Introduction

In India, cotton diseases like root rot, the wilt, bacterial blight and the anthracnose cause a heavy losses. The losses due to diseases is estimated to 10.4% of cotton lint. The storage of cotton seed is likely to get heated and deteriorate in quality by the attack of micro- organism. Under warm and humid conditions of storage, cotton fibres are liable to be attacked by fungi and bacteria.

Diseases at Different Crop Stages

Areolate or Grey mildew

Ramularia areola


Occurrence The disease is known to occur in United States, India etc., It occurs usually in low lying moist localities and minor importance though in M.P becomes serious in wet years.


Symptoms
• It usually appears on plants, which are coming to maturity.
• Irregular, angular, pale translucent spots measuring 1-10mm (3-4mm) in diameter and bordered by the veinlets, appear on the older leaves.
• As infection starts, leaves become yellowish brown end finally fall off prematurely.

Disease Cycle
• The disease cycle spreads mainly through air - borne conidia.
• The method of perpetuation during the off - season is unknown.

Epidemiology
• It mainly develops in the moist localities and also low-lying wet soils. When the temperature is 25-30°C the conidial germinate.

Control Measures
• Destroy the infected-plant debris.
• Avoid continous cultivation of cotton.
• Spraying wettable sulphur 3g or Karathane 1g or benomyl 1g per litre of water controls the disease. Spray 2-3 times at weekly interval till it is controlled.

Wilt

Fusarium vasinfectum


Occurrence Originated from Mexico - Central America and spread to other growing areas through the infected seeds.

• Prevalent in black soils of Madhya Pradesh, Karnataka, South Gujarat, Uttar Pradesh and the Punjab.
• Wilt pathogen is rapidly destroyed in alluvial soils during the hot months preceding the monsoons.
• The disease is found in the areas where soil temperature during the growing period of cotton ranges from 20° - 30°C. This disease occurs on all seasons.

Symptoms
• Affects the host at all the stages of growth.
• Earliest symptoms to appear on the seedling is the yellowing and browning of the cotyledons.
• In young and grown up plants the leaves loose their turgidity first turn yellow and then brown and finally drop off.
• The tap root of a wilted plant is usually stunted and laterals are less abundant.
• Browning and blackening of vascular tissues. (Outstanding diagnostic symptoms)
• Discoloration of woody tissues can be traced upon the surface of the host as a black streaks.
• Discolouration of leaves starts from the margins and spread towards midribs.
• Wilting may be complete or partial.

Epidemiology
• Soil temperature and soil moisture greatly influence the incidence of the disease.
• The temperatures between 24-28°C are optimum for attack on early stages of crop.

Disease Cycle
• The fungus enters the young host roots through the incidental wounds and openings caused by insects and eel worms.
• The fungus moves up through xylem vessels by the growth of the mycelium and movement of spores.
• The mycelium partially or completely plugs the xylem vessels and stops or lowers the upward flow of water along the dissolved salts.
• If the bolls are present, the mycelium may grow through peduncle into the seed.
• This results stunting and wilting of the plant.
• Fungus survive in soil as a saprophytic plant.

Control Measures



Grow resistant varieties. Provide good drainage. Follow crop rotation 3-4 years once. Treat the seeds with 4g Trichoderma viride formulation + thiram 3g/kg seed. Destroy the infected - plant debris. Apply 2kg of Trichoderma viride commercial formulation with 50kg farmyard manure along with the rows in the mainfield reduces the disease incidence.


Cultural Methods
• Mixed cropping of cotton with immune offers some protection to the crop against the wilt.
• Potash reduces wilt while nitrogen and Phosphorus increase it in certain limits.

Root Rot

Rhizoctonia bataticola



Occurrence The disease occurs in irrigated and rainfed areas of Gujarat, Punjab, Madhya Pradesh, Telangana and Andhra Pradesh.

• High soil temperature favours the development of the disease. June, July are months in which diseases attacks severely.
• This disease occurs on all seasons.

Symptoms



The sudden and complete wilting of the plant (first and the most prominent symptoms). Bronzing or yellowing of the leaves. Plants succumbed to this disease exhibit roots particularly the bark, broken down into shreds. Affected plants occur in more or less circular area and can be easily pulled out. Except the tap root, few secondary roots are fresh which holds the plant and other roots are decayed. Tip of the root of a freshly wilted plant is slightly moist and sticky.


