Here is a list of ten major diseases of cotton: 1. Angular Leaf Spot or Black Arm Disease 2. Vascular Wilt Disease 3. Grey Mildew or Dahiya Disease 4. Anthracnose Disease 5. Root Rot Disease 6. Boll Rot Disease 7. Leaf Spot or Blight Disease 8. Reddening or Lalya Disease 9. 2, 4-D Injury Disease 10. Tobaaco Streak Virus Disease.
Diseases of Cotton:
- Angular Leaf Spot or Black Arm Disease
- Vascular Wilt Disease
- Grey Mildew or Dahiya Disease
- Anthracnose Disease
- Root Rot Disease
- Boll Rot Disease
- Leaf Spot or Blight Disease
- Reddening or Lalya Disease
- 2, 4-D Injury Disease
- Tobaaco Streak Virus Disease
1. Angular Leaf Spot or Black Arm Disease:
Causal Organism:
Xanthomonas axonopodis pv. malvacearum (Smith) Vauterin.
Symptoms:
Small water-soaked spots appear on the under surface of cotyledons, which may dry and wither. Such spots also appear on the leaves. They become angular bound by veinlets and turn brown to black in colour. Several small spots may coalesce. The infected petiole may collapse. Elongated, sunken and dark brown to black lesions appear on stem, petioles and branches.
The young stems may be girdled and killed in the black arm phase. Sunken black lesions may be seen on the bolls. Young boll may fall-off. The attacked stem becomes weak. Bacterial slime is exuded on the brown lesions. Discolouration of lint may take place.
Etiology:
The bacterium is rod-shaped. It occurs singly or in pairs, is capsulated but forms no spores, and is motile by one polar flagellum. Stain reaction is gram negative. The bacterium is aerobic.
Perpetuation:
The pathogen can remain as slimy mass inside the seed or on the fuzz. The disease may be carried over through infected leaves, bolls and twigs on the soil surface. The secondary infection is through water, wind.
Management:
1. Field sanitation.
2. (a) Externally seed borne infection can be eradicated by delinting the seed with Cone H2SO4 for 5 minutes, wash with lime solution to neutralise the effect and finally washing with running water to remove the residue and drying seeds.
(b) Internally seed borne infection can be eradicated by soaking seeds overnight in 100 ppm streptomycin sulphate or Agrimycin.
3. Secondary spread of the disease can be controlled by spraying the crop with streptomycin sulphate 100 ppm + Copper oxychloride (0.25%) at an interval of 15 days.
2. Vascular Wilt Disease:
Causal Organism:
Fusarium oxysporum f. sp. vasinfectum (Atk.) Snyder and Hansen.
Symptoms:
Wilt is restricted to black cotton soils with pH 7.6-8.00. It is rare in light to loam soils. The disease appears at all the stages of plant growth. In seedling stage, there is yellowing of cotyledons, browning of petioles, followed by death and falling of affected leaves. In young and adult plants, there is loss of turgidity, drooping of leaves and tender shoots, yellowing, browning and finally death of the plants.
Etiology:
Mycelium is septate, hyaline and intravascular. The fungus is facultative parasite and produces three types of spores, Micro-conidia, which are one or bicelled, oblong, hyaline and borne on short conidiophores, Macro-conidia are 3 to 6 celled, chlamydospores, which are hyaline, spherical and thick walled.
Perpetuation:
Primary infection takes place through soil borne inoculums.
Management:
1. Field sanitation, crop rotation and mix cropping are useful for reducing the incidence.
2. Use of resistant varieties. G. arboreum and G. herbaceum are susceptible whereas G. hirsutum and G. barbedanse are immune.
3. American varieties are resistant to wilt in India.
3. Grey Mildew or Dahiya Disease:
Causal Organism:
Imperfect stage. Ramularia areola Atk.
Perfect stage. Mycosphaerella areola.
Symptoms:
The fungus usually attacks the older leaves causing irregular to angular, pale, translucent spots. They are usually restricted by the vein lets and appear mostly on the lower surface of the leaf though occasionally on the upper surface. A few to over a hundred spots may be found on a single leaf. In severe infections the leaves turn yellowish brown and fall off prematurely.
Etiology:
A frosty or whitish grey mildew growth consists of conidia and conidiophores and mycelium of the fungus. The mycelium is endophytic, septate. The conidiophores are short, septate, branched at the base. The conidia are borne singly or in short chains at the tip of the conidiophores and are colourless, irregularly oblong, with pointed, rounded, or flattened end, unicellular to three septate.
Management:
1. Destruction of infected plant debris.
2. Dust the crop with 300 mesh sulphur at the rate 20 kg/ha or spray crop with 0.05% propiconazole.
4. Anthracnose Disease:
Causal Organism:
Imperfect Stage:
Colletotrichum gossypii Southw.
