually leading to the defoliation.

Mode of Spread 

The pathogen spreads through Wind borne conidia and wind splashed rain

Survival

The pathogen is externally and internally seed borne

Epidemiology

High humidity and intermittent rains favour the disease

Management

• Remove and destroy the infected plant debris
• Spray mancozeb or copper oxy chloride at 2 kg/ha
• Grow resistant varieties like K 7,8, MCU 5 and Jayadhar

7. BOLL ROT – Complex disease- F.monoliforme, C.capsici, A.flavus, A.niger

Symptoms

Initially the disease appears as small brown or black dots which later enlarge to cover the entire bolls. Infection spreads to inner tissues and seeds rot. Finally the bolls never burst open and fall off pre maturely

Mode of Spread 

The pathogen spreads through air borne conidia

Survival

The pathogen survives in the infected bolls and debris in the soil

Epidemiology

Heavy rainfall during the square and boll formation stage helps the disease

Management

• Adopt optimum spacing and apply recommended doses of fertilizers
• Early sowing and growing early maturing cultivars
• Spray COC 2.5 kg/ha + Fenvalerate 75 g a.i /ha or mancozeb 2 kg/ha from 45^th day at 15 days interval

8. STENOSIS / SMALL LEAF – Phytoplasma (Mycoplasma like organism)

Symptoms

Affected plants are stunted and produce numerous small leaves in clusters. Dormant buds are stimulated resulting in profuse vegetative growth. Large number of flower buds and young bolls fall off pre maturely.

Mode of Spread 

Draft transmissible and No insect vector has been identified 

Management

• Rogue out the infected plants periodically
• Cotton varieties developed from G.hirsutum and G.barbadense are found to be resistant

9. BACTERIAL BLIGHT – Xanthomonas campestris pv. malvacearum

Symptoms

    Five common phases of symptoms are noticed

a) Seedling blight

Small, water-soaked, circular or irregular lesions develop on the cotyledons

b) Angular leaf spot 

Angular, reddish brown spots restricted by veins and vein lets appear

c) Black vein 

The infection causes blackening of the veins and veinlets, gives a typical “blighted appearance”

d) Black arm 

    The stem hangs typically as dry black twig due to cracking and breaking. 

e) Boll rot

Bolls are attacked by the bacterium at any stage of their development. The infected area appears first as small dark green water soaked areas. The boll lesions gradually enlarge, turn black and become shaken as the tissues die. Even if the bolls do open, the lint in the infected portions is discoloured yellow due to the ooze of the bacterium.    

Mode of Spread 

Primary spread is through seed borne bacteria and Secondary spread takes place through wind, rain splash and irrigation water

Survival

The pathogen survives in the infected plant debris in soil

Epidemiology

High temperature (30–40⁰ C), rains, followed by bright sunshine favour the disease

Management

• Remove and destroy the infected plant debris
• Delint the cotton seeds with conc.H₂So₄ at 100 ml/kg of seed
• Soak the seeds in 1000 ppm streptomycin sulphate
• Spray streptomycin sulphate + tetracycline mixture 100 g with COC @ 2 kg/ha or spray COC alone @ 2.5 kg/ha
• Grow resistant varieties like Sujatha, 1412 and CRH 71.