Dean Malvick, Department of Plant Pathology, St. Paul
The soybean crop is growing well across most of Minnesota. As of July 25, 80% of the state's ~7.4 million acres of soybeans was flowering and 25% was setting pods. Most of the soybean crop in Minnesota was rated in good (58%) or excellent (27%) condition. With the frequent rains this season, however, disease problems are appearing in some fields and others may be brewing. The leaf diseases downy mildew, bacterial blight, and Septoria brown spot are common now in Minnesota, but fortunately none of these diseases typically cause significant yield reductions. Many areas in Minnesota have also had favorable conditions for development of Phytophthora root and stem rot, stem canker, white mold, and sudden death syndrome.
Bacterial blight is a common soybean disease that is most common during wet, cool weather. This disease usually occurs at low levels that don't result in yield loss. Bacterial blight is most often seen on leaves in the mid to upper canopy. Infections begin as small, water-soaked spots that turn brown and become surrounded by yellow halos. The spots often merge to form large, dead patches that may fall out to give leaves a ragged appearance. Plants can be infected at any time, and development of this disease slows or stops in dry, hot conditions.
Downy mildew is a widespread disease that occurs during periods of high humidity and moderate temperatures. The disease is typically superficial and causes no yield loss, but can cause reduced yields under rare conditions. Young leaves are most susceptible and infected leaves are most often seen on the tops of plants. Symptoms start as small, light green spots (not water-soaked) on tops of leaves. The spots enlarge and turn pale to bright yellow. Tan tufts of fungal growth develop on the bottoms of leaves under the yellow spots. Downy mildew is most common after flowering begins. Downy mildew is a distinctly different disease than powdery mildew, which appears white on the surfaces of leaves and usually occurs after mid-August.
Septoria brown spot is also common in soybean fields in Minnesota. Symptoms usually appear first on the lower leaves and then progresses to the mid-to-upper canopy during wet periods throughout the summer. Initial symptoms are small dark brown spots, and they often enlarge and grow together into irregular brown areas that often are associated with yellow patches concentrated on one side of a leaf. The infected leaves may drop, especially in the lower canopy. Septoria brown spot is most common when conditions and leaves are wet and warm for extended periods of time. It rarely develops to high levels of severity that result in significant yield loss, but yield loss can occur if infection starts early after flowering begins and continued wet conditions favor disease development. Similar to other foliar diseases of soybean, Septoria brown spot typically does not require management because it rarely causes significant losses.
Phytophthora root and stem rot. This has been a very favorable year for Phytophthora root and stem rot in Minnesota, and this disease is more widespread than it has been for several years. This disease is favored by wet and warm soil conditions, especially saturated conditions early in the growing season. In July and August, infected plants develop brown lesions and root rot, and a chocolate-brown discoloration of the stem extends up the stem from the soil line. Leaves turn yellow and brown and typically stay attached. Plants are often killed in patches or in sections of rows. Cultivars with resistance genes such as Rps1c, Rps1k, Rps3 and field tolerance should be planted to reduce Phytophthora root and stem rot. However, one or more of these Rps genes are not effective in many areas due to the presence of forms of the pathogen that overcome the resistance. I have several confirmed reports this year of varieties with Rps1c and 1k being killed by Phytophthora rot.
Stem canker is also developing in fields across southern Minnesota this year. The frequent rain early in the growing season has favored stem canker. This disease often kills or damages scattered plants, but large areas in fields can be killed. Symptoms often develop from mid-July to harvest. Reddish-brown lesions develop on the outer stem at the nodes, and these can develop into brown cankers that spread up and down the stem and kill plant parts above the lesions. The external stem lesions lesions may extend to the soil line and be confused with Phytophthora rot. Reddish-brown discoloration also occurs inside the stem, whereas brown stem rot symptoms in the pith are tan to brown without a reddish tint.
White mold (also called Sclerotinia stem rot) can be a serious problem and has started to develop in soybean fields in southern and central Minnesota. Wet and cool weather during flowering is required for this disease to develop. White mold is also favored by high plant populations, high fertility, narrow rows, and fields where plants dry slowly. This disease is difficult to manage and resistant varieties are a partial solution. Foliar fungicides may be helpful, but have generally performed inconsistently and should be applied soon after flowering begins. The chances of effective disease control appears to be low if initial fungicide applications are done after pods start to form near the tops of the plants and the plants completely fill the rows.
Sudden death syndrome (SDS) is likely to appear soon many fields in Minnesota. Weather conditions this year have been very favorable for this destructive disease to develop. SDS typically starts showing up in early August and becomes more obvious in mid to late-August. It started developing in my research plots in Waseca by July 23 this year, which is earlier than normal. Leaf symptoms often begin on the lower parts of the plant as scattered yellow, diffuse spots between veins. The spots typically expand between veins to become brown lesions surrounded by yellow areas. In addition, tan to gray discoloration develops in the vascular tissue just under the epidermis of the lower stem. The pith in the stem remains white, which distinguishes SDS from brown stem rot. Please see a separate article soon with more information on SDS and a request for samples to be sent in to me (free of charge) to expand our knowledge of where SDS is occurring in Minnesota.
For more information and photographs on these and other soybean diseases, please see the Minnesota Crop Diseases web site (www.extension.umn.edu/cropdiseases/soybean/index.html)