Questions about which soybean diseases will be problems this year in Minnesota come up often.
Although there are no good predictive systems, the wet weather conditions this summer are favorable for sudden death syndrome and Phytophthora root and stem rot. Phytophthora rot occurs across the state. SDS occurs most often in southern Minnesota, but appears to be spreading north each year.
Symptoms of SDS typically appear first in early to mid-August. Symptoms begin on leaves as mottled, yellow spots between veins in the middle parts of the plant canopy. As the disease develops, yellow patches expand between veins and the yellow leaf tissue turns brown. The leaves curl and detach from the petioles. Tan discoloration develops in the vascular tissue just under the surface of the lower stem. The pith remains white, which distinguishes SDS from the more common disease brown stem rot. SDS also causes root rot, and blue fungal growth can sometimes be seen on roots in moist soil.
SDS tends to develop when soybean planting is followed by 2-3 weeks of wet and cool soil, and heavy rains occur frequently in June and July. Compacted soil, poor drainage, high populations of the soybean cyst nematode, and high yield environments also favor development of SDS. Susceptible soybean varieties suffer the greatest yield loss, but no varieties are completely resistant to SDS. As was true in 2010, these conditions occurred in many Minnesota fields this year.
Phytophthora rot has been reported in a number of fields in southern Minnesota at low levels over the past two weeks. This disease is favored by wet and warm soil conditions, and is most common in low, poorly drained areas where saturated soil occurred early in the season. Clay and compacted soils also favor this disease. It can kill plants throughout the growing season from the time of planting nearly until harvest. Disease management is based on using soybean cultivars with major gene (Rps) resistance, but races (pathotypes) of the pathogen exist across Minnesota that overcome the major resistance genes.
Symptoms of Phytophthora root and stem rot include root rot and a dark chocolate-brown discoloration of the stem that extends from the soil line into lower parts of the plant. Leaves turn yellow, wilt, and typically stay attached. Plants are often killed in patches or in sections of rows. This disease can be confused with stem canker, which usually starts at a node and spreads up and down the stems of infected plants.
More information and photographs for SDS and Phytophthora root and stem rot, and a fact sheet on SDS can be found on the Minnesota Crop Diseases web site (http://www.extension.umn.edu/cropdiseases/soybean/index.html)
-By Dean Malvick, U-M Extension