Colletotrichum capsici (Syd.) Butl & Bisby.
Perfect Stage:
Glomerella gossypii (Southw) Edgerton
Symptoms:
In seedling stage, small, reddish circular spots appear on the cotyledons and primary leaves. When the lesions are on the collar region, the stem may be girdled, causing seedlings to wilt and die. In mature plants, the fungus attacks the stem, causing it to split and shred the bark.
The symptoms are prominent on bolls as water-soaked, circular, slightly sunken reddish-brown spots which turn black in colour. As a result of boll infection, they open prematurely. The lint becomes stained, hard and compact.
Etiology:
The mycelium is septate. The fungus produces acervuli in which the setae are present. Conidia are hyaline, single-celled, sickle shaped.
Perpetuation:
Primary infection takes place through infected seeds and plant-debris, secondary infection by means of conidia disseminated by wind.
Management:
1. Seed treatment with Thiram 3 g or Carbendazimm 1 g + Thiram 3 g/kg of seed.
2. Spray the crop once or twice with copper oxychloride (0.25%) or Zineb (0.25%) after boll formation.
5. Root Rot Disease:
Causal Organism:
Sterile Stage:
Rhizoctonia bataticola (Taub.) Butl.
Imperfect Stage:
Macrophomina phaseolina (Tassi) Goid.
Symptoms:
The symptoms may appear in seedling stage, where brownish spots on the cotyledons are seen. At collar region there is browning, which may extend downward. The fibrous roots undergo decaying. The bark of the roots show rotting and shredding. Affected plants can be pulled out with ease. The disease appears in patches in the field.
Etiology:
The hyphae are thick, septate and produce black and irregular sclerotial bodies. Crop residue with sclerotia act at primary source of infection.
Management:
1. Seed treatment with Thiram or Captan at the rate 3 g +1 g Carbendazim/ kg seed.
2. Soil mulching after rains.
3. Mixed cropping with legumes and sorghum.
4. Soil disinfection with 0.1% Carbendazim helps in controlling the disease.
6. Boll Rot Disease:
It is a complex disease caused by several fungal pathogens viz., Fusarium moniliforme, Colletotrichum capsici, Aspergillus flavus, A. niger, Rhizopus nigricans, Nematospora nagpuri and Botryodiplodia sp.
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 rotting of seeds and lint occur. The bolls never burst open and fall off prematurely.
In some cases, the rotting may be external, causing rotting of the pericarp leaving the internal tissues free. On the affected bolls, a large number of fruiting bodies of fungi are observed depending upon the nature of the fungi involved.
Predisposition:
Heavy rainfall during the square and boll formation stage, punctures caused by the insects, especially red cotton bug Dysdercus cingulatus, close spacing and excessive nitrogen application.
Etiology:
Many kinds of fungi are involved. The fungi survive in the infected bolls in the soil. The insects mainly help in the spread of the disease. The fungi make their entry only through the insect punctures. The secondary spread of the disease is also through airborne conidia.
Management:
Adopt optimum spacing. Apply the recommended doses of fertilizers. Spray Fenvalerate 75 g a.i./ha + Copper oxychloride 0.2% or Carbendazim 0.1% or Mancozeb ) 0.2% from 45th day at 15 days interval. Two or three sprays are necessary.
7. Leaf Spot or Blight Disease:
Causal Organism:
Alternaria macrospora Zimm.
Symptoms:
The disease may occur in all stages but more severe when plants are 45-60 days old. Small, pale to brown, irregular or round spots, measuring 0.5 to 6 mm diameter, may appear on the leaves. Each spot has a central lesion surrounded by concentric rings.
Several spots coalesce together to form blighted areas. The affected leaves become brittle and fall off. Sometimes stem lesions are also seen. In severe cases, the spots may appear on bracts and bolls.
Etiology:
The fungus produces dark brown, short, 1-8 septate, irregularly bend conidiophores with a single conidium at the apex. The conidia are obclavate, light to dark brown in colour with 3-9 transverse septa and four longitudinal septa, with a prominent beak.
The pathogen survives in the dead leaves as dormant mycelium. The pathogen primarily spreads through irrigation water. The secondary spread is mainly by air-borne conidia.
Predisposition:
High humidity, intermittent rains and moderate temperature of 25-28°C.
Management:
Remove and destroy the infected plant residues. Spray 0.2% Mancozeb or Copper oxychloride at the initiation of the disease. Four to five sprays may be given at 15 days interval.
8. Reddening or Lalya Disease:
American and Egyptian cottons are susceptible to this disease. Severe attack results in low yield and poor quality of crop. Butler (1908) reported for the first time the occurrence of this disease in India.
Red leaf is caused due to soil being puddled or ill drained or bad soil condition. High humidity and cloudy days accompanied by low temperature have been reported to increase the incidence of disease. It is also reported due to soil deficient in magnesium.
