Brown patches develop mainly on the leaf tips (haulms) and around the leaf edges. These haulms of infected plants develop brownish-black patches which may collapse very readily. In wet and humid weather white fluffy fungal growths maybe visible around the edges of these patches . This is more common on the lower leaf surfaces as these fungal spores are spread by rain splash. Infected tubers develop slightly sunken, dark patches on the skin often with reddish-brown discolouration which may spread into the flesh. Blighted potatoes should not be stored or used as seed potatoes the following year. If badly infected the tubers will not be edible.
Blight affects potatoes but also affects tomatoes too. The blight season for potatoes begins around late May to mid June when the levels of air humidity and temperature are suitable for its development and spread. During wet summers the incidence of blight can be worse and prolonged.