Angular leaf spot/ bacterial blight/boll rot/black arm of cotton

C/O: xanthomonas campestris pv malvacearum

Symptoms:

Attack all stage of plant.

Seedling blight:

• Small, water-soaked, circular or irregular lesions develop on the cotyledons.
• Later, the infection spreads to stem through petiole and cause withering and death of seedlings.

Angular leaf spot

• Small dark green water soaked area developed on the lower surface of leaves.
• Angular spot are restricted by vein and veinlet and spot are visible on both surface of leaf.
• At lesion are older, they turn to radish brown color.

Vein blight or vein necrosis or black vein:

• The infection of veins causes blackening of the veins and veinlets, gives a typical ‘blighting’ appearance. On the lower surface of the leaf, bacterial oozes are formed as crusts or scales.
• The affected leaves become crinkled and twisted inward and show withering.

Black arm:

• On the stem and fruiting branches, dark brown to black lesions are formed, which may girdle the stem and branches to cause premature drooping off of the leaves, cracking of stem and gummosis, resulting in breaking of the stem which hang typically as dry black twig to give a characteristic “black arm” symptom.

Square rot / Boll rot:

• On the bolls, water soaked lesions appear and turn into dark black and sunken irregular spots.
• The infections on mature bolls lead to premature bursting of bolls.
• The bacterium spreads inside the boll and lint gets stained yellow because of bacterial ooze.

Etiology:

• Short rod with a single polar flagellum.
• It is gram negative, non-spore forming.
• The bacterium is aerobic, capsule forming and produces yellow colonies in culture medium.

Favorable Conditions:

• Optimum soil temperature of 28 ⁰C,
• high atmospheric temperature of 30-40 ⁰C
• relative humidity of 85 per cent
• early sowing, delayed thinning, poor tillage
• late irrigation and potassium deficiency in soil.
• Rain followed by bright sunshine during the months of October and November are highly favourable.

Disease cycle

• Seed borne and soil disease
• Bacteria rest on seed coat of cotton and infected plant parts and soil.
• Primary source of inoculum: infected seed, volunteer plant, self-sown cotton
• Transmission: wind, wind blown rain splash, irrigation water, insects and other implements
• collateral hosts: Thurbaria thespesioides, Eriodendron anfructuosum and Jatropha curcas.
• bacterium enters through natural openings or insect caused wounds.

Management:

• Early thinning, good tillage, early irrigation, early earthing up and addition of potash to the soil reduces disease incidence. • Grow resistant varieties like HG-9, BJA 592,
• Grow resistant varieties like HG-9, BJA 592, G-27, Sujatha, 1412 and CRH 71. Suvin is tolerant.
• Gossypium herbaceum and G. arboreum are almost immune. G. barbadense, G. hirsutum, G. herbaceum var typicum and G. herbaceum var acerifolium have considerable resistance.
• Seed Treatment: Delint the cotton seeds with concentrated sulphuric acid at 125ml/kg of seed.
• Treat the delinted seeds with Carboxin at 2 g/kg seed or soak the seeds in 1000 ppm Streptomycin sulphate overnight or treat the seed with hot water at 52-560C for 10-15 minutes.
• Spray with Streptomycin sulphate (Agrimycin 100), 500 ppm along with Copper oxychloride at 0.3%.