With most of the soybean and corn crop emerged and growing across Minnesota - it is a good time to assess fields for seedling disease problems and the potential benefits or failures of seed treatments. The recent fluctuating temperatures and abundant rainfall that resulted in surplus topsoil moisture in about 21% of the state last week (USDA-NASS data) have created good conditions for seedling diseases and root infection by a complex mix of pathogens in many fields. Scattered problems with seedling diseases have been reported.
Seedling infection can result in dead plants before or after emergence, stunted and discolored plants, wilting, and complete or partially rotted and discolored roots. The problems often occur in a circular or random pattern in the field. Seedling infection can also lead to damage that may not be seen until mid to late summer, as is common with Phytophthora rot and sudden death syndrome. Disease is just one of many stresses that seedlings are encountering in fields. Close inspection and thorough diagnosis are often required to accurately identify the cause.
The pathogens that cause these problems are widespread and persistent in Minnesota fields. Favorable conditions for infection and plant damage include wet and compacted soils, cool or warm soil depending on the pathogen, and poor seed quality. Slow plant emergence and growth, crusted soil, and fertilizer or herbicide injury may also enhance the problems. Seedling diseases can begin soon after the seed is planted and can continue for a month or more.
The most common soilborne fungi and fungal-like pathogens of soybean appear to be Fusarium, Pythium, Rhizoctonia, and Phytophthora. Each of these groups of pathogens, except for Phytophthora, also infect corn. These four pathogen groups are not composed of a single type of pathogen. We have been learning that these pathogens are more complex than previously known.
Recently we have learned that there are at least 10 different species of Fusarium in Minnesota that can cause root rot on soybean and possibly corn. There also appear to be over 15 different species of Pythium that infect soybean roots in Minnesota based on a 2011 survey of seedlings. The importance of these different species is unknown, but they may be infect under different environmental conditions, may have different levels of aggressiveness to soybean and corn, and might have different levels of sensitivity to seed treatment 'fungicides'. Several types of Rhizoctonia are known that preferentially infect different crops such as soybean, corn, and sugarbeet. Many different pathotypes (races) of Phytophthora occur in Minnesota, some of which can overcome Rps resistance genes in soybean.
Infected roots often show symptoms that can be caused by multiple pathogens and cannot be conclusively diagnosed without laboratory testing. For example, tan/brown, soft-rot symptoms on seedling roots caused by Pythium and Phytophthora are very similar. In contrast, the reddish- brown, often sunken lesions caused by Rhizoctonia are easier to recognize. The symptoms associated with Fusarium infection are typically light to dark brown lesions.
Scouting fields to identify when, where, and which diseases occur can assist in managing them in future crops and in understanding efficacy and or failures of seed treatments. Timely scouting is important because seedling diseases often develop rapidly and seedlings can degrade quickly. Intact, non-degraded plants are needed for diagnosis by the University of Minnesota Plant Disease Clinic (pdc.umn.edu/) or other diagnostic laboratories. Accurate diagnosis helps to focus management plans, which differ for different pathogens and diseases. More information on soybean seedling diseases can be found at Minnesota Crop Diseases web site (www.extension.umn.edu/cropdiseases/soybean/index.html).