Downy mildew outbreak in onions requires immediate action

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Weather conditions this spring and summer have been very favorable for onion downy mildew and several fields both on the west and east sides of the state are now infected. Downy mildew is caused by the fungus, Peronospora 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 seven hours, even when temperatures are 50 to 54ºF.

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 75ºF. 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 fungus 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. The fungus 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.

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. All onion growers must consider their crops at high risk for downy mildew this year. Intensive spray programs will likely be needed as long as the weather remains cool and wet. Downy mildew is not a disease that can be reined in once it is established in a field with the weather that we’ve been experiencing. Mancozeb-based fungicides can be effective and may be used in alternation or in combination with Ridomil-based or strobilurin-based (Pristine, Quadris) fungicides (NOTE: See E-312 for formulations and rates). Keep in mind that weather conditions are also favorable for botrytis and purple blotch and spray programs must provide protection against these blights in addition to downy mildew. Unfortunately, I do not have data on the newer downy mildew fungicides including Revus, Forum, and Phostrol, but a field trial is ongoing.

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

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