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Downy Mildew: Another Threat to Plant Stand

Tuesday, January 1, 2008
filed under: Disease

Another contributor to an inconsistent plant stand is the seedling disease downy mildew. Typical symptoms include dwarfing and yellowing of the leaves, and either a premature death of the seedling or a stunted plant that is not going to yield any seed.

For years the sunflower industry depended on the fungicide metalaxyl (Apron®) to provide immunity to the disease. However, in the 1990s downy mildew developed resistance to Apron, and the struggle to find a substitute began.

Wet and cool soil conditions around germination can result in the seedling becoming infected. The incidence of downy mildew has been most often found in the northern reaches of the production area (the Dakotas and Minnesota), where infection levels of 30% or higher have been common.

Fortunately, USDA Agricultural Research Service scientists identified resistant genes that provide immunity to all races and released lines to the private seed companies. Many of hybrids on the market today have these resistant genes and are often listed with the ‘DM’ designation. Seed company literature will identify their hybrids as ‘resistant to all races’. Additionally, the CruiserMaxx™ seed treatment has a cocktail of fungicides, including Dynasty®, which provides good suppression of the disease. There also are other fungicides in the EPA registration pipeline that will provide improved resistance to the disease.

Dr. Tom Gulya, USDA-ARS sunflower research plant pathologist, is a strong advocate of two-way protection — i.e., fungicides and genetic resistance — to delay the possibility of a new race developing.

For sunflower producers in North Dakota, Minnesota and Manitoba, the only real protection against downy mildew when environmental conditions are right for the disease is a package that includes a resistant hybrid and the Cruiser seed treatment.
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