Cotton: Gossypium spp.; RHIZOCTONIA ROOT ROT; WILT;VERTICILLIUM WILT; ANTHRACNOSE

Cotton: Gossypium spp. Family: Malvaceae   1. RHIZOCTONIA ROOT ROT The root rot of cotton is one of the most severe disease in India and other cotton growing countries such as Africa, Venezuela and USA. The disease is severely found in Gujarat, Haryana, Karnataka, Madhya Pradesh, Punjab, Rajasthan, Tamil Nadu and Uttar Pradesh. The disease is mainly found in sandy soils. Symptoms: The disease affected cotton plants shows sudden, complete wilting and spread in concentric circles in the field. The entire root system of the disease affected plants rot. The severely affected plants has only tap root attached with it and other lateral roots decayed and remain in the soil. The tissues of tap root showed discolouration, shredded bark, moist and sticky. The fungal sclerotia may be appeared on the woody tissues rotting bark as minute black dots. Causal Organism: The root rot disease is caused by fungal pathogen Rhizoctonia bataticola (Taub.) Butler and R. solani Kuhn. The fungal hyphae are present in the host tissue inter and intracellularly. The thick and septate hyphae produced sclerotia which are about 100 µm in diameter. The sclerotia of R. bataticola and R. solani are black irregular and dark brown irregular respectively. Disease Cycle: The both the pathogen are facultative parasites and live as a saprophyte on the organic matter in the soil for several years. Once the vulnerable plants come in contact with the pathogen, it dominates inside the roots and multiplies rapidly in the cortical tissues or may also reach up to pith or xylem tissues. The diseased plant roots showed increased accumulation of calcium, iron and some toxic substances which possibly may cause wilt symptoms in the plants. The fungal pathogen also multiplies on some other crop plants. When cotton crop is sown in the next growing season, the pathogen again active and infect the crop. The disease is favoured by 35 °C soil temperature, 15-20 percent soil moisture and alluvial or sandy soils. Disease Management: The seed treatment with Quintogene, Carbendazim and Oxathiin can effectively reduce the seed and soil borne infection. The pre and post sowing drenching of soil with Quintogene and Carbendazim gives maximum disease control. Certain agronomic practices such as mix cropping and growing of shady plants in the field may reduce the disease incidence. The bacterium Pseudomonas fluorescens, commonly found in rhizosphere of cotton seedlings, generally increased seedling survival. The cultivation of resistant varieties such as KH-33-146, 15-Kw-2 (MB) and 9-Kw-2 (MB) is the best way to control the disease completely. 2. WILT The wilt of cotton is one of the major diseases found wherever the crop is grown. It is believed that the disease was originated in Mexico or Central America and spreading to many African countries, some parts of France, Italy, India, USSR, West Indies and Yugoslavia. In India, the disease was reported first time from Nagpur by Evans in 1908. The disease is now appearing in almost all cotton growing areas of India. The disease is severely found in heavy soils with 20-30 °C soil temperature. The disease is not common in loamy soils of indo-Gangetic plains, Maharashtra, Tamil Nadu, Karnataka and other states while in Punjab and Jammu & Kashmir, it is absent due to unfavourable soil temperature. It is estimated that the disease causes 40-60 percent yield loss because of reduced growth, small bolls and poor lint quality. Symptoms: The plants of all ages are affected by the disease. The first symptoms of the disease appear on the young seedlings as a vein clearing and interveinal tissue necrosis. The young seedlings show yellowing and browning of the cotyledons and produce brown ring on the petiole. The affected seedlings soon wilt and die. If the disease appear some later stages, the leaves become flaccid and results in the drooping and wilting, starts from older to young leaves of whole plant. The complete defoliation and the barren stem in the field may be seen oftenly. The basal stem of the affected plants shows black coloured discolouration. The black coloured streaks are commonly seen below the bark, extending up to root and shoot tips. The transverse section of the root, shoot, branches, leaves and bolls shows discolouration. The disease affected plants shows retarded growth. Causal Organism: The wilt disease is causing by Fusarium oxysporum f. sp. vasinfectum (Atk.) Snyder & Hansen. The fungal mycelia is present inter and intracellularly in the host tissues. The fungal mycelium plugs the xylem vessels partially or completely and checks the water and mineral supply from root to shoot. The pathogen produces micro and macro conidia in the host and on culture media. The macro conidia are hyaline, 1-5 septate, thin walled, falcate with tapering ends and measuring 27-48 x 2.5-4.5 µm. The micro conidia are hyaline, elliptical to spherical, thin walled and 1-2 celled and measuring 6.4-10.6 x 2.3-3.2 µm. The conidia germinate and produce hyphae and mycelium. Disease cycle: The fungal pathogen enters in the host roots through wounds and openings caused by nematodes and insects. The pathogen multiplies inside the root tissues and reached up to xylem vessels and formed abundant mycelia. The mycelia plug the vessels partially or completely which interrupted nutrient supply to the upper shoot system thus stunting plant growth. The interrupted nutrient supply leads to wilt. The fungal pathogen induces the tyloses formation in the xylem vessels; prevent the upward flow of nutrients. The fusaric acid produced by the pathogen may be another cause of wilt symptoms which is not host specific. The disease is favoured by 20-30 °C soil temperature with optimum temperature 24-28 °C. The dry and hot period followed by