Fungicides for soybeans

The registration of fungicides for use in growing soybeans has gained momentum since the first case of Asian soybean rust was found in the U.S. during the fall of 2004. This past year, a large amount of white mold in soybeans spurred more manufacturer fungicide registration research for soybean production, but it isn't likely that an ag retailer or a fungicide manufacturer is going to turn huge profits targeting white mold.

However, soybean growers aiming to preserve a soybean varieties' full yield potential likely will be using more fungicides to improve the crop's health and control a wide number of "nibbler"

Those nibbler diseases are ones that individually don't have much impact on yield, but two or three of them attacking a soybean plant during the growing season can reduce yield.   

White mold has commonly been a severe problem only about once every five to 10 years. When it jumps from being a nibbler disease to a major concern, it can easily reduce yields by 50 percent. Iowa State University contends that at least 10 percent of plants need to be heavily infected throughout a field before a yield reduction occurs that would justify a fungicide application. Of course, at that determination the infection is too late to control.

"Ninety percent of the white mold infection occurs through the flowers. So, most of the infection is happening at flowering," said Alison Robertson, Ph.D., Iowa State University, assistant professor, plant pathology. The plant growth stage for fungicide application typically is R1.

2009 Doesn't  Translate

White mold was a big problem across the Corn Belt in 2009 with the worst disease pressure in the eastern half. The 2009 growing season was wetter and cooler than average. "White mold is always dependent on the environment," said Carl Bradley, Ph.D., University of Illinois, assistant professor, plant pathology. "If we have another year like last year, we'll have a white mold problem. If we are warmer and drier, we won't have a problem this year."

One year's white mold pressure has no correlation to what will happen the following year; if the weather isn't appropriate for white mold development in 2010, it isn't going to happen.

White mold overwinters as sclerotia (little rough, black seed-looking bodies) and germinate during a cool spring. "Sclerotia only germinate under shaded conditions," Robertson said. "So, often soybeans that are planted early reach flowering when canopy has closed over the rows. That means there is shading under the canopy, and that increases the risk of white mold." The sclerotia can stay in the soil for years before ideal weather allows them to germinate.

The good aspect about white mold is that hot weather stops it in its tracks. "White mold is really a cool weather disease. When temperatures are higher than 85 degrees, it kind of shuts down," explained Bradley.

White Mold Illinois Trial   

Bradley had a fungicide trial against white mold near DeKalb, Ill., in 2009. "Conditions were perfect for white mold, and it actually kind of overwhelmed most of the treatments out there," he said.

Topsin M was the only fungicide for years that was registered for use on soybeans to help control white mold. Bradley tested Topsin, Domark and Endura, which are labeled for use against white mold on soybeans. The fungicides (Proline, Headline, Quadris, Stratego) labeled for use on soybeans, but not claiming white mold control, were also in Bradley's trial plot. Of the products listed, the only one that showed a significant difference in preserving yield was Endura. Endura, however, was applied twice, which may have given it an advantage over other fungicides that were applied only once. 

What did the best to keep white mold suppressed was a high rate of Cobra herbicide; it has a label claim of suppressing white mold. The fungicide-treated beans yielded between 21.6 to 24.4 bushels per acre. The Cobra-treated beans yielded 42.1 bushels per acre.

Products used in the plot under experimental use because they aren't labeled for soybeans didn't fare much better than the soybean-registered fungicides, but a few show promise. Bradley warns that one trial doesn't prove a lot, especially under such high white mold pressure. He says fungicides can have a positive impact in many instances.

Preventing and Scouting

Fungicides for control of white mold and other diseases are almost exclusively for preventive treatments, and that won't change with products in the current trials pipeline.

Contans biofungicide was recently announced as being labeled for white mold in soybeans, again as a preventive. Contans is different than most fungicides, as it should be applied to the soil in the fall after harvest or in the spring before planting. Its primary targets are the sclerotia in the soil.

In general, most fields in a soybean rotation in the Corn Belt probably have at least some white mold sclerotia in the soil. To protect soybean yield, Robertson suggests growers, crop consultants and agronomists use good management practices.

She said, "A fungicide treatment is only a preventive treatment applied at flowering. You have to kind of take into account the variety of soybean and the white mold field history. You have to get out in the fields, check on canopy closure and flowering and look at the weather forecast for the next two, four, six, eight or even 12 weeks. Then you weigh your risks along with the economics of the price of soybeans and the price of a fungicide application. You decide how high your risk is and whether it is going to pay to apply the fungicide. It really can be a tough management decision."


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