Epidemiology
• Plants suffer high mortality at soil temperatures > 39°C.
• Adequate supply of water, favours the development of disease when the plant is in early growth.

Disease Cycle
• The plants are vulnerable to infection the fungi dominate inside the root.
• The hyphae multiply rapidly in the cortical tissues and extend to the pith and sometimes enter the xylem vessels.
• There is an increased accumulation of calcium and iron in a diseased plant roots and certain toxic substance play a role to wilt.
• Fungus remain saprophytically on host root in soil.

Control Measures



Inter cropping of cotton with moth bean (vigna acomitifolia) reduce the incidence. (moth bean helps in reducing the temperature and retention of high soil moisture due to shading effect).


Chemical Control



Adjustment of time of sowing when soil temperature is not high.

• Mixed cropping with Moth bean (Vigna aconitifolia) check the disease intensity.
• Seed treatment with benomyl or carbendazim @2.5g/kg seed reduces disease incidence.
• Apply 2kg of Trichoderma viride commercial formulation with 50kg farmyard manure along with the rows in the main field reduces the disease incidence.

Root Knot Nematode

Occurence

• It is serious disease of the cotton crop in United States.
• In India it is reported from Punjab state.

Symptoms
• Lack of vigour is the first sign of the disease though ordinarily the affected plants appear to be normal.
• They become stunted and lose the chlorophyll.
• Diseased plants show knots or galls on the tap and lateral roots.
• Their tissue is soft and usually decay off.



Control Measures

Rotation with resistant crops like Sorghum, Grasses, small grains, Corn, Peanuts. Nematicides should be used.

Bacterial Blight

Xanthomonas malvacearum


Occurrence

• This disease is originated in India.
• The disease has been found in Bombay, M.P, Telangana, Andhra and Maharashtra.
• It reduces the photosynthetic activity of the plant.
• The lint of the bolls may also get stained there by lowering its market value.
• It occurs in December and January.

Symptoms
• The disease usually affects parenchymatous tissues and vascular bundles.
• Four common symptoms are developed.
• Angular leaf spots on the leaves (Water soaked leisions that are angular in outlines)
• Black arm lesions on the stem.
• Boll rot and gummosis (Boll blight) round leisions, water soaked.
• Seedling infections.
• Leaf lesions appear as minute, water soaked spots on the under surface of young leaves.
• The disease on the stem and petioles appear in form of elongated greyish to sooty black lesions which under favourable environmental conditions.
• The disease appears first as small, round, water - soaked, raised spots. The lesions gradually became irregular in shape, turn brown to deep black in colour and become in the center.
• The disease first appears on the margins of the ventral side of the cotyledons as small water soaked, circular or irregular but not angular spots.

Epidemiology
• The primary infection depends upon soil temperature and moisture.
• The secondary spread of diseases should have the presence of moisture.

Disease Cycle
• The primary infection is mainly through seed - borne bacteria.
• Bacteria remain on the fuz of seed coat and multiplies after sowing and infects the seedlings.
• In case of seeds, bacteria may get inside the seed coat through the micropyl to infect the seedlings.
• The secondary, bacteria entering the host through natural openings or insect caused wounds.
• As the weather conditions favours, it multiplies and affect the growth of the plant.

Control Measures


Sanitation

• Removal and destruction of volunteer plants from the fields are very essential, as they are the chief source of infection.

Cultural method



	Crop rotation, late sowing, early thinning, good tillage, early irrigation and addition of potash to the soil reduce the disease.
	The seed is immersed in sulphuric acid and should be arranged in wooden tubs or in earthen jars.

• Seed treatment with 2gms of carbendizam per kg of seed.
• As it occurs in months of September and November, 100 mg of tetracycline, streptomycin sulphate, plantamycin, poshamycin; should be sprayed for 3 or 4 times in every 15 days gap.
• Chemicals linked with copper should be sprayed at their recommendation of 3gm per litre of water to reduce the diseases.
• Destroy the infected-plant debris.
• Crop rotation, late sowing, early thinning, good tillage, early irrigation and addition of potash to the soil minimise the disease.
• Use healthy and resistant seed.
• Acid delinting followed by seed dressing with carbendazim @3-4g/kg of seed eliminates seed borne inoculum.
• Foliar spray of combination of Pausha mycin/Plantomycin 100mg+3gms of Copper Oxychloride per lit of water for 3 or 4 rounds at 15 days interval from the time of disease appearance.