Important diseases of cotton, nature of damage and their management aspects are as follows:
1. Alternaria Leaf Spot (Alternaria macrospora):
Nature of Damage:
Small brown, round spots surrounded by a purple margin appear on leaves. On older leaves the necrotic center of the spots may be marked by a pattern of concentric zonation. Several spots coalesce to form large necrotic areas particularly near leaf margin. Under humid weather conditions the spots appear as sooty black leading to premature defoliation. The leaf stalk and bolls become infected with spherical or elliptical purple spots.
Control Measures:
Grow certified acid delinted seeds of resistant varieties. Follow recommended agronomic practices for land preparation, stubble management, seed rate, fertilizers and irrigation management. Do not extend the normal crop period and avoid ratooning. Avoid dense cropping that helps in reducing the disease incidence by allowing free air current. Remove and destroy crop residues after last picking.
Conserve bio agents (species of Gossypium) like Aspergillus fumigatus, A. Niger, Drechslera halodes, Fusarium culmorum, F. moniliforme, Monilia sitophila, Rhizopus stolonifer and Trichoderma viride. Spray mancozeb 2.5 g or copper oxychloride 3g/litre 4-5 times at 15 days interval. Dress the seeds with vitavax or aureofungin or seedex or difolatan or indofil M-45 @ 2-3 g/kg of seeds. Spraying with 0.2 per cent difolatan (captafol) or mancozeb at 20 days interval from first appearance is effective in managing the disease.
2. Anthracnose (Colletotrichum gossypii):
Nature of Damage:
The fungus attacks the seedlings, bracts and bolls. On cotyledons small reddish spots are formed. At collar region elongate reddish brown lesions are formed. Bolls of all stages are attacked, initially small water soaked, circular, sunken, reddish brown spots appear and their centers become black as they grow.
The pinkish-brown spots appear mainly on the underside of leaves. In case of severe infection, area under necrotic region increases and often results in defoliation. Dense canopy with warm humid weather favours the disease development.
Control Measures:
Grow certified acid delinted seeds of resistant varieties. Follow recommended agronomic practices for land preparation, stubble management, seed rate, fertilisers and irrigation management. Do not extend the normal crop period and avoid ratooning. Avoid water logging.
Avoid dense cropping and collatoral weeds such as Aristolochia bracteate and Hibiscus diversifolius. Remove and destroy crop residues after last picking. Destroy the infected plant debris. Acid delinted and treat the seeds with carbendazim or benomyl. Spray carbendazim 1.5 g/l or benomyl 1.5 g/l or mancozeb 3g/l.
3. Cercospora Leaf Spot (Cercospora gossypina or Mycosphaerella gossypina):
Nature of Damage:
First reddish dots appear which ultimately enlarge but retain a narrow reddish margin enclosing a white to light brown centre of dead tissue. Later on spots become irregular surrounded by a chlorotic halo. Leaves turn yellow and defoliate prematurely.
Control Measures:
Grow certified acid delinted seeds of resistant varieties. Follow recommended agronomic practices for land preparation, stubble management, seed rate, fertilizers and irrigation management. Do not extend the normal crop period and avoid ratooning. Remove and destroy crop residues after last picking. Spray mancozeb 2.5 g/l or copper oxychloride 3g/ litre 4-5 times at 15 days interval.
4. Grey Mildew/Areolate Mildew (Ramularia areola):
Nature of Damage:
The disease first appear on the lower canopy of older leaves when bolls set. Irregular, angular translucent spots (areola) are formed by the veins of leaves.
Leaves become yellow, turn to brown and defoliate prematurely. The pathogen survives mainly in plant debris and volunteer plants. High humid favour disease development.
Control Measures:
Deep ploughing. Destroy crop residues. Grow certified acid delinted seeds of resistant varieties. Crop should be rotated with cereals, and preference should be given to tolerant varieties. Crop residues should be removed and the fields must be deeply ploughed in order to bury and destroy the remaining plant tissues. Spray 3 g wettable sulphur or 1 g carbendazim per litre.
Dusting by 8-10 kg of sulphur powder effectively controls the disease or application of 1% Bordeaux DF or 3% copper fungicide. Foliar application of sulphur dust/wettable sulphur @ 10 kg/ha or 2-3 g/l at 10 days interval from the day of first appearance, or sulphex 80 WP (0.25%) or benomyl, carbendazim @ of 200-300 g a.i./ha is effective in controlling the disease significantly.
5. Helminthosporium Leaf Spot (Helminthosporium spiciferum):
Nature of Damage:
Lower leaves of seedlings turns light yellow which increase in size, become dark brown and surrounded by a dark purple border. The centre of the spots become ashy and shed off leaving shot holes.
