Several soybean diseases have popped up over the past several weeks in fields 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 soybean variety selection and possible management of these diseases in the future.
That advice comes from Daren Mueller, Iowa State University Extension plant pathologist, and Tristan Mueller, operations manager for the Iowa Soybean Association’s On-Farm Network. They offer the following information on how to properly identify these diseases and provide management advice on how to control them.
Some of the more common diseases found in field scouting in Iowa over the past few weeks include:
Sudden death syndrome showing up in many Iowa areas
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 you can now see 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, an article in the ISU Integrated Crop Management newsletter discussed the increased risk of SDS in 2014. Read this article to review that information.
Figure 1: The top photo shows yellow patches of sudden death syndrome in a soybean field. The bottom photo shows how leaves of SDS 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. This article from earlier in the season explains more about how to tell SDS from BSR, as well as discusses BSR management issues.
Top dieback is a disease whose cause is unknown
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 is a foliar disease, becoming more common
This foliar disease is becoming more common as the 2014 growing 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 underside of the leaf (Figure 2).
Figure 2: Fluffy white growth on the leaf underside is a sign of downy mildew infection.
Frogeye leaf spot showing up especially in southern Iowa
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, you need to look for distinct lesions on stem
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 by Craig Grau.
White mold is showing up in fairly low incidence
There have been reports of white mold across most of Iowa this summer; however, most cases have been fairly low incidence of the disease. Additional information on white mold can be found in this article.
Other diseases seen in Iowa this summer 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, you can submit plant samples to the Plant and Insect Diagnostic Clinic at Iowa State University. See this article for instructions on how to do so.
Figure 4: Possible symptoms of soybean dwarf virus. Samples are being tested for confirmation that this virus is the cause.
Daren Mueller is an assistant professor in ISU’s Department of Plant Pathology and Microbiology; he can be reached at [email protected] or (515) 460-8000. Tristan Mueller is operations manager for the Iowa Soybean Association’s On-Farm Network and can be reached at [email protected]. or (515) 334-1075.