Disease Management

1. Leaf Curl Virus

Symptoms

• Young and delicate leaves on top are affected first. Initial symptoms are thickening and darkening of fine veins, later on more veins are thickened causing netted appearance, upward or downward curling and cupping of leaves. Small leaf shaped structures called 'enations' may develop on the lower side of leaves on the main and lateral veins. In severe disease incidence plants remain stunted with bushy appearance with shedding of buds and squares causing a few or no boll setting.

Etiology

• Leaf curl disease is caused by 'Gemini Virus' which is transmitted by an insect white fly Bemisia tabaci (Genn.) and by grafting.

Varieties

• Most of the hirsutum varieties and hybrids are susceptible to this disease. However, RS-875, RS-810 and RS-2013 & desi cotton variety RG-8 or RG-18 varieties developed at Agricultural Research station, Sriganganagar of Rajasthan Agricultural University, (RAU), Bikaner, showed resistance to CLCUV under field conditions. Gossypium arboreum cotton varieties were free from disease.

Disease Management

• Grow G. arboreum (Desi cotton) Variety RG-8, G. hirsutum varieties RS-875, RS-810, RS-2013, LRA-5166 and hybrids GK-151 and LHH-144 which are resistant to leaf curl disease.
• Do not grow American (G.hirsutum) cotton near or with in orchards and lady's finger (Bhendi) fields.
• Follow clean cultivation and remove volunteer plants and weed hosts like Abutilon sp. (peeli buti), Sida sp. (Kangi buti), Althea rosea (Hollyhock) and Zinnia sp.
• Remove diseased plants at vegetative phase to check secondary spread of the disease.

Spray recommended systemic insecticides viz., Imidacloprid, Triazophos, Methyl demeton, Phosphamidon, Acetamiprid, Dimethoate, Ethion or neem based insecticides for the control of white fly vector at 4-5 leaf stage of plants.

2. Root Rot

Causal organism : Rhizoctonia solani kuhn and R. bataticola (Taub) Butler.

• Root rot is one of the most serious diseases of cotton in north west Rajasthan. The arboreum (desi) cotton is more seriously affected than hirsutum cotton.

Symptoms

The main symptom is sudden and total wilting of green plants, all the leaves droop down within a day. Diseased plants are easily pulled out from the soil.

• The disease spreads in circular area or in patches.
• The root tip of a freshly wilted plant is slightly moist and sticky. The bark (cortical tissue) is broken into shred, rots and is removed form the cortical woody cylinder (vascular bundle), the wood in severly infected plants turns brown or black.
• Sometimes yellowing and shredding of roots may extend above the ground level.
• In old wilted plants fungal growth and brown or black sclerotia may be seen near the soil surface around the plant.
• The disease mostly appears after first irrigation when the plants are 35-45 days old.

Disease Management

• Due to the soil inhabitant nature of the pathogen it is difficult to control the disease. However, by adopting a few cultural, chemical and biological control methods, the incidence of the disease may be reduced significantly.

Cultural Control

• It is known that Rhizoctonia solani and R.bataticola show a well marked periodicity in their parasitic activity in irrigated areas. Sow arboreum cotton between first week of April to first week of May.
• Grow American cotton in the infested fields.
• Sow one row of moth bean phaseolus acontifolius crop between two rows of cotton, and uproot them by the end of August.

Chemical Control

• Seed treatment with Emisan or Bavistin (carbendazim) @ 1 gm or 2 gm / kg seed respectively and application of 24 kg commerical zinc sulphate in soil per hectare.

Biological Control

• Soak seed in plain water for sometime, slightly dry in shade and treat with Trichoderma harzianum @ 10 gm / kg seed or In root rot sick fields native Trichoderma harzianum @ 10 kg/ha pre-incubated in 200 kg well rotten FYM for 10-15 days be applied or mix in the soil before sowing followed by seed treatment @ 10 g / kg seed for better control of the disease.

3. Wilt

Causal organism : Fusarium oxysporum f.sp.vasinfectum

• The disease causes losses due to the death of seedlings and growing plants, stunting of growth, development of smaller bolls and production of immature lint. The disease usually occurs in scattered patches. Circular patches which progressively increase in size are called as "wilt sick". The disease is sporadic in nature and attacks desi cotton (G.arboreum) in a few fields at Sriganganagar and in Mewar cotton track.

Symptoms

• The wilting may be gradual or sometimes yellowing, withering and drying of leaves followed by drying of entire plant or some of the branches.
• Wilting may be partial or complete. The leaves loose their turgidity, first turn yellow then brown, start wilting and finally drop off.
• Discoloration of the leaves starts from the margin inwards and spreads towards the midrib. An abcission layer is formed at the base of petiole and the leaf falls off. Older leaves towards the base are affected first followed by younger ones towards the top.
• In completely wilted plants cotton stalks with base branches are left in the fields.
• Most characteristic symptom of the disease is the browning and blackening of vascular tissues. In severe cases of disease discoloration may extend through out the plant starting from roots and extending to the stem, leaves and even bolls.

