Downy mildew and foliar diseases of onion

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.

Purple blotch and stemphylium leaf blight

Purple blotch first appears as small water-soaked lesions that quickly develop white centers. As they age, the lesions turn brown to purple, surrounded by a zone of yellow. Lesions can coalesce, girdle the leaf, and cause tip dieback (Figure 1). Occasionally, bulbs are infected through the neck or wounds on the scales. Spores of Alternaria porri can form repeatedly on lesions with cycles of low and high relative humidity. When free water is available, spores can germinate in 45-60 minutes at 82-97°F. Spores can form after 15 hours of relative humidity greater than or equal to 90 percent and can be spread by wind, rainfall and irrigation. Fungal growth is favored by temperatures of 43-93°F with an optimum temperature of 77°F. Old and young leaves injured by onion thrips are more susceptible to infection. Symptoms can appear one to four days after infection, and new spores can appear by the fifth day. The pathogen can overwinter in onion debris.

Stemphylium leaf blight is an occasional foliar problem in Michigan that can be confused with purple blotch. Symptoms begin as small, light yellow to brown, water-soaked lesions that develop into elongated spots that turn dark olive brown to black with spore development. Coalescing spots can blight leaves but rarely affects the bulb. The pathogen normally invades dead and dying tissue. Disease development is favored by long warm periods with leaf wetness. Fungicides effective against purple blotch are also effective against Stemphylium leaf blight.

Onion downy mildew

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 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 (Figure 1. Years 2000, 2003, 2004). 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.

Figure 1. (click to view larger)
downy mildew 2008

Figure 2. (click to view larger)
figure 2

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

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