Downy mildew detected on onions so take action now

Preventive action is needed now to preserve your onions from downy mildew.

The wet, weather conditions of this spring have prompted the development of onion downy mildew which was confirmed yesterday (July 14) in Michigan. This is NOT the same downy mildew that was confirmed on cucumbers last week. Downy mildew on onion is caused by the fungusPeronospora destructor and is not present every year. In many years, conditions are just too warm for downy mildew to develop. Spores are easily blown long distances in moist air and can germinate and begin to grow on onion tissue within 1.5 to 7 hours, even when temperatures are 50 to 54°F.

This mildew can reproduce in 11 to 15 days, providing spores for infection of nearby onion plants. The spores are short-lived and very sensitive to drying, so in dry weather, most conidia die without causing new infections. This fungus is favored by relatively cool weather, and can form spores at night temperatures from 35 to 75°F. In order to form its spores, it must have fairly high humidity for eight to nine hours during two consecutive nights (at least 60 to 90 percent, depending on how high the temperature), or it must have very high humidity (95 percent or higher), during at least eight hours of the previous night. The higher the night temperatures, the higher the humidity must be to support sporulation.

This mildew sends its spore-bearing stalks out through the leaf stomates (the breathing pores on the leaf surface). The leaf appears green and normal except for a velvet-like growth that appears purplish gray (see Photo 1). The mildew only forms one crop of spores on any given piece of leaf tissue, and once the spores have been formed and released, the tissue collapses and dies. This is what causes the blasted appearance in a mildew-infected field.

Spores of downy mildew

Photo 1. Spores of downy mildew on an onion leaf.

The mildew typically begins in one area of a field and can spread to surrounding plants. If the weather turns dry after a disease outbreak, plants can produce new leaves and may recover somewhat. However, if the humidity returns, the fungus can revive and the new growth becomes diseased. Premature death of infected leaves reduces the bulb size. Problems can occur on the bulbs due to green and succulent necks which can be a target for fungal and bacterial pathogens in storage. Overwintering spores (called oospores) can form in dying onion foliage. Oospores have thick walls and a built-in food supply so they can withstand unfavorable winter temperatures and survive in the soil for up to five years.

Foliar fungicides can be helpful, especially when applied preventively at high rates and short intervals. Fungicides registered for use on onion in Michigan can be found in the MSU Extension Bulletin E-312, Insect, Disease and Nematode Control for Commercial Vegetables. On online version can be found at the MSU Extension Vegetable Team website. Mancozeb-based fungicides are the most commonly used products for limiting downy mildew on onion and can be used in combination with or in alternation with other fungicides such as Ridomil-based fungicides, Tanos, Forum, Revus, or Reason (Note: See E-312 for formulations and rates). I do not have efficacy data from Michigan for the newer onion fungicides including Tanos, Forum, Revus, and Reason. Therefore, I suggest that these newer fungicides be used in a tank-mix with mancozeb. Each fungicide component of the tank mix should be at its full labeled rate. Cutting the labeled rates of the tank mix partners is a recipe for disaster.

An earlier study evaluating fungicides for managing downy mildew was conducted at the Michigan State University Muck Soils Research Farm. Onion ‘Daytona’ seeds were planted on May 14 at a seed spacing of 1 inch. Rows were spaced 18 inches apart on three-row beds centered 64 inches apart. Fungicides were applied with a CO2 backpack sprayer equipped with three XR8003 flat fan nozzles spaced 18 inches part and calibrated to deliver 50 gal/A at a nozzle pressure of 52 psi. Eight applications were made at weekly intervals on July 17, 24 and 30; August 7, 14, 22; and September 5. Disease was assessed using a 1-10 scale (1=no disease; 10=complete defoliation) and was evaluated on August 14 and 21, and September 11. Bulbs were graded and weights of small (less than 2-inch diameter), medium (2- to 3-inch diameter), and large (more than 3 inch-diameter) were recorded.

Disease was detected on August 7 and advanced slowly through mid-August. Disease in the untreated plots progressed rapidly during a 21-day period between August 21 and September 11. At the time of final disease evaluation, all fungicide programs that included Manzate 75DF were most effective in suppressing disease and resulted in significantly better disease control when compared with the untreated (Photo 2). When compared to the untreated, Manzate 75DF applied alone (Photo 3), and Ridomil Gold MZ 68WP alternated with Manzate 75DF resulted in significantly higher yields and percentages of large bulbs (more than 3 inches) (Table 1).

For further information on onion diseases, please visit my Research on Onion/Tomato website.

Untreated onions

Photo 2. Downy mildew disease on untreated onions.

Onions treated with Manzate 75WG

Photo 3. Downy mildew disease on onions treated with 3 lb of Manzate 75WG.

Table 1

Table 1. Effectiveness of fungicides in controlling onion downy mildew.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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