Onion downy mildew confirmed in the state
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.
Downy mildew on onion has been confirmed in Michigan. Although the disease was detected on only one plant, this pathogen can multiply rapidly and spread through a growing region if left untreated.
Downy mildew of onion is caused by the pathogen, Peronospora destructor, and first infects older leaves, occurring as pale, elongated patches that may have a grayish-violet fuzzy growth. Symptoms of the disease are best recognized when dew is present in the morning. Infected leaves become pale green, then yellow and can fold over and collapse. Premature death of onion leaves reduces bulb size. The downy mildew pathogen initiates infection during cool temperatures (less than 72°F) and wet conditions. Multiple infection cycles can occur in a season. Spores are produced at night and are easily blown long distances in moist air. They can germinate on onion tissue in one and a half to seven hours when temperatures are 50°F to 54°F. High daytime temperatures and short or interrupted periods of humidity at night can prevent sporulation. Overwintering spores, called oospores, can form in dying plant tissue and can be found in volunteer onions, onion cull piles, and in stored infected bulbs. 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.
Onion downy mildew study results
Research studies in Michigan (Figure 1) have shown that weekly applications of mancozeb (available as Dithane, Manzate, or Penncozeb) protect against downy mildew when spray coverage is good and sprays are begun before disease appears. Some growers choose to include Ridomil Gold MZ in alternation with mancozeb although this program is more costly. We have tested Pristine 38WG in rotation with Ridomil Gold MZ for downy mildew control. The program with Pristine and Ridomil Gold MZ in alternation was effective for downy mildew, but is very costly. It is likely that Pristine alternated with mancozeb would also be effective, but needs to be tested. In addition to downy mildew, Pristine 38WG has activity against the leaf blights including purple blotch, Stemphylium, and Botrytis. New products such as Tanos, Revus, and Ranman have been effective some years of our studies but have not been tested as a stand alone product with much success. New unregistered products such as Quadris Top (azoxystrobin/difenoconazole), Inspire Super, and newly released numbered products were very effective in controlling downy mildew in our 2008 research studies. At this time none of these products are currently labeled for use on onions. In the 2004 Ridomil Gold MZ alternated with either Manzate or Pristine provided the most consistent control of downy mildew. Revus and Tanos provided early season control of downy mildew but disease levels increased as the season continued and pressure approached epidemic levels. Ranman alternated with Omega provide a modest level of control, however neither of these products are registered for use on onions.
Onion leaf blight and purple blotch study results
Historically any product containing chlorothalonil (Bravo, Equus, Echo) and iprodione (Rovral), were effective in controlling both onion leaf blight and purple blotch. Some of the new stroblilurin products such as Quadris, Pristine, and Flint are also very effective for purple blotch control.
In our recent studies (Figure 2), Quadris Top, Inspire (difenoconazole), Omega, and numbered compounds were effective in controlling both onion Botrytis leaf blight and purple blotch. The product Switch was also effective in controlling Botrytis leaf blight and is the only product tested in these studies that is currently labeled for use on onion.