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Soybean disease control starts with soil sampling

Two of the greatest soilborne concerns in the Mid-South soybean world, according to Mississippi Extension Plant Pathologist Tom Allen, are reniform and root-knot nematodes. “Nematodes are a tremendous problem in the Mid-South due to the level of acreage dedicated to continuous soybean or soybean following continuous cotton,” he says.

Identifying reniform and root-knot nematodes, and then planning a management strategy, begins with taking soil samples in the fall typically after harvest, and then submitting these samples to a laboratory with the capabilities to test for nematodes. Then, based on the soil sample reports, and depending on the particular nematode present, a grower can decide how best to manage the pest.

If root-knot nematodes are the issue, Allen suggests either planting a tolerant soybean variety or grain sorghum, a non-host for root-knot. On the other hand, if reniform nematode populations are building in your soybean field, he suggests rotating to corn the following year.

However, Allen cautions, “A one-year rotation is typically not long enough to reduce nematode numbers. In addition, corn can be a host for root-knot nematode depending on the field’s history and particular root-knot species present.”

Another disease rearing its ugly head in the Mid-South is frogeye leaf spot. “Frogeye is showing up a lot more in 2012, mostly in susceptible Maturity Group IV soybean varieties.”

One of Allen’s concerns, he says, is the occurrence of resistance that has developed to the strobilurin  fungicides in neighboring states, including Illinois, Kentucky, Louisiana, Missouri, and Tennessee.

The strobilurin-based products comprise the major class of fungicides used to manage soybean diseases. To prevent the development of resistance in the frogeye leaf spot fungus population, his first two plans of action are to plant resistant varieties, and to change fungicide management strategies.

Triazole fungicides will manage frogeye leaf spot. However, the application timing is a bit different, as this class of chemistry is applied at disease onset instead of as a preventative treatment.

In 2012, frogeye leaf spot was mostly making its appearance at the R5 growth stage, but the development of this disease is purely environmentally driven, according to Allen.

To identify frogeye leaf spot, Allen suggests scouting for lesions that are round and can be as small as one-sixteenth-inch to as large as one-fourth-inch with purple margins. In the most mature lesions, the center of the lesion will be gray or have fallen out. As the disease advances, the center of the lesion turns more grey in color than purple. At that stage, its appearance is similar to that of a cigarette burn.

In comparison, the biggest disease concern for Southeast soybean producers in 2012 is soybean rust. “In most years, soybean rust is an issue, and we see some isolated cases of the disease. With all of the rainfall we’ve had this year, it is the most significant disease issue,” says University of Georgia Plant Pathologist Bob Kemerait.

In addition to a multitude of summer storms and the abundant rainfall that accompanied these tropical systems, the relatively cooler summer southeastern growers have enjoyed in 2012 further favors soybean rust development. “I consider the storms the vehicle for infection to occur and the cooler temperatures the driver for further spreading soybean rust infection,” he says.

On a more localized basis, the excessive rainfall experienced in late summer has increased incidences of anthracnose and pod and stem blight in the region’s soybean acreage, according to Kemerait.

“Our soybeans are typically treated with a fungicide after a problem is noted, and in the case of soybean rust growers usually wait until the soybean rust sentinel plots trigger a suggested fungicide application,” he says.

“Historically, producers have hoped they could avoid spraying, but with commodity prices where they are, more growers with potentially high-yielding soybean crops are automatically treating with a fungicide when they make a pass across the field.”

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