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Downing Downy Mildew

Saturday, December 1, 2001
filed under: Disease

Downing Downy Mildew

New seed treatments and hybrids in the pipeline, though no new products expected for 2002

Downy mildew could be found in over a third of sunflower fields surveyed in North Dakota late September by the National Sunflower Association. That may sound high, but historically, that incidence is about normal, even before a new strain of the fungus began showing up in the Northern Plains that is resistant to the fungicide seed treatments Apron-XL and Allegiance, according to Tom Gulya, USDA-ARS plant pathologist, Fargo, N.D.

Overall, the disease was generally a minor problem with minimal impact on yield, just like last year. Still, it can be a problem for some growers. While average severity of downy mildew was 1.8% in the NSA field survey, disease severity within individual fields ranged from 0.5% to 42% in a field in Ward County, N.D.

Downy mildew was minor in South Dakota as well, generally limited to the north central part of the state. There, downy mildew infection in some sunflower fields reached as high as 25%, meaning that 25% of plants in infected fields were rendered nonproductive because of downy mildew, according to Marty Draper, SDSU plant pathologist.

Downy mildew infects sunflower in the seedling stage, and infection is favored by cool, water-logged soils. Symptoms 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.

Fungicide seed treatments such as Apron-XL and Allegiance remain beneficial in protecting developing sunflower seedlings from other diseases that cause dampening off and seedling blight. Their only Achilles heel, so to speak, is the Apron-tolerant downy mildew strain that became evident a few years ago. It may be impossible to replace the cost-effective downy mildew control that Apron provided, but several promising fungicides are in the pipeline.

Gulya has been evaluating several new fungicide seed treatments for the control of Apron-XL and Allegiance-resistant downy mildew in greenhouse and field tests. Azoxystrobin (a product from Syngenta marketed as Quadris), zoximide (a Dow/Rohm & Haas product to be marketed as Zoxium) and fenamidone (an experimental product by Aventis) have proven effective against the new strain of downy mildew. Combinations of different chemistries in particular seem to be most effective. The best treatment to date in trials is the combination of fenamidone and ethaboxam, the latter a Korean fungicide used on downy mildew and late blight that currently is not marketed in the U.S. or Europe.

Another question about these new compounds is their effect on stored, treated seed. A study will be conducted this winter to test the effect of these products on sunflower germination.

None of these new seed treatment products is expected to be commercially available for 2002. Even when one or more of the products do become registered for use, they will probably need to be mixed with another fungicide to minimize cost and optimize treatment effectiveness, Gulya says.

Resistant hybrids will ultimately be the real answer to downy mildew. Gulya says some seed companies are developing hybrids that are resistant to the new strain.

Producers should avoid planting sunflower in fields infected by downy mildew within the past few years, says Gulya. “Since it’s a soil-borne fungus, it does not move around much,” he says. Further, avoid planting sunflower in fields with poor drainage or low areas, which lead to waterlogged soil favored by the fungus.—Tracy Sayler

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