Iowa Soybeans: Diseases Are Taking Off

Several soybean diseases have popped up over the past several weeks across Iowa. While some diseases are quite severe in certain fields, others are only scattered problems in parts of the state. Regardless, it is important to scout and identify what diseases are present and to keep track of where the diseased spots are occurring in the field.

This can help with variety selection and possible management of these diseases in the future. Some of the more common diseases found in our field scouting over the past few weeks include:

Sudden death syndrome (SDS)

Severity of SDS has increased in many parts of the state. While it is still mostly pockets within a field, it is the cause of many of the yellow spots observed in soybean fields (Figure 1). Remember to check your soybean cyst nematode (SCN) counts this fall in fields with SDS to see if SCN is contributing to the SDS problem. In July 2014, we discussed the increased risk of SDS in 2014 in this article

Figure 1. Yellow patches of sudden death syndrome in a soybean field. Leaves of infected plants can fall off but petioles will stay attached to the plant.

Brown stem rot (BSR). This disease is still mostly only in northern Iowa but has also been reported in parts of eastern Iowa. Remember to split stems to distinguish between SDS and BSR.

Top dieback

While the cause of top dieback is unknown, many yellow spots in fields are the result this disease. Top dieback appears as yellowing on the outside margins of leaves in the upper canopy (much like potassium deficiency, but in the upper canopy). The cause of this disease is still up for debate, but potassium deficiency, SCN, and the Phomopsis/Diaporthe disease complex (fungi associated with stem canker and pod and stem blight) may be culprits. Several years ago, a very thorough article was written about top dieback.

Downy mildew

This foliar disease is becoming more common as the season goes on. Downy mildew can be identified by the light green, irregular shaped lesions on the top side of the leaf and fluffy white growth on the under side of the leaf (Figure 2).

Figure 2. Fluffy white growth on the leaf underside is a sign of downy mildew infection.

Frogeye leaf spot

Another foliar disease that is showing up in parts of Iowa, especially in the southern counties, is frogeye leaf spot. Frogeye is fairly easy to identify by the gray lesions with purple borders.

Stem canker

There have been a few isolated reports of stem canker, which is another disease that is more common after very wet springs. Look for distinct lesions on the stem to identify this disease (Figure 3).

Figure 3. Stem canker can be identified by examining stems for sunken, gray-brown lesions with reddish margins. Note green tissue below lesions. Photo courtesy Craig Grau.

White mold

There have been reports of white mold across most of Iowa; however, most cases have been fairly low incidence of the disease.

Other diseases seen in Iowa include Septoria brown spot and bacterial blight (both have been around for most of the growing season), soybean vein necrosis virus (SVNV) in southeast Iowa, and possibly soybean dwarf virus (Figure 4).

Many soybean diseases have similar symptoms to other diseases. If diagnosis is proving to be difficult, samples can be submitted to the Plant and Insect Diagnostic Clinic at Iowa State University.

Figure 4. Possible symptoms of soybean dwarf virus. Samples are being tested for confirmation that this virus is the cause.

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