NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

August 13, 1998

Fungicide-Resistant Strain of Sunflower Downy Mildew Found in Region

A strain of the sunflower downy mildew fungus totally resistant to Apron®, a seed-treatment fungicide, is appearing in scattered fields throughout the Dakotas and Minnesota, says a researcher with the USDA's Northern Crop Science Laboratory in Fargo. This Apron-resistant fungus infects the roots of sunflower seedlings and causes a systemic disease that becomes a major problem in cold, water-logged soils.

"Apron has been used in the United States since 1985, and it has provided total control of downy mildew," says Tom Gulya, a USDA research pathologist stationed on the North Dakota State University campus. "Two years ago, however, researchers in France found isolated fields with infected plants, despite having Apron seed treatment."

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 the underside of leaves. Many of these plants die within a few weeks. Those that survive produce upright-facing heads. Gulya says sunflower producers should not confuse small leaf spots caused by airborne downy mildew spores with systemic infection. Small, angular, quarter-inch-sized yellow spots do not cause a systemic infection and do not result in stunted plants. Only seedlings infected through the roots will result in a systemic infection.

Gulya says yield losses due to systemic infection may be negligible, especially if diseased plants are scattered, since neighboring healthy plants can compensate for the loss of individual plants early in the season. But if large areas of a field are uniformly infected with downy mildew, yield losses could be sizeable.

"There were more calls this year about seeing downy mildew on sunflower than we have experienced in previous years," says Art Lamey, extension plant pathologist at NDSU. "This is what first alerted us to the fact that there might be a problem."

Based on leaf samples coming primarily from fields in the Red River Valley, Gulya determined that the fungus is indeed resistant. Leaf samples have also come from fields in other parts of the Dakotas and Minnesota. Gulya has tested 40 samples, and all but two were resistant to Apron, which means that 95 percent of the fields with downy mildew have the Apron-resistant strain present.

"In some fields I visited with extension agents there were some pretty high levels of downy mildew," says Lamey. "This was the case in an area where there had been about two inches of rain just shortly after planting, which would be the type of situation that would favor the early development of downy mildew. And, a lot of these problems were showing up in areas where we have had wet conditions for several years running, which would be the kind of situation that would favor the development of resistant strains."

Once plants are systemically infected, there is no rescue treatment available, so the availability of effective seed-protectant fungicides is crucial. In greenhouse tests, Gulya has identified two fungicides that will protect seedlings both from the "old" strain of downy mildew and the Apron-resistant strain. Because these fungicides are not labeled for use on sunflower, Gulya and Lamey are requesting that the Environmental Protection Agency grant Section 18 Emergency Labels, permitting use of these seed treatments.

"Since the current situation represents a bona fide crisis and Apron is currently the only seed treatment available for downy mildew on sunflower, we expect the EPA to approve the request, perhaps by as early as Nov. 1," says Gulya. "That would allow seed companies time to treat planting seed for 1999 before it goes to their seed salesmen."

Meanwhile, Gulya and NDSU researchers and specialists will continue monitoring sunflower fields and talking with producers to get an accurate idea of how widespread the resistant strain is, and its effect on this year's crop. In areas that received low or normal rainfall at planting, they expect the disease to be virtually nonexistent. If next spring does not bring excessive rains leading to flooded fields, then downy mildew won't be a concern. Likewise, concern about the Apron-resistant strain will end once a new fungicide is labeled for sunflower.

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Sources: Tom Gulya (701) 231-1316 and Art Lamey (701) 231-7056

Editor: Dean Hulse (701) 231-6136