Symptoms:
Leaves of the affected cotton plants turn yellow or red during all the stages of growth. The discolouration starts at the margins and extend inwards. The red colour may also develop as patches in interveinal portion of the leaves. In severe cases, the whole leaf is involved. The affected leaves roll down wards, begin to dry and ultimately shed. The affected plant exhibits early fruiting and excessive shedding of the bolls.
Management:
1. Application of F.Y.M. or compost or nitrogenous manures which may help to improve the physical condition of soil ultimately minimising the incidence.
2. Spray 2% diammonium phosphate or 1% urea. Avoid hot winds by providing wind breaks like shevari.
3. The chlorophyll in the leaf cell decomposes and probably results in the formation of anthocyanin pigment as soon as temperature falls below 34°C.
9. 2, 4-D Injury Disease:
2, 4-D is a broad spectrum selective post emergence herbicide used for control of herbaceous annual dicot weeds. But crops like cotton, tomato, tobacco are extremely sensitive to 2, 4-D which can cause harmful damage to crops (cotton).
Symptoms:
Leaves are greatly modified. They become narrow and deeply lobed.
1. Scorching, rolling and puckering of leaves which hamper normal photosynthesis in plants.As a result of 2, 4-D injury there can be elongation of leaves giving string like slender appearance.
2. Disruption of phloem tissues and dislocation of normal translocation of food material-so plants looks like wilted.
3. Alternation of nucleic acid (protein metabolism)—Growth ceases
4. Blocking of gaseous exhange between leaf and atmosphere so ultimately plant die.
5. Malformation is also observed in crop.
6. If 2, 4-D concentration is high then the symptoms are seen within 2-3 days.
7. If 2, 4-D concentration is low then the symptoms are seen within 8-10 days.
Management:
Urea (2%) spray may show little recovery if the damage is in early stage.
10. Tobaaco Streak Virus Disease:
Cotton is an important fiber crop. Cotton streak disease was newly recognized in India, identified to be caused by the Tobacco streak virus (TSV). The virus was recognized on 26 coded Bt entries at Cotton Research Station, Mehboob Bagh, Parbhani. Incidence of TSV was noted for the first time in Maharashtra.
Disease observations were recorded from July-06 to November-06 at an interval of 15 days. Data revealed that viral disease incidence was recorded at increasing rate up to the end of September 06 and further incidence was noted in decline rate from October to November 2006.
Maximum disease incidence was recorded in the month of September observation in the range of 36.59 to 83.88%. Lowest incidence was noticed on entry 6108 and highest on entry 6126.Reddy (2002) first reported occurrence of Tobacco streak virus on peanut in India.
Per cent incidence of cotton streak virus on different Bt cotton hybrids under natural condition at Cotton Research Station, Mehboob Bagh, Parbhani were recorded during the year 2006-2007. Gross plot size 5.4 x 6.0 (six rows with 10 dibbles) and Net plot size 3.6 x 4.8 (Four rows with 8 dibbles) were used. 90 x 60 cm spacing was followed. Fertilizer dose 100: 50: 50 NPK was applied. Date of sowing was 1.07.2006.
Symptomatology:
A serious disease with putative virus etiology was found to occur in the transgenic cotton growing region of Southern Maharashtra. Symptoms comprised of chlorotic and necrotic spots and leaf distortion. A generalized leaf and stem necrosis extending to mid- veins and petioles resulting in stunting was observed in early stage of crop development.
The virus was identified as Tobacco Streak Virus (liar virus) based on the reaction to virus specific antibody in ELISA. This virus has also been recorded to infect other hosts like sunflower, groundnut and soybean and was found to be transmitted by thrips. Incidently thrips population was unusually high in virus infected fields. Virus is also reported to be transmitted by other arthropods and insects of the order Thysanoptera.
Detection:
The virus was idendified as Tobacco Streak Virus (liar virus) based on the reaction to virus specific antibody in ELISA. This virus has also been recorded to infect other hosts like sunflower, groundnut and soyabean and was found to be transmitted by thrips.
Incidently thrips population was unusally high in virus infected fields. Virus is also reported to be transmitted by other arthropods and insects of the order Thysanoptera.
Incidence of Cotton Streak Virus:
Observations on cotton streak virus, a new viral disease identified as tobacco streak viral disease of liar group were recorded on 26 coded Bt entries at CRS, Nanded and data are presented in table 7.1.
Disease observations were recorded from July 06 to November 06 at an interval of 15 days. Data revealed that viral disease incidence was recorded at increasing rate up to the end of September 06 and further incidence was noted in decline rate from October to November 2007.
Maximum disease incidence was recorded in 5th (30.09.06) observation in the range of 36.59 to 83.88%, lowest incidence was noticed on entry 6108 and highest on entry 6126.