Onion Smudge: Concentric rings on infected bulb

Pathogen: Colletotrichcum circinans

Symptoms

On the outer bulb scales, over the surface of the bulb, masses of minute stromata form beneath the cuticle. These bodies, dark green at first, blackening with age, are sometimes scattered over the surface of the bulb but are more frequently concentrated in smudge-like spots, roughly circular, or arranged in concentric rings 1-2 cm diameter or more.

Disease Development

Disease spread is most rapid during moderate temperatures with high humidity, rainfall or overhead irrigation. Infection can be more severe if necks are still succulent when stored. Storing uncured onions at temperatures and humidity which are too high can promote disease development and spread.

Disease Management

Thorough curing of the bulb after harvesting and storing the bulbs in well ventilated rooms can control the disease. The disease can be managed by thorough curing of bulbs after harvesting and storing them in well-ventilated rooms.