Disease Alert!

December 7, 2010 09:11 AM
Disease Alert!

Soybean diseases are like the proverbial box of chocolates—you can’t predict which ones are going to show up next year. What you can do is understand each field’s history, know what conditions foster each disease, select resistant or tolerant varieties and be prepared to apply a fungicide if necessary.

“Really look at the resistance package as you choose varieties for 2011,” urges Ohio State University plant pathologist Anne Dorrance. “In Ohio, the past two years were good examples of different weather environments. Since we can’t predict next season’s weather and which pathogens it will favor, having resistance to several diseases will help maintain yields.”

Plant pathologists in several major soybean growing areas cite an array of disease threats that popped up this year and that may or may not return, largely depending on the weather.

In Ohio, Dorrance says, seedling diseases such as phytophthora, pythium and fusarium hit early, and the effects were aggravated by flooding injury. Later, drought aggravated the effect of soybean cyst nematodes.

The heat also fostered charcoal rot. “Some southern Ohio growers have experienced three or four years of drought, so they need to learn more about charcoal rot,” Dorrance says.

Sudden death syndrome (SDS) showed up in many Ohio soybean fields. “The majority of those fields contained soybean cyst nematode [SCN], combined with the fungus Fusarium virguliforme, which causes sudden death syndrome,” Dorrance says. “These two pests have been linked since the pathogen was first identified in the U.S. The fungus colonizes the roots and leads to root rot, causing the roots to decay.”

Overall, Illinois experienced only average disease pressure, says University of Illinois plant pathologist Carl Bradley. But individual growers suffered yield losses from a variety of pathogens.

Sudden death spreads. “Sudden death syndrome continued its sprawl across the state,” Bradley says. “It was most severe in west-central and northern Illinois, especially in areas that stayed wet and in fields that were planted earlier than normal.”

Cool, wet conditions and early planting also set the stage for seedling diseases and for frogeye leaf spot, which was present in some Illinois fields. “We observed frogeye leaf spot across the state, but it was severe only on highly susceptible varieties,” Bradley says.

Frogeye leaf spot is becoming less of a threat in the Midwest as growers adopt resistant varieties, Dorrance and Bradley agree. But there may be a cloud on the horizon. Bradley’s detection of fungicide-resistant strains of Cercospora sojina, the causal agent of frogeye leaf spot, suggests the disease should not be taken for granted.

Bradley found the strobilurin fungicide–resistant strains in plant samples from a Tennessee soybean field. The active ingredient strobilurin is used in several popular fungicide products.
So far, the fungicide-resistant strains have not been found to be widespread. But their existence reaffirms the importance of planting varieties resistant to frogeye leaf spot. “Resistant varieties remain the best way to manage the disease,” Bradley says.

If you plant a susceptible variety and are considering applying a fungicide, choose a triazole fungicide rather than a strobilurin product, Bradley advises. If you need to spray a strobilurin fungicide for other foliar diseases as well as frogeye leaf spot, apply a product or a tank mix containing both classes of fungicides.

Assess the risk. To reduce the chance of developing resistance to frogeye leaf spot, apply foliar fungicides only when they are needed, based on disease risk and scouting observations, Bradley adds.

Finally, he says, “soybean cyst nematode continues to rob yield from growers who are dismissing or mismanaging the disease.”

In Minnesota, “soybean diseases ranged from worse than average in some areas to less than average in others,” reports University of Minnesota plant pathologist Dean Malvick. “SDS and phytophthora rot were significant in many fields, and pod and stem blight and brown stem rot [BSR] were problems in some areas.

“BSR was more of a problem in Minnesota in 2008 and 2009 than in 2010,” Malvick says. “There is a high risk it will return in a significant way in 2011. White mold was significant in some fields, but thanks to warm summer conditions, it did not develop into a widespread, severe problem. SCN is always a problem.”

Some disease damage went undetected in Minnesota fields, Malvick adds. “Excellent growing conditions in most of the fields concealed the effects of patches of disease,” he says.

In Iowa, SDS and brown spot, two diseases often associated with each other, were widespread, reports Iowa State University plant pathologist X. B. Yang. “The SDS outbreak was among the worst I’ve ever seen,” he says. Yang began warning growers to expect SDS as soon as he saw cool, wet planting conditions shaping up this past March.

Disease management. The widespread reports of SDS, which often causes yield losses of 10 bu. to 30 bu. per acre, suggest it should rank high in your disease management planning.

“Everything you can do to manage SDS must be done before or at planting,” Yang says. “There is no chemical that controls the disease.

“Selecting resistant varieties is very important,” Yang continues. “There are no SDS-resistant varieties, but there are varieties with tolerance. Use local information when you choose varieties; information from other states can be misleading. Remember where you saw healthy looking, SDS-free fields in low or flat ground around your farm last season. Find out what varieties were grown there and consider planting them yourself in 2011.

“Plant fields with a history of SDS last, when the weather is warmer and the soil is drier,” Yang adds.

Managing nematodes and planting SCN-resistant varieties will help reduce the occurrence of SDS and the amount of damage. “Often, SDS shows up in a field four or five years after SCN becomes a serious problem,” Yang says.

Correcting soil compaction and using vigorous, fast-emerging seed helps fight off the impact of SDS, according to Yang, Malvick, Dorrance and other pathologists. Improving drainage can reduce the severity of the disease, Dorrance adds.

With seedling diseases such as phytophthora, pythium and fusarium, fungicide seed treatments can help protect stands. “Consider applying a seed treatment especially if you are planting early, when cool, wet conditions favor the diseases,” Bradley says.

If you have had problems with white mold, remember it can take several years to reduce fungal populations, warns Farm Journal Associate Field Agronomist Missy Bauer. “Be cautious if you will be rotating to soybeans in fields that had heavy white mold pressure in 2009,” she says. “In those fields, plant the more resistant varieties.

Consider planting in wider rows or reducing population, or even changing your crop rotation.”
Yang sums up: “2011 may bring different diseases, which means employing different management strategies. Understanding the risks each season, and in each field, is key to minimizing yield loss.”

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