Know your onion diseases and how to manage them

Recent weather conditions favor anthracnose and purple blotch. Know how to identify and manage them.

Anthracnose caused by the fungal pathogen Colletotrichum has been detected in an onion field last week and confirmed June 20. This new disease problem was found in several onion fields in 2010 and was responsible for severe leaf blighting in some fields. The lesions are bleached in color and may be seen along the length of the leaf, including the plant tissue near the bulb. While anthracnose may be confused with purple blotch, a primary difference between the two is the color of the lesions. The anthracnose lesion may appear lighter in color than purple blotch. Last year it was noted that in some cases both the anthracnose and purple blotch pathogens could be found in the same lesion.

The warmer and wet conditions in recent days favor the development of anthracnose and other commonly-occurring leaf blights such as purple blotch. If fungicide sprays have not been started, it may be prudent to do so soon. Recommended products for anthracnose and purple blotch include those with mancozeb, such as Dithane, Manzate Pro-Stick and Penncozeb, chlorothalonil, such as Bravo, Equus, and Ech, and the strobilurin fungicides, such as Quadris and Cabrio. In a field study that was conducted last year for anthracnose, neither Rovral nor Tanos limited anthracnose compared to the untreated control.

For more information on onion diseases, see Dr. Hausbeck’s proceedings from the 2010 Great Lakes Fruit, Vegetable and Farm Market Expo, or go to her Onion Research Page.

Anthracnose lesion
Anthracnose lesion caused by Colletotrichum, confirmed June 20. Photo credit: M. Hausbeck, MSU.

Anthracnose on bulb
Anthracnose disease became severe in some fields in 2010. Symptoms included bleached lesions near the base of the bulbs. Photo credit: M. Hausbeck, MSU.

Purple blotch
Purple blotch lesions tend to be darker in color. Photo credit: M. Hausbeck, MSU.

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