Most of the soybean and corn crop is emerged and growing well across Minnesota. Seedling disease problems in scattered soybean and corn fields have been reported in early June and more are expected due to wet and flooded fields. Abundant (or excessive) rainfall and fluctuating temperatures and have created excellent conditions for seedling diseases. This is a good time to check fields for seedling disease problems and efficacy of seed treatments.
Infection of seedlings before or after emergence can result in dead plants, rotted and discolored roots, stunted and discolored plants, and wilting. The problems often occur in patches in fields. Seedling infection can also lead to damage that may not fully develop until mid to late summer, as with Phytophthora root and stem rot and sudden death syndrome. Disease can cause serious damage, but it is just one of many stresses that seedlