A Favorable Year for Downy Mildew
This growing season seems to be one of those years favorable for downy mildew in some areas of the Northern Plains growing region.
“I’ve been getting a lot of calls about it. There were fields ripped up because of downy mildew, and I don’t remember the last time that’s happened,” says Marty Draper, South Dakota State University extension plant pathologist.
Downy mildew infects sunflower in the seedling stage, and infection is favored by cool, water-logged soils. “It seems to be a greater problem under no-till,” Draper says. “Rain saturated soils and under no-till, the water stuck around long enough to cause infections.”
Symptoms of downy mildew include dwarfing and leaf yellowing (chlorosis), with the appearance of white cottony growth of fungal spores on the undersides of the leaves. Plants that survive will often produce heads that face straight up, producing little or no seed. Plant stunting and leaf distortion are also symptoms of herbicide drift injury, especially 2,4-D and related phenoxy compounds. However, herbicide damage never exhibits the white appearance (fungal growth) on the underside of the leaves, nor the chlorosis typical of downy mildew.
Occasional plants with systemic infection do not result in yield losses, with nearby plants compensating in yield. When large areas of plants have systemic infection, however, then yield loss occurs.
In 1998, the mildew fungus was found to have developed resistance to Apron seed treatment in many parts of eastern North Dakota, western Minnesota and northeastern South Dakota. Current fungicide seed treatments such as Apron-XL and Allegiance remain beneficial in protecting developing sunflower seedlings from diseases that cause dampening off and seedling blight, but offer fair control at best of tolerant strains of downy mildew.
Other fungicide seed treatment products are in the pipeline, including one likely to be introduced for the next growing season that would offer downy mildew suppression, but not complete control.
Resistant hybrids are the ultimately the best answer to downy mildew, and the good news on that front is that several seed companies are already offering downy-mildew resistant hybrids, with more on the way. “There are hybrids out there holding up well with basically no downy mildew in them, and downy mildew resistant germplasm that’s looking excellent,” says John Swanson, product manager for Croplan Genetics. “Growers in areas affected by downy mildew this year should ask seed companies about downy mildew resistant hybrids adaptable to their growing conditions, and to the markets they’re looking at.”
Bruce Due, district agronomist for Mycogen Seed, says that it takes time for seed companies to build seed stocks in response to specific crop needs. “Next year there will still be a limited amount of downy-mildew resistant hybrids on the market. Realistically, we’re about two years away from where the majority of oil sunflower acres could be planted to resistant hybrids.”
Fields with poor drainage or low areas, which lead to waterlogged soil favored by the downy mildew fungus, are the best candidates for planting resistant hybrids.
“You need the pathogen, plant host, and a favorable environment for infection to occur. We can’t predict what conditions will be like next year, so there’s no reason to move away from the recommended three to four-year sunflower rotation,” says Due. “Use the tools available to manage downy mildew, hybrids and/or seed treatments. Realize that the best you might expect from a seed treatment is 60 to 80% suppression of downy mildew. But put that in perspective, say you have a field with 25% downy mildew, but you wipe out 70% of that with a seed treatment, and you’re looking at only sporadic plants with downy mildew. That’ll get us through until more resistant hybrids are on the market.” – Tracy Sayler
Monitor Sunflower for Sclerotinia
How might delayed sunflower plant maturity affect risk to Sclerotinia infection? Very little, says Tom Gulya, USDA-ARS plant pathologist, Fargo. What matters are weather conditions until harvest, he says, with prolonged periods of heat and moisture more conducive to disease development.
The best time to monitor sunflower for Sclerotinia is about four weeks after flowering. Look for plants that are dead or wilted. If the plants died from Sclerotinia wilt, there will be a soft rot at the base of the stalk. The rot is brown at first, but later may have a bleached appearance. As dry-down proceeds, it becomes harder to evaluate infected plants. In wet weather, a cottony white growth may form on the surface of the stalk. Later, irregular-shaped hard black bodies, the sclerotia, form inside the stalk and sometimes also on the surface of the stalk.
More information can be found in the NDSU bulletin “Sclerotinia Heat Rot in Sunflower” online at http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/pp1193w.htm or in the NDSU bulletin “Sclerotinia Diseases of Sunflower” online at http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/pp840w.htm.
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