Utah Pests News Spring 2008

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Onion Powdery Mildew – Should We Worry?

  Figure 1.  Visible mycelium of the powdery mildew fungus on onion.

Although considered a minor disease on onions, powdery mildew is showing up in more onion fields in the western United States. The disease does not appear to be of major concern at present. In Utah, occurrences of the disease have been more noticeable and prevalent in the latter part of the season, after crops have matured. Consequently, crop yields have not been significantly affected.

While there are no recommendations for chemical control, there are reports that some onion cultivars are more susceptible than others. I am unaware of any onion breeding program actively screening for resistance to the disease; however, there are likely onion breeders doing preliminary investigations.

Onion powdery mildew is caused by the fungal pathogen, Leveillula taurica (Lev.) G. Arnaud. This pathogen has the ability to cause disease on over 1,000 different plant species including onions, eggplant, tomatoes, potatoes, peppers, and cucumbers. The pathogen was first noted in the U.S. in Florida in the mid 1970s, then later in California. It was observed in Washington in 1996 and over the years has appeared infrequently according to a publication by Dr. Lindsey du Toit in the online Plant Health Progress journal.  Du Toit’s report describes the disease on onions in a variety trial planted in Washington; however, the pathogen was also detected in Idaho at about the same time.

Powdery mildew has been reported on onions in Israel, southeastern Europe, and California. In Utah, Extension Vegetable Specialist Dr. Dan Drost identified the disease on onion grown in a Box Elder County variety trial in September 2007. 

Early infection symptoms appear as whitish lesions on leaves and later, when conditions are conducive, the fungus will produce visible mycelium (Figure 1) with prolific numbers of spores (Figure 2). Primary spores, called conidia, are pointed or lanceolate (Figure 3) and secondary spores are more cylindrical and rounded on the proximal ends (Figure 4).

Figure 2. The fungus that causes powdery mildew (Leveillula taurica) forms prolific spores on plant tissue.
Figure 3 (right). The primary spores (conidia) are lanceolate.
Figure 4 (far right). Secondary spores are more cylindrical and rounded.

The pathogen can survive from one season to another in crop residues, serving as inoculum to infect susceptible crops the following season. It can also be introduced into an area on infected planting stock. Keep your eyes peeled for this disease. How ever you slice it, my belief is that powdery mildew on onion is not worth shedding tears over just yet.

-Kent Evans, Extension Plant Pathologist (No longer at USU)