Blackarm or Bacterial blight of Cotton

   Among the plant diseases caused by bacteria, blackarm of cotton is a severe one. It was first described in 1891 by Atkinson from the United States.The disease has been reported from South America, Egypt, Sudan and other African countries, U.S.S.R., China, Australia, Sri Lanka, Pakistan and India. Cotton yield losses due to disease of 10% have been reported.

   Etiology

  Causal organism: Xanthomonas campestris pv. Malvacearum

            The bacterium is a short motile rod, formed singly or in pairs and equipped with a single polar flagellum. It is gram negative, non acid fast   non spore forming and measures 1-1.2 x 0.7 x 0.9µ. in culture, it produces a yellow, slimy colony on nutrient agar medium and coagulates an         peptonises milk.

   Disease cycle

            The primary source of infection originates from the seed coat and fuzz. The secondary spread is favoured by wind splashed rain and dew.

   Symptoms

• Angular leaf spots on the leaves
• Venial Blight
• Black arm lesions on the stem.
•   Boll rot and gummosis (Boll blight) 
• Spots on cotyledons are circular, dark green and greasy
• Leaf lesions - minute, water soaked spots on the under surface of young leaves.

Water soaked lesions on leaves                           Vienal blight                              Symptoms on boll

    Epidemiology

            Optimum soil temperature of 28^OC, high atmospheric temperature of 30-40^OC, 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.