Control Measures:
Grow certified acid delinted seeds of resistant varieties. Follow recommended agronomic practices for land preparation, stubble management, seed rate, fertilisers and irrigation management. Do not extend the normal crop period and avoid ratooning. Avoidance of dense cropping helps in reducing disease incidence as well as intensity.
Remove and destroy crop residues after last picking. Spray mancozeb 2.5 g/l or copper oxychloride 3g/l 4-5 times at 15 days interval. Foliar spray of thiobendazole or copper oxychloride @ 0.2 per cent controls the disease. However, a number of other fungicides such as zineb, ziram and captan are effective in controlling the secondary spread of this disease.
6. Fusarium Wilt (Fusarium oxysporum f. sp. vasinfectum):
Nature of Damage:
The symptoms first appear on the cotyledons vein starts darkening, followed by peripheral chlorosis. The cotyledons become progressively more chlorotic and then necrotic before being shed. In older plants, the first external evidence of infection is yellowing of margin of one or more lower leaves. As the disease progresses within the plant, more leaves develop chlorosis, which characteristically appears in patches between the main veins whereas rest of the leaf remaining green. Leaves droops, dry and shed off.
Ultimately plant dies causing 100% mortality. The fungus is soil borne. Nematodes attacking cotton predisposes the crop to wilt disease.
Control Measures:
Follow deep ploughing. Grow resistant varieties. Intercrop with moong bean/ragi/soybean/ groundnut/maize/cowpea/onion. Fields with long history of disease should be avoided for growing the crop. Use of nitrogen fertilizers, particularly ammonium nitrate should be discouraged while use of potassium fertilizers should be encouraged. Destroy crop residues. Treat the seed with 4 g Trichoderma viride formulation/thiram 3g per kg seed. Apply 2 kg of Trichoderma viride commercial formulation with 50 kg farm yard manure along with the rows. Spray of 0.4% solution of oxathilin. Seed treatment with carbendazim @ 2 g/kg seeds should be given.
7. Root-Rot (Rhizoctonia solani R. bataticola):
Nature of Damage:
Circular yellowish black patches are seen on seedlings or wood. Root tips are discoloured and black dot size sclerotia develop on the woods which rot. The most common symptom is dry or wet dark rot of the lower stem.
On split opening, the affected plant can be easily distinguished by discoloured stele of main root and pith of stem. In severe cases, there is dissolution of stem and root tissues. Sudden and complete wilting of the plant. The plant suffers high mortality at soil temperatures 39°C.
Control Measures:
Intercrop with moth bean. Grow resistant variety and follow deep ploughing. Avoid mono-cropping and cultivation of cucurbitaceous and solanaceous crops, in adjoining fields. Fields having long history of disease should be avoided for sowing. Field should be deeply ploughed and left for solarization. Early sowing and harvesting is recommended to avoid extreme temperatures. Sowing in April or June instead of May reduces disease incidence.
Crop should be rotated. Intercropping using Vigna acontifolia reduces the incidence quite significantly. After harvesting, either plant debris should be completely buried or removed. Treat the seed with benomyl or carbendazim @2.5 g/kg seed. Apply 2 kg of Trichoderma viride commercial formulation with 50 kg farm yard manure along with the rows.
Seed dressing with bio-pesticide preparation of Trichoderma viride or Gliocladium virens or with brassicol + captan and carbendazim slurry effectively reduces the disease incidence. Green manuring with Sesbania acubeata + planting during second week of July, and application of ammonium sulphate and intercropping with moth (Vigna aconitifolia) considerably reduce the disease incidence.
8. Bacterial Blight (Xanthomonas axonopodis pv. malvacearum):
Nature of Damage:
The larvae feed gregariously on the under surface of the leaves and skeletonize them leaving only the mid-rib and veins in severe cases. They also attack flowers, buds and squares causing considerable loss.
Control Measures:
Plough deeply to expose the pupae and hibernating larvae. Mechanical collection when larvae are feeding in groups, i.e., the younger larvae. Collection and destruction of egg masses. Set-up pheromone traps. Release egg parasitoids Trichogramma sp. (1.5 lakh/ha) and larval parasitoids Chelonus blackburni or Telenomus remus or Bracon sp. Release of predators Chrysoperla carnea @ 50,000 /ha. Spray Spodoptera NPV @ 250 LE/ha. Spraying of insecticides endosulfan 35 EC @ 600-750 ml/ ha effectively reduces the population. Spraying synthetic pyrethroids fenvalerate 20 EC @ 400-500 ml/ha or cypermethrin 10 EC or decamethrin 2.8 EC @ 600 – 700 ml/ha is also effective.