Soybean crop conditions have been highly variable across Minnesota, as has been the soybean disease situation. Rhizoctonia root and stem rot was widespread in June and early July. Seedling diseases of various types and Phytophthora root and stem rot were also reported early in the season. Since then several mid to late season soybean diseases have appeared in multiple fields, including pod and stem blight at significant levels, and sudden death syndrome, brown stem rot, and white mold.
Pod and stem blightIn the past 2-3 weeks, concerns have been raised in across southern Minnesota about pod and stem blight killing all or parts of plants in many fields. Pod and seed infections have also been seen. Many samples have been sent to St. Paul for diagnosis. There are no characteristic stem symptoms caused by this disease, but rows of black dots (fungal structures) often form in rows on dying stems (see photos below). The pathogen that causes pod and stem blight overwinters in debris from soybean and some weeds, and infection is favored by wet weather at all growth stages. Our studies have shown that this pathogen is widespread in soybean plants across Minnesota, even when they appear healthy.
The conditions that lead development of damaging disease seems to be uncertain, although wet weather, virus infection, early maturity, and other types of plant stress may be contributing factors. Soybean varieties vary in susceptibility to pod and stem blight, and results with fungicides have been variable.
Sudden death syndrome (SDS)SDS can be one of the most significant soybean diseases. Leaf symptoms of SDS have been reported this year in soybean fields scattered across southern Minnesota. Although some fields were significantly damaged, this was not a severe year for SDS overall. The last year with widespread SDS problems in Minnesota was 2010.
Why have we not seen more SDS in the past few years and what conditions lead to it becoming a significant problem? SDS is a complicated disease that is controlled by many different variables. One of the most important factors based on observations and irrigated field studies is rainfall (or irrigation) in June, July, and August. For several years, adequate rainfall in many areas in May and June has favored early season root infection, but SDS foliar disease didn't develop. The same was true this year. This suggests that even with favorable conditions for infection, rainfall in July and early August is critical for development of SDS.
Using Waseca, MN as an example, only 1.7" of rain fell from July 1 to August 20, 2014. In contrast, in 2010 when SDS was widespread in the Waseca area, there were 8.5" of rain in the same period. Further evidence for the importance of rainfall in July and August was reported in Iowa, where they found that rainfall averaged over 13" during this period in SDS years and under 8" in non-SDS years from 1993 - 2011. This information helps us to understand when and where outbreaks of SDS occur. The key to managing SDS is planting soybean varieties with resistance to SDS, although there may be additional options in the near future.