Purple Blotch

Purple blotch Common name Purple blotch Causal agent It is a fungal disease caused by fungi belonging to the genus Alternaria. Most long-day onion cultivars are infected, especially Sweet Spanish onions, while all short-day ones are infected. Scientific name Alternaria porri Symptoms & Signs Purple blotch disease is characterized by small, water-soaked lesions on older leaves. As the disease progresses, the lesions increase in size and change their color to brown and purple. The same pattern of infection is also seen in the onion bulbs, leading to a decrease in size and rotting. The lesions may also have a bull’s eye pattern. Infected leaves wilt and eventually girdle. Transmission The fungal spores can transfer to healthy crops by means of wind, rain, irrigation, or by contact. The infection enters the plant by means of an opening caused by injury or a previous disease. It thrives in humid weather with temperatures between 77-85°F (25-29°C). Time of concern Throughout the growing season Common hosts Onion Garlic Leeks Shallot