Take Note Of Seedling Diseases In This Cool, Wet Spring

Take Note Of Seedling Diseases In This Cool, Wet Spring

Current growing conditions change some of the rules for soybean disease management and increase the risk of seedling disease in fields without seed treatment.

Farmers in Iowa have had a good planting season.  As of Monday May 10, Iowa State University Extension agronomists reported near completion of corn and 50% of the soybean acres were planted in Iowa. "So far this has been one of the most trouble free planting seasons I remember, but with some similarities to last season," says X.B. Yang, an ISU Extension plant pathologist.
Before planting, climatologists had predicted that the spring of 2010 would be cooler than normal.  As predicted, May has been cooler and somewhat wet, with frost in the first week. The cool spring has disadvantages for some farmers. Early planted soybean fields in southern Iowa reported frost damage which required replanting. Stress from frost could add root rot problems for soybean plants that were not killed by frost, especially in fields which are wet, he says.

Watch fields for "damping off" of soybean seedlings this spring

Seedling damping off will be a disease to watch for as it likes a cool and wet planting season, notes Yang. The most common diseases in a cool planting season are Pythium damping off and Fusarium root rot. If replanting has to be done because of frost damage or diseases, fungicide treated seed should be used because the risk of seedling diseases will be higher in these replanted fields, especially in fields that had diseases.
"More and more growers are using fungicide seed treatment when they plant beans, which has benefits in a cool, wet spring like this year," he says. Farmers who use seed treatment are likely to have few seedling disease worries, as seedling diseases have an increased presence in these conditions over a normal season. Years of university data suggest that replanting can be avoided and yields increased with the use of fungicide seed treatments in a spring when risk of seedling diseases is high. Because of record amount snowfall this past winter, many growers had anticipated a wet spring and made plans to use treated seed.

Many of the normal disease management rules won't work in 2010

Many rules previously used to manage soybean diseases will not work in current growing conditions, says Yang. One example is the use of planting date to reduce the risk of sudden death syndrome (SDS) in a field. In a normal season, the earlier the planting date the higher the risk of having SDS in the summer, and with later plantings the better the chance of having less SDS in the field.

However, this rule did not work in the cool, long spring of 2009, a planting season similar to this year, he says. Soil temperatures were cool after mid-May. In the fall, SDS was found in many fields planted after mid-May. With 50% of Iowa's soybean acres yet to be planted this spring, late planted soybeans may be at risk to be infected by SDS fungus because of the cool soil temperatures.

"Some growers are likely to see SDS again this fall, but there is not much we can do about it now," notes Yang.

The good news that comes with a cool and wet spring is that this type of weather will reduce the risk of insect borne diseases, such as bean pod mottle virus. Cool temperatures will hammer the development of insects, such as bean leaf beetles and therefore reduce the movement of bean leaf beetle virus. "The spreading of the virus from plant to plant depends on the number of bean leaf beetles," says Yang. "A cooler spring slows down the beetle population development and, therefore, reduces the risk of the virus the beetles carry."

TAGS: Extension
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