|C. circinans spores|
|Colletotrichum dematium f.sp. circinans (Berk.) Arx 1957
Vermicularia circinans Berk. 1851
Onion smudge (Colletotrichum circinans) occurs in many parts of Europe and the United States. It is generally of minor importance. Smudge occurs on the common field onion, onion sets, shallots, and leeks. It has not been observed on garlic.
Onion smudge may appear at all growth stages but it is more common on bulbs approaching maturity and during storage. The disease affects the scales and lower portions of the unthickened leaves which constitute the neck of the bulb that appears as minute, sunken, yellowish spots that can enlarge and join together. Infected leaves have oval, greenish or yellowish-gray spots with yellow halos that turn dark as they age.
No satisfactory controls are available for white onions; therefore, coloured varieties should be used where smudge is a persistent problem. At least two substances responsible for colour in the outer scales are toxic to the fungus. These are protocatechuic acid and catechol. Crop rotation, use of seed or healthy sets, proper drainage and a good foliage fungicide program helps. The crop should be harvested promptly; exposure to rain should be avoided between harvest time and storage.