Grey Mildew of Cotton
Symptoms usually appear towards the end of the growing season. Young leaves show circular, water-soaked, pale green spots. Eventually they become chlorotic and wither with the spots turning red to brown. Halos are rarely present. On older leaves, small, light green to yellowish, angular spots, that are limited by the veins, appear on the upper side. During periods of high humidity, the underside is covered with silvery white fungal growth. Severely affected leaves become necrotic, curly and dry, show a red-brown color and drop prematurely. The defoliation weakens the plant and its productivity. Harvesting losses increase as infected bolls lose strength, open prematurely or break during pulling and threshing at harvest.
The symptoms are caused by the fungus Mycosphaerella areola, which survives as spores or fungal hairs (mycelium) in infected plant tissue. The main source of inoculum are actually the residues of previous seasons. Temperatures between 20-30 °C, high nocturnal humidity and low diurnal humidity long periods of time promote the infection and the progression of the disease. The spores are produced in leaf lesions and subsequently become airborne, resulting in secondary infections. The disease is most severe under wet, humid conditions.
Seed treatment with Pseudomonas fluorescens (10g/kg seeds) and spraying of a solution of 0.2% every 10 days reduces the infection significantly. The antifungal bacteria Bacillus circulans and Serratia marcescens have been used to control other species of Mycosphaerella and reduce the incidence of related diseases in other crops.
Always consider an integrated approach with preventive measures together with biological treatments if available. Apply wettable sulfur in the initial stage of the disease. In later stages, sulfur powder is recommended. Fungicides containing carbendazim or benomyl can also be applied.