Disease Management

Cultural Control

• In the disease infected fields grow American cotton (G.hirsutum).
• Do not grow Desi (G.arboreum) cotton in the infected fields for 4-5 years to reduce inoculum load in soil and thereby the disease incidence.

Chemical Control

• Apply commercial zinc sulphate @ 24 kg/ha in soil at the time of sowing.
• Seed treatment with carbendazim (Bavistin) @ 2 gm / kg seed is effective in reducing disease infection.

Biological Control

• Soak seed in plain water for sometime, slightly dry in shade and treat with Trichoderma harzianum @ 10 gm / kg seed or In root rot sick fields native Trichoderma harzianum @ 10 kg/ha pre-incubated in 200 kg well rotten FYM for 10-15 days be applied or mix in the soil before sowing followed by seed treatment @ 10 g / kg seed for better control of the disease.

4. Bacterial Blight (Angular Leaf Spot)

Causal organism : Xanthomonas campestris pv malvacearum, X.axonopodis pv malvacearum.

• It is one of the most serious diseases of cotton in all cotton growing areas.
• American cotton (G.hirsutum) is most susceptible to the disease.
• Disease is more serious in humid and rainy weather.

Losses

• The disease causes losses by destroying the chlorophyll of leaves and stem which reduces photosynthetic activity of the plant and by defoliation of leaves, shedding, rotting and distortion of bolls.
• It causes significant loss in the seed cotton yield, seed index, oil percentage and ginning out-turn.
• Fibre quality parameters viz length, maturity, fineness, bundle strength and uniformity ratio remain unaffected.

Symptoms

• All the aerial parts of the plant are affected.
• There are four phases of the disease. Seedling blight of cotton (Seedling phase), Angular leaf spot of cotton (Leaf phase), Black arm of cotton (Stem phase) and Boll rot (Boll phase).
• In young seedlings first symptoms appear on cotyledons. This infection comes from the seed which may carry the inoculum on the fuzz.
• Small water soaked irregular lesions appear on the margins of the cotyledons, these lesions turn brown or black and cotyledons shrivel.
• Most important symptoms are appearance of many minute water soaked angular spots scattered on the ventral surface of leaves, these spots turn brown and then are converted into black dead lesions on both sides of the leaf.
• Very often one or more veins in part or whole becomes water soaked and then turn brown or black, this symptom is called as "Black vien" or "Vein blight". This may appear independently or together with angular leaf spots.
• The bolls are affected by the bacterium during hot and humid weather. Small circular water soaked spots appear on young and old bolls, they turn brown or black and depressed in the center, resulting into premature opening of bolls or rottening of bolls due to the growth of saprophytic fungi. This phase is called as boll blight or boll rot.

Disease Management

Cultural control

• Sow less susceptible or tolerant varieties.
• Sowing of American cotton between 1^st May to 20^th May or early have less or no disease.

Chemical control

• Seed treatment of delinted seed by soaking in 100 ppm solution of streptocycline or plantomycin for two hours, and of fuzzy seeds by soaking for 8-10 hours.
• Spray the crop at first appearance of disease with streptocycline @ 50ppm and copper oxychloride @ 0.3%. One or two more sprays may be given at an internal of 10-12 days depending on the intensity and weather conditions.

5. Leaf spots:

1. Alternaria leaf spot

Causal organism : Alternaria macrospora

Symptoms:

The disease causes blightening of leaves. In early stage of infection pale green spots with irregular margins develop on leaves later on these enlarge and concentric rings and cracked centre develops, severe infection causes drying and falling of leaves.

2. Myrothecium Leaf spot

Causal organism : Myrothecium roridum

• This disease is more prevalent in this area.

Symptoms:

• The disease appears on leaves, bracts as well as on bolls.
• The characteristic symptoms are the appearance of circular to semicircular light brown to tan coloured spots with violet to reddish brown margins, later on shield shaped small fruiting bodies (sporodochia) are produced in the central necrotic portion of the spot. The center dries and drops off causing big shot holes in leaves.
• High humidity and intermittent rains are congenial for the development of the disease.
• In Rajasthan both the leaf spots appear alone or in combination on both hirsutum and arboreum cotton.
• In northern part of the state it appears late in the season at the maturity of the crop, therefore, the loss due to these spots is negligible.

Disease Management

• For the control of both the types of leaf spots spray 0.2 % Mancozeb or Copper oxy-chloride or 0.1 % carbendazim.

TIRAK

• Physiological disorder due to imbalance in the nutrients and unfavourable weather conditions.

Symptoms

• The bolls do not open properly due to which lint does not comes out properly.
• Spell of high temperature and low humidity during flowering and fruiting phase of plant growth causes this problem.

Disease Management

Cultural Control

• Cotton may not be grown in the drought prone areas.
• Green manuring and use of farm yard manure and use of balanced fertilizers NPK may be useful.

Regular irrigation should be ensured during flowering and boll formation stage of the crop.