The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona Cooperative Extension in Yuma, Ariz.

Botrytis leaf blight on onions

By Mike Matheron, UA Extension Plant Pathologist

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 1/8th inch in diameter surrounded by a light green halo from 1/16 to 1/8 inch wide. 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 ¼ 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 degrees F., the fungus can develop rapidly and cause blighting and eventual leaf death. Botrytis leaf blight can cause quality loss on green onions where foliage condition is a primary market factor. On a dry bulb crop, premature leaf death can result in yield loss due to smaller bulb sizes.

Effective management of Botrytis leaf blight with 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 reduce infection. A rotation with non-susceptible crops will also prevent the buildup of the pathogen in the soil.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.