Onion disease alert

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

Downy mildew was verified by my lab yesterday afternoon from a field in the mid-Michigan area. This disease is not unexpected given the rains and cooler, moist weather that we’ve experienced this year. Unfortunately, downy mildew is an especially devastating disease because it spreads rapidly and is not readily controlled. 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.

Research studies 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. In a field test for purple blotch and Stemphylium leaf blight, Pristine alternated with Bravo Weather Stik was a stand-out treatment. Newer products for downy mildew control include Acrobat and Reason. While I do not have Michigan field data for these products, I would expect them to perform well in a rotation for downy mildew. However, Acrobat and Reason have action only against downy mildew and will not control botrytis or purple blotch.

Purple blotch and stemphylium leaf blight

Many Michigan onion fields are showing symptoms of purple blotch. This disease got an earlier start than usual with the wet weather and warm night temperatures that we experienced a few weeks ago. 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. 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°F - 97°F. Spores can form after 15 hours of high relative humidity (greater than 90 percent) and can be spread by wind, rainfall and irrigation. Fungal growth is favored by temperatures of 43°F - 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 and has not yet been detected this year. 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 affect the bulb. The pathogen normally invades dead and dying tissue. Disease development is favored by long warm periods of leaf wetness. Fungicides effective against purple blotch are also effective against Stemphylium leaf blight.

The following products are recommended for purple blotch: Pristine, a strobilurin (Quadris or Cabrio), and Rovral tank-mixed with Bravo.

Botrytis leaf blight

Botrytis has been detected this year in onions. It is caused by a fungus (Botrytis squamosa) that survives in onion trash or in soil by means of sclerotia (hard, black survival structures) which germinate in the spring, forming ascospores that begin the disease cycle.

Germinating sclerotia are small and very difficult to find, but they have been reported onion leaf trash. Once the initial infections occur, spread is controlled by weather conditions. This fungus forms its conidia (the spore stage that spreads the disease) only on dead or dying tissue, and only after 60 to 72 hours of continuous high humidities (75 to 100 percent) at temperatures averaging between 54°F and 75°F.

Conidia formed under these conditions are spread by wind currents, land on healthy tissue, and infect after a minimum of six hours of leaf wetness. The longer the leaves remain wet, the more infection that occurs. The fungus forms rather small, white lesions about the size of a pinhead surrounded by a light green halo. Most of these lesions do not enlarge, but a small proportion will enlarge, girdle the leaf, and cause blighting.

The higher the humidity and the longer the periods last, the more leaf blighting that will occur. Bravo has always been excellent for leaf blight control, but mancozeb and Rovral also give good control. In previous years, tank mixes of Rovral with either mancozeb or Bravo have been especially effective where botrytis leaf blight and purple blotch are both present. Pristine will also do a good job when both botrytis leaf blight and purple blotch are a problem.

Bacterial diseases

Bacterial diseases are showing in many onion fields, especially those that experienced heavy rains earlier this summer. It is likely that naturally-occurring soil bacteria were washed into the bulb and leaf whorls where they have multiplied and now causing a rot. It will be difficult, if not impossible, to reach those inner onion tissues that are infected with the bacteria. The only product that is proven and recommended for bacterial diseases is copper. I’ve heard a lot of talk about spraying bleach and I do not recommend that growers do that. Bleach will become inactivated as soon as it comes in contact with leaf tissue or soil. It has no residual and will not protect the leaf tissue. Copper is a proven bactericide with a residual action which is a fact that is supported by years of studies on onions and other crops that are affected by bacteria. To lessen the concerns of phytotoxicity on onion leaf tissue, spray copper at a time of day when the solution can dry readily. The longer that copper stays in solution, the more likely that it may cause some plant tissue burning. When using copper sprays to combat bacterial disease, the spray interval should be at least every seven days.

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