monsoon seasons is favourable for fungal disease initiation grown in black soils. The fertilizers such as nitrogen, phosphorus and manganese increase the disease incidence while the zinc and potash reduces the disease. The fungal pathogen can live in the soil as a saprophyte for many years and chlamydospores remain in the soil for several years as resting spores. The pathogen is internally and externally seed borne. The fungal hyphae and chlamydospores play major role in disease dissemination. Disease Management: Since the pathogen is internally and externally seed borne, the seed treatment with systemic fungicides such as Carbendazim, organomercurials, Thiram and Topsin M can effectively reduce surface contamination. The phenolic enzyme activity induced in the seeds treated by Chlorothalonil, Thiabendazole and Carboxin which helps in the reduction of disease intensity. The soil drenching with Bavistin and Benlate can control the wilt disease to some percent. The agronomic practices of cultivation such as changing in sowing dates, application of potash and zinc amendments can also reduces the disease to some extent. The Indian tetraploid cotton varieties are immune to Indian races of Fusarium oxysporum f. sp. vasinfectum. The sowing of resistant varieties is best economical and beneficial way to control the disease. Some resistant/immune Indian varieties are C-C-1-35, JLA-101, AKH-590, SM-143, LD-327 and LD-254.   3. VERTICILLIUM WILT The Verticillium wilt is a major problem of cotton in USA and previous USSR which was reported from India in 1968 on G. hirsutum from south India. The disease appear in the plantation in November-December when crop in boll stage. The disease not only causing yield loss but also deteriorate fiber quality which may be reached up to 90 percent depending upon disease severity. Symptoms: The first symptoms of the disease are appearing on the leaves as pale yellow irregular areas on the margins and central veins with mottling. The yellow areas of leaves become paler, whitish and necrotic. The affected leaves soon shows brown colour, dry and finally shed off. The infection moves in upward direction and complete defoliation may be seen. The longitudinal section of the infected stem reveals pinkish to pinkish brown discolouration of the vascular tissues. The root system is also interrupted by the pathogen. Causal Organism: The fungal pathogen causing wilt disease is Verticillium dahliae Kleb. The hyphae are hyaline, septate and measuring 1.2 x 4.0 µm in diameter. The verticillate conidiophores arise in groups measuring 10.6-38.7 x 1.1-2.6 µm bears single conidia at the top which are unicellular, oval, ellipsoidal, hyaline, verticillate and measuring 2.5-7.5 x 1.1-3.1 µm. The fungal pathogen also produced black coloured sclerotia which are globose to elongate produced within the infected host tissues. Disease Cycle: The disease is soil borne and spreads through sclerotia persists in the soil and conidia carried on the seeds. The disease is disseminated from disease to healthy plants by contact of leaves, stem or roots. The increased availability of nutrients of the host plants has favoured disease development. Disease Management: The crop rotation with cereal crops is an effective method to control the wilt of cotton. The cultivation of resistant varieties can control the disease like Sujatha and CBS-156. The seed treatment with systemic fungicides or organomercurial giving effective disease control. The disease spread may be checked by soil drenching with Benomyl or Benlate.   4. ANTHRACNOSE The anthracnose disease of cotton is found worldwide including India and reported from every cotton growing area.  In severe conditions, the disease is causing considerable yield loss. Symptoms: The disease appears on the cotyledons and primary leaves as small reddish circular spots. The lesion found on the collar regions, girdled the stem, resulting in wilting and death of the seedlings. The splitting and shredding of bark is seen on mature affected plants. The bolls of affected plants show water-soaked, circular, slightly sunken and reddish brown lesions. The lesions increased in size and turn black with red margins. The lesions may coalesce to cover large areas on the bolls. The infection of bolls is reached to the lint and seeds, causing rot and remain smaller in size with deformity. The lint is transformed into a mass of brittle fibre. Causal Organism: The pathogens causing anthracnose of cotton are Colletotrichum capsici (Syd.) Butler & Bisby and C. gossypii Southw. The pathogen is producing inter- and intra-cellular mycelium which producing many acervuli on the affected plant parts. The conidiophores are club shaped, hyaline, single shaped which bearing falcate, hyaline, single celled and thin walled conidia. The conidium of C. gossypii is smaller than the conidium of C. capsici. Both the species produced dark, thick walled and septate setae. Disease Cycle: The disease is primarily seed borne and pathogen become active when seed is sown. The secondary spread of the disease is found through air and soil borne conidia. The disease can be spread rapidly in the field under moist and wet environmental conditions, causing seedling wilt, stem lesions and boll rot. The infected bolls produced contaminated seeds on harvesting. Disease Management: The surface contamination of seeds can be eliminating by treating it with concentrated sulphuric acid, organomercurials, sulphur or other seed dressing fungicides. Since the pathogen can be viable for only one year on the seed surface, the sowing of two year old seeds can be useful for disease control. The eradication of collateral weed hosts from around the field can be useful method in checking the disease. The spray of 1 per cent Bordeaux mixture or other fungicide during boll formation is useful in disease check.