Etiology and control of the following Cotton diseases: Angular leaf spot
Angular Leaf Spot of Cotton:- Also called as Bacterial Blight of Cotton.
Pathogen:- Xanthomonas campestris p.v malvacearum
Symptoms:- The bacterium attacks all stages from seed to harvest. Five common phases of symptoms:
1. Seedling blight:-
> Small, water-soaked, circular or irregular lesions on the cotyledons.
> Infection spreads to stem through petiole and cause withering and death of seedlings.
2. Angular Leaf Spot:-
> Small, dark green, water soaked areas develop on lower surface of leaves.
> The spots become angular restricted by veins and veinlets and are visible on both the surface of leaves.
> Later they turn reddish brown colour and infection spreads to veins and veinlets.
3.Vein necrosis or Vein banding:-
> 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 leaves become crinkled and twisted inward and show withering.
> The infection also spreads from veins to petiole and cause blighting leading to defoliation.
4. Blackarm:-
> On the stem and fruiting branches, dark brown to black lesions are formed.
> Girdle the stem and branches to cause premature drooping off of the leaves.
> Cracking of stem and gummosis, resulting in breaking of the stem and hang typically as dry black twig to give a characteristic “black arm” symptom.
5. Square rot or boll rot:-
> On the bolls, water soaked lesions appear and turn into dark black and sunken irregular spots.
> Infection slowly spreads to entire boll and shedding occurs.
> Infection on mature bolls lead to premature bursting.
> The bacterium spreads inside the boll and lint gets stained yellow because of bacterial ooze and loses its appearance and market value.
> The pathogen also infects the seed and causes reduction in size and viability of the seeds.
Survival and Mode of Spread:-
> The bacterium survives on infected, dried plant debris in soil for several years. The bacterium is also seed-borne and remains in the form of slimy mass on the fuzz of seed coat.
> The primary infection is through seed-borne bacterium.
> Secondary spread is through wind, windblown rain splash, irrigation water, insects and other implements.
Favourable Conditions:-
> Optimum soil temperature of 28˚C.
> High atmospheric temperature of 30-40˚C.
> Relative humidity of 85 percent.
> Poor tillage, late irrigation.
> Potassium deficiency in soil.
> Rain followed by bright sunshine during the months of October and November.
Management:-
> Delint the cotton seeds with concentrated sulphuric acid at 100ml/kg of seed.
> Treat the delinted seeds with Carboxin or Oxycarboxin at 2 g/kg or Carboxin 37.5% + Thiram 37.5% WS @2.5 g/ kg.
> Remove and destroy the infected plant debris.
> Rogue out the volunteer cotton plants and weed hosts.
> Follow crop rotation with non-host crops.
> Early thinning and early earthing up with potash.
> Spray Streptomycin sulphate @ 100g +Copper oxychloride@500 g/acre.