Alternaria leaf spot of Cotton

Alternaria leaf spot is one of major foliar disease. This disease occurs in almost all the cotton growing countries of the world. Hybrids are more susceptible to this disease.  Disease infect on leaves resulting in suppression of plant growth and reduction of yield. High severity of the infection causes strong defoliation of cotton, sharp decrease of yield and crude fiber quality.

Etiology 

                 Causal organisum : Alternaria macrospora

            Conidia are light-brown, 22-27 x 9-11 µ. They affect cotton cotyledons in seedlings, also bolls and their fiber. Mycelium of A. macrospora is dark-brown. Conidiophores are light brown, single or in groups. Conidia are red-brown, 90-180 x 15-22µ in size. 

Disease cycle

            The undecomposed crop residues and infected seeds provide the primary source of inoculum, giving rise to infected cotyledons, which support the early stages of an epidemic. Primary infection of lower canopy leaves can be initiated from conidia splashed up from infected crop residues or blown into the crop from other foci of infection. Alternaria spp., also attacks the bolls and grow on exposed lint if bolls open in wet weather, giving rise to contaminated seed. The disease cycle is completed when infected leaves fall to the ground.

Symptoms

·          Small, pale to brown, round or irregular spots

·         Leaves become dry and fall off.

·         Cause cankers on the stem.

·         Infection spreads to the bolls and finally falls off.

      Brownish spots on leaves               Symptoms on boll               Severely infected field with Alternaria leaf spot 

            Epidemiology

            Favourable condition for pathogy was high humidity, intermittent rains and moderate temperature of 25-28^OC. 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.