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Downy Mildew Dilemma

Friday, December 1, 2000
filed under: Disease

Downy Mildew Dilemma

It was a minor problem in 2000. But with few treatment options in store for 2001 to control the new Apron-resistant strain of this disease, it appears that Mother Nature will be depended upon again next spring for suppression

Downy mildew generally was a minor problem in sunflower this year, with dry conditions after planting helping to keep the soil-borne disease in check. Hopefully, Mother Nature will cooperate again next spring. Although several products that promise suppression are in the pipeline, treatment options appear to be limited in the 2001 growing reason.

Downy mildew most often infects sunflower in the seedling stage, and infection is favored by cool, water-logged soils, according to the North Dakota State University Extension Service. 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, with little or no seed. Some hybrids are resistant to two or more races, but at least two dozen races are known to occur in the United States.

Fungicide seed treatments (such as Apron or Allegiance ) are commonly used to protect germinating seedlings against downy mildew. However, a new strain of the fungus has been discovered recently in the Northern Plains that is resistant to Apron and Allegiance . Researchers in France noted this Apron-tolerant mildew strain a few years ago also, so this problem is not unique to the U.S. NDSU extension plant pathologist Art Lamey describes the loss of this formerly effective product for downy mildew control as a “nasty dilemma.”

“There’s no doubt about it. It’s going to be hard to find a replacement for Apron,” he says.

Tom Gulya, USDA-ARS plant pathologist, Fargo, N.D., has been studying ways to control this new strain for the past two years. From a group of 30 potential fungicides, he is now concentrating on three leading candidates. He’s finding just how difficult replacing Apron will be.

Azoxystrobin (a Zeneca product marketed as Quadris or Protégé), zoximide (a new Rohm & Haas product to be marketed as Zoxium) and fenamidone (an experimental product by Aventis, to be marketed in the near future) are three chemicals that show promise as new seed treatments for downy mildew control. Of the three, azoxystrobin is likely to be commercialized soonest. It is already on the market under the trade name Quadris, used as a foliar spray to treat potato late blight, and also as Protégé, a seed treatment for use on soybeans and cotton.

However, its manufacturer, Zeneca (AstraZeneca) is now in the midst of a merger with Novartis. The merger is expected to be complete by the end of the year, creating a new combined company called Syngenta AG. The negotiations have effectively placed product registrations of Novartis and Zeneca on hold. Although it’s possible that azoxystrobin may be available in 2001 under special circumstances, the product probably won’t receive a full label for downy mildew control on sunflower until the 2002 growing season, at the earliest.

Even when azoxystrobin does become registered for use, it will probably need to be mixed with another fungicide to minimize cost and optimize treatment effectiveness, Gulya says. Whereas Apron offered systemic control of downy mildew, most of the experimental products in the pipeline only inhibit infection, or when the disease is severe, delay it. Thus, a likely scenario will be a combined treatment with both Quadris and Zoxium.

Genetics may prove to be the real answer to the new race of downy mildew. Some seed companies are already developing hybrids that are resistant to the new strain. However, those same companies are hesitant about increasing, releasing, and marketing these new resistant hybrids. One reason is that the new hybrids in some cases have undergone only limited field evaluation for yield, oil and other agronomic traits. Another concern is demand. Recognizing that downy mildew was a non-issue for most sunflower growers this year, seed companies are also concerned about releasing hybrids that won’t sell.

“We have material in our nurseries, but we’re trying to sort out the demand yet for that product. It’s an evolving issue. Right now, we don’t think farmers will be beating on our door for it,” says one sunflower breeder. That doesn’t mean hybrids resistant to the new strain won’t be on the market at all in 2001, however.

Producers should avoid planting sunflower in fields infected by downy mildew within the past few years, says Gulya. Secondly, ask seed companies on the availability of new hybrids with “multi-race” mildew resistance if the disease has been a problem. Lastly, avoid planting in fields with poor drainage or low areas, which lead to waterlogged soil favored by the fungus.

Gulya points out that neither genetic resistance nor a new fungicide are fail-proof measures that will last indefinitely. Given enough time, it is not unusual for crop pests, including fungi, to evolve new strains that overcome resistance genes or new fungicides. “Thus, combining resistant hybrids with a new, dual-fungicide seed treatment may be the best means of controlling mildew, when these options become available,” he says.– Tracy Sayler

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