Introduction Anthracnose Blackarm Grey Mildew Leaf Curl Leaf Spots Little Leaf Root Rot Verticillum Wilt Wilt

Introduction The cotton crop sufffers from a number of diseases which can conveniently be classified into foliar and soil borne diseases. The important foliar diseases are bacterial blight caused by Xanthomonas axonopodis Cv.Malvacearnum, leaf spots, caused by Alternaria macropora, Myrothecium roridum and Helminthosporium speciforum. Gray mildew, a foliar disease caused by Ramularia areola which was initially restricted to G.arboreum, is also affecting G.hirsutum. Leaf curl is another devastating foliar disease. The soil borne diseases include root rots caused by Rhizoctonia solani and R.batalicolaand wilts caused by Fusarium oxysporum and Verticillium dahliae. Root rot and wide spread in the north zone, Fusarium wilt in the black soils of Gujarat, Madhya Pradesh, Maharashtra, Karnataka and Gujarat and Verticillium wilt in Tamil Nadu. Disease Management Strategies Bacterial blight and fungal leaf spot Spray the crop using stretocyclin (0.01%) and copper oxychloride (0.25%) Grey mildew Spray wettable S (0.2%) or carbendazim (0.1%) Boll rot Spray copper oxychloride (0.25%) or carbendazim (0.1%) along with the recommended insecticide. Root rot Treat seed with carbendazim 2.3 g/kg seed before sowing. Drench the plants in the affected patch with carbendazim (0.1%). Intercropping with moth bean and application of ZnSO4 kg/ha should be recommended where the disease is prevalent. Seed treatment with antagonists like Trichoderma viride and T.harianum. Top Anthracnose : Colletotrichum gossypii and C. capsici Mode of survival and spread Primary inoculum seed-borne ; sec. Spready by soil borne and air borne conidia; survival on collateral hosts. First seedlings attacked when reddish circular spots on cotyledons and primary leaves; when lesions on collar region the stem is girdled and seedlings wilt and die; boll spotting most common and serious; circular, slightly sunken, reddish brown lesions, deep seated, discoloring the lint to yellow/brown, causing it to rot; fibre brittle. Control Seed treatment with organo -mercurials; removal of collateral weed hosts ; spraying plants 2 times with copper fungicides before boll formation. Top Black arm/ angular leaf spot/bacterial blight : Xanthomonas malvacearum Mode of survival and spread Primary infection seed borne on the surface; on voluntary plants; sec. spread through wind, water and other physical and biological agents; also on several collateral hosts. Symptoms All above ground parts are attacked; on seedlings water soaked lesions on cotyledon which cause distortion and withering of cotyledons and death of seedlings ; water soaked lesions on leaves of mature plants bound by veinlets and turn dark brown to black; on stem brown to black linear sunken lesions which guide the stem; on bolls also sunken spots which are dark brown to black, deepseated and damage the lint fibre and seed. Control Acid delinted with conc. Sulphuric acid or organo- mercurials ; internal infection checked by soaking seeds in 100 ppm streptomycin sulphate overnight; secondary spread in field checked by spraying 500 ppm of streptomycin sulphate at 3 week interval ; use resistant varities when available. Top Grey mildew or areolate mildew : Ramularia areola Mode of survival and spread Spread by airborne conidia ; survival method not known. Irregular to angular pale translucent spots usually restricted by veins appear usually on the lower surface of older leaves; in severe infections, leaves turn yellowish brown and fall off prematurely. Control Spraying twice with 1% copper fungicide or dusting twice with sulphur. Top

Leaf curl

• The plant disease is now threatening cotton cultivation in north zone. Leaf curl was first noticed in Nigeria in 1912. It is causing considerable damage in cotton fields in north India since, 1993.
• The disease is caused by 'Gemini virus' which is transmitted by Whitefly (Bemecia tabaci). The initial symptoms begin on the top newly emerging leaves with the darkening and thickening of leaves. As rains get thickened the leaves either cup upwards or curl downwards. Later leaf shaped enations appear on the lower side of the leaves from the main vein. Plants remain stunted. Excessive shedding of buds and bolls occur.
• The G.arboreum cottons are not affected by this disease. Growing G.arboreum cotton or resistant varieties like RS 8975, LRA 5166, LII 144 are some reliable methods to curtail this disease. Removal of alternate hosts like Hibiscus esculentus, Abutilon sp., Sida alba, Althea rosea and Zinnia sp, should be recommended. The vector Bemesia tabaci should be contained by timely spraying to check further spread of the disease.

Agronomic practices

• Deep ploughing during summer season.
• Use of certified seeds of cotton hybrid.
• Complete sowing in a shortest span of time.
• Retain only one plant/hill by adopting thinning.
• Follow proper dosage of fertilizer and time of application as per the recommendation.
• Take up foliar spray of DAP @ 2.0 per cent, for 2-3 times at an interval of 15 days during peak flowering period.
• Clipping of terminal shoots at 130 days of crop growth.

Disease Management

Summer ploughing

• Fusarium wilt & Rhizoctonia root rot - Carbendazim at 1 g/lit & 1-2 lts/plant applied to affected as well nearby healthy plants.

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Leaf spots

• Helminthosporium gossypii
• Cercospora gossypiana
• Alternaria macrospora

Mode of survival and spread

• Survival on infected plant debris; on colateral hosts; spread by windborne conidia.

Symptoms

• Leaf spots of various shapes, color and sizes, characteristic of the respective pathogens; causing leaf fall.

Control

• Prophylactic sprays with copper or organic fungicides 2-3 times at 15 days interval.

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Stenosis or little leaf : Mycoplasma

Mode of survival and spread

• Graft transmissible

Symptoms

• Plants stunted, leaves reduced in size and clustered along the stem; buds are stimulated to produce numerous small leaves in clusters; flowers and bolls not formed and if formed small and ovaries abortive ; roots poorly developed.

Control

• Not know.

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Root rot : Rhizoctonia botaticola and R. solani

Mode of survival and spread

• Survives on decaying organic matter in soil ; several collateral hosts.

Symptoms

Sudden and complete wilting of plants; the disease spreads in field in concentric circles; when affected plants uprooted, the complete root system is rotten & basal stem shows shredding of bark.

Control

• Sowing of resistant varieties; adjustment of sowing dates; mixed cropping with sorghum and moth.

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Verticillum wilt : Verticillium dahliae

Mode of survival and spread

• Soil-borne sclerotia ; seed borne conidia; contact between healthy and diseased roots.

Symptoms

• Mottling of leaves with yellow areas between veins and on margins; necrosis of leaves, which become brown, dry and finally shed off; cross section of infected stem shows discolor-ation of vascular system.

Control

• Grow resistant varieties; spot application in soil of ceresan at .1% (6.5kg/hect); or Benomyl or Benlate at 0.05%; using acid delinted seeds after their treatment with vitavax (0.4%).

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Wilt : Fusarium oxysporum f.sp. vasinfectum

Symptoms

• It has been known to occur in black soil regions of Karnataka,Gujarat - with a predisposing soil temperature of 20-300c. Leaves yellow and drooping; plants stunted, gradually wilt and die.
• Discoloration starts from margin and spread to midrib.The vascular tissues become brown; affected seedling soon die.
• The attack is typically characterised by patchy appearence of field due to scattered attack.
• The diploid cottons (G. arboreum, G. herbaceum) are susceptible to this disease.

Mode of survival and spread

• Soil-borne ; survival on infected plant debris

Control

Grow resistant varieties; soil application of carbendazim (0.2%) helps in managing the disease.

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