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Botrytis Leaf Blight on Onions (Dec. 15, 2010)

Botrytis leaf blight, caused by the fungus Botrytis squamosa, is a disease that can occur in Arizona and California desert onion plantings. Initial symptoms on leaves are the appearance of small white sunken spots about 1/8 inch in diameter surrounded by a light green halo ranging from 1/16 to 1/8 inch in width. The presence of this halo is diagnostic for leaf blight and can help distinguish between leaf blight lesions and necrotic spots caused by herbicides, mechanical damage, or insect feeding sites. With age, the halo may disappear and the lesions may expand in size to a width of about inch and a length of about 5/16 inch. Development of these spots requires a wet leaf surface (due to rainfall, dew, or sprinkler irrigation) for at least 20 hours. Under prolonged moist conditions at temperatures ranging from 54 to 76 F, the fungus can develop rapidly and cause blighting and eventual death of leaves. Botrytis leaf blight can cause quality loss on green onions, where foliage condition is a primary market factor. On a dry bulb crop, premature death of leaves can result in yield loss due to smaller bulb sizes. Effective management of Botrytis leaf blight by use of fungicides requires initiating applications when the disease symptoms first become visible. In growing areas with frequent periods of foliar moisture due to rainfall, dew, or sprinkler irrigations, establishing plantings in single rows spaced at least 12-inches apart, compared to multiple-row plantings, can facilitate more rapid leaf drying and thus reduce infection. Rotation with nonsusceptible crops will also prevent the buildup of the pathogen in soil.

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