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Apron-Resistant Downy Mildew Strain Appears

Tuesday, September 1, 1998
filed under: Disease

Though termed a "major" disease of sunflower during the 1970s and 1980s, downy mildew has received relatively little publicity during the 1990s. That's because seed companies incorporated resistance to initial downy mildew races into their hybrids - and also because, over the past several years, commercial seed sold in the Dakotas and Minnesota commonly has been treated with Apron™ fungicide as a protectant against newer forms of this fungus.

During the summer of 1998, however, downy mildew emerged from its relative obscurity - and not in a good way. Scientists have confirmed the widespread presence of a strain of downy mildew that is resistant to Apron. Of 40 samples tested by USDA-ARS plant pathologist Tom Gulya, 95 percent (all but two) were resistant to Apron. These samples were gathered from throughout the eastern third of North Dakota as well as northwestern Minnesota.

That doesn't mean 95 percent of sunflower fields in the Dakotas and Minnesota have downy mildew in them. Gulya estimates the disease was present in about half of the region's fields in 1998 - though usually not at an economic level. For a few fields, however, damage from downy mildew was extensive.

Gulya's findings suggest that given the right conditions - i.e., cool, water-logged soils during the early part of the growing season - downy mildew has the potential to manifest itself in sunflower fields across a wide area of the Northern Plains. And, he emphasizes, Apron will be ineffective against the new resistant strain.

That's the bad news. The good news is that the region's seed companies and growers should have an alternative in place for the 1999 planting season. Gulya and North Dakota State University extension plant pathologist Art Lamey have requested Section 18 (Emergency) labels from EPA for two other fungicides which will protect seedlings from both the "old" types of downy mildew as well as the Apron-resistant strain. Gulya is very optimistic the request will be approved by EPA sometime this fall, giving seed companies time to treat sunflower planting seed over winter.

The more promising of the two fungicides is "Protege," a product of Zeneca Ag Products. Protege is already marketed (as "Quadris") for uses such as late blight control in potatoes and Aphanomyces root rot control in sugarbeets. "Protege is not as effective as Apron once was," Gulya admits, "so we've not been able to come up with a fungicide that will give 100-percent control." By the 2000 planting season, however, Gulya expects Protege will be available in combination with another product, resulting in close to 100-percent control of downy mildew in sunflower.

The USDA pathologist adds that seed companies simultaneously are working to develop additional resistance in hybrids. "A truly resistant hybrid would not require fungicide - and, it would resist both the old and new types of mildew," he states.

The Apron-resistant strain seemed to come "out of nowhere" in 1998. Gulya believes that the fungus had been evolving over time "but at such a low rate that we weren't able to observe it."

The stimulus behind this year's mildew problem was the excessive rainfall received across much of eastern Dakota and northwestern Minnesota during late spring and early summer. Downy mildew cannot be detected through soil sampling, Gulya explains. "We need the plant to serve as an indicator, and the plant will express mildew only if you have water-logged soil," he notes. So for the disease to show up, "we need sunflower planted in a field that has the fungus - and we need a lot of water."

Sunflower plants with a systemic infection of downy mildew are stunted, display yellow areas on the leaves and have a coating of white spores on leaf undersides. Many infected plants will die within a few weeks, while those that survive typically are very short and produce upright-facing heads.

Gulya says small leaf spots caused by airborne downy mildew spores should not be confused with a systemic infection. The small, angular, quarter-inch-sized yellow spots from airborne spores do not cause systemic problems, nor will they produce stunted plants. The systemic infection occurs only when seedlings are infected through their roots.

Along with the likely availability of the Protege seed treatment for 1999 and the ongoing breeding for hybrids resistant to all known types of downy mildew, there's another reason for Northern Plains growers not to be discouraged by news of the Apron-resistant strain: Normal or low rainfall around planting time next spring should result in virtually no downy mildew problems.

Of course, seed treatments are actually an insurance policy, Gulya points out. So instead of hoping Mother Nature will drop just the right amount of early season rains on your fields, he favors the use of an effective seed treatment next spring.

The good news about this particular production problem, he reiterates, is that "we have a solution, all the seed companies know about it, and we can respond relatively quickly. We do not perceive this causing a serious problem next year." - Don Lilleboe

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