This year's early season rain has increased the risk of soybean disease in Iowa. Farmers who are scouting their fields in late July, and into August, need to keep an eye out for symptoms.
"We knew it was coming and now the waiting is over," says Daren Mueller, an Iowa State University Extension plant pathologist. "For the past month we have seen Septoria brown spot and bacterial blight showing up in some soybean fields in Iowa. The week of July 20 we identified sudden death syndrome (SDS), frogeye leaf spot and some other diseases for the first time this year."
Sudden death syndrome: The 2015 season is primed for being another "SDS year" because of the weather conditions experienced to date in Iowa. "As we've learned from SDS outbreaks in years past, this disease likes it wet," says Mueller. "The early wet weather helps increase the root rot phase of the disease."
Figure 1: This map shows areas of the state with more rainfall, the areas which may have a higher risk of soybean diseases this summer.
SDS has been identified in different parts of the state this summer, says Mueller. It's especially prevalent this year in central and southeast Iowa. SDS foliar symptoms appear as yellowing and death of tissue between leaf veins (See Figure 2). Remember, there are a few other diseases that may be confused with SDS such as brown stem rot and stem canker. Look for lesions on the outside (stem canker) and browning in the pith (brown stem rot) to distinguish from SDS.
Figure 2: This yellowing and death of soybean leaf tissue is due to sudden death syndrome disease.
Frogeye leaf spot: "We do not normally see frogeye leaf spot this early in Iowa, but it has already been identified in a central Iowa field," says Mueller. Frogeye leaf spot is fairly easy to distinguish from other leaf spot diseases of soybeans. It has a dark purple ring at the margins of lesions with a light tan center (Figure 3). Lesions are usually found in the middle to upper canopy.
Figure 3: Frogeye leaf spot lesions on soybean leaves.
Phyllosticta leaf spot: Phyllosticta leaf spot is a minor disease. "And we've received one report in Iowa so far this year, of this disease," says Mueller. Look for the dark spots within the lesion to help identify this disease (Figure 4).
Figure 4: Phyllosticta leaf spot lesion with dark specks on the soybean leaf.
NOTE: Daren Mueller is an Iowa State University Extension soybean pathologist and assistant professor. He can be reached at 515-460-8000 or [email protected]. Tristan Mueller is Operations Manager for Agronomic Research with the Iowa Soybean Association's On-Farm Network and can be reached at [email protected] or 515-334-1075.