Mary Hausbeck, Department of Plant and Soil and Microbial Sciences, Michigan State University-July 23, 2014
The state of Michigan has confirmed downy mildew on onions. In Michigan, this disease occurs every three to four years. This is a particularly devastating disease because if left untreated, it can multiply quickly and spread throughout the growing area.
Downy mildew is caused by the destruction of the pathogen Peronospora, which can defoliate crops prematurely. It first infects the earlier leaves and appears in the early morning of the off-season. It may grow as grayish-purple fuzzy growth with faint slender spots. The infected leaves turn light green and then yellow, and can be folded and folded. The lesion may be purple-purple. The affected leaves turn light green first, then yellow, and can fold and collapse. The symptoms of the disease are best recognized when dew appears in the morning.
Premature death of onion leaves will reduce bulb size. Infection can occur systemically, and the stored bulbs become soft, wrinkled, watery and amber. Asymptomatic bulbs will germinate prematurely and form light green leaves. The bulb can be infected by secondary bacterial pathogens, causing decay.
Downy mildew pathogens begin to infect in cool temperatures, below 72 degrees Fahrenheit, and in humid environments. There may be multiple infection cycles in a season. Spores are produced at night and can easily blow a long distance in humid air. When the temperature is 50 to 54 F, they can germinate on the onion tissue in one and a half to seven hours. High temperature during the day and short or intermittent humidity at night will prevent spore formation.
Overwintering spores, called oospores, can form in dying plant tissues and can be found in volunteer onions, onion culling piles, and stored infected bulbs. The spores 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.
Purpura is caused by the fungus Alternaria alternata, a common onion leaf disease in Michigan. It first manifests as a small water-soaked lesion and rapidly develops into a white center. As we age, the lesion will turn brown to purple, surrounded by yellow areas. The lesions will coalesce, tighten the leaves, and cause the tip to recede. Sometimes the bulb of the bulb becomes infected through the neck or wound.
Under the cycle of low and high relative humidity, spores in the lesion can form repeatedly. If there is free water, the spores can germinate within 45-60 minutes at 82-97 F. Spores can form after 15 hours when the relative humidity is greater than or equal to 90%, and can be spread by wind, rainfall, and irrigation. The temperature is 43-93 F, and the optimum temperature is 77 F, which is conducive to the growth of fungi. Old and young leaves damaged by onion thrips are more susceptible to infection.
Symptoms will appear one to four days after infection, and new spores will appear on the fifth day. Purple spots can defoliate onion crops prematurely, impair bulb quality, and may lead to rot caused by secondary bacterial pathogens. The purple spot pathogen can survive the winter over the fungal thread (mycelium) in the onion fragments.
When choosing a biocide, please alternate between products with different modes of action (FRAC code). The following table lists the products labeled for downy mildew and purple spots on onions in Michigan. The Michigan State University extension says to remember that pesticide labels are legal documents regarding the use of pesticides. Read the labels, as they change frequently, and follow all instructions exactly.
*Copper: badge SC, champion product, N copper count, Kocide product, Nu-Cop 3L, Cuprofix hyperdispersant
*Not all of these products are marked with downy mildew and purple spots; DM is especially recommended for controlling downy mildew, PB is especially recommended for controlling purple spots