By Dean Malvick, Department of Plant Pathology
Cool weather and low rainfall have held the soybean crop back in parts of Minnesota this summer. Now, significant diseases are a concern in some areas. Several diseases have started to appear recently that can significantly damage soybeans, especially sudden death syndrome (SDS), brown stem rot (BSR), and white mold. These diseases are favored by weather conditions that have occurred in large areas of Minnesota this season. The information we gather now can help to explain why yield may be low in some fields and can assist with targeting disease management where these diseases occur most often. There are no effective fungicide or other treatments that can be used this season to reduce damage from these diseases.
SDS has been reported at significant levels in Dodge, McLeod, and Waseca counties, and it is undoubtedly also appearing in other areas in Minnesota. Based on results from previous years, SDS probably occurs across much of the southern half of Minnesota, but it could occur anywhere in the state. Foliar symptoms of SDS have been developing for at least the past two weeks. The last two weeks of August and the first week of September is a good time to scout for SDS. Look for this disease first in low or compacted areas of fields and near field entry points. Initial symptoms are yellow areas on leaves, and these yellow areas turn into brown patches. Reports have indicated different levels of disease severity among some varieties. This would be a good time to look for different factors that could affect SDS severity, including soybean variety, tillage, drainage, crop rotation, and planting date. The map of the confirmed distribution for SDS in Minnesota, as well as more information and photos for SDS and other soybean diseases, can be found at the Minnesota Crop Diseases web site (www.extension.umn.edu/cropdiseases/soybean/index.html).
Foliar and stem symptoms of BSR are also starting to appear. BSR occurs in fields throughout Minnesota, including all areas where SDS occurs. When scouting fields or attempting to diagnose whether plants are infected with SDS or BSR, keep in mind that the foliar symptoms can look very similar for these two diseases. The key for BSR is to split the bottom 6" of the soybean stems and look for brown pith, especially at the nodes. Plants with BSR will have brown pith, but plants with SDS will have white pith. Keep in mind that plants with BSR always develops brown pith, but foliar symptoms (brown tissue between veins) do not always develop. The only way to know how much BSR is in a field is to split stems, and internal BSR stem infections alone can cause yield loss. BSR tends to be more severe when temperatures are cool (60- 80°F) during reproductive growth stages. When leaf symptoms develop, they can be most severe when soil has been wet at early flowering to pod fill growth stages and dry during maturation. The conditions in large areas of Minnesota appear to have been favorable for BSR this season.
White mold may also be significant in some parts of Minnesota based on growing conditions this year. We have had reports of white mold damaging and killing in several areas of Minnesota. This disease is favored by cool temperatures and rainfall from mid- July through August. All of Minnesota has had cool conditions, and the areas that received enough rain to maintain wet soil for short periods in July and early August are set to develop white mold. Look for scattered and patches of dead and dying plants, and the presence of white moldy growth and sclerotia (black to gray round to oblong structures up to 1/4") on and inside the stems. As with the other diseases, look to see if you can associate the more severe areas with factors that may affect white mold such as soybean variety, manure or fertility history, tillage, row spacing, or high plant population.
Finally, downy mildew is common in a number of areas. Although this soybean leaf disease rarely if ever reaches levels that damages yields in Minnesota, it can be fairly easy to see. Downy mildew typically only infects leaves at this time of the season, although later it can infect pods. Symptoms of downy mildew on the tops of leaves are irregular yellow to brown spots, and the bottom of the leaves under the spots have tan to gray tufts of fungal growth that can be seen easily with a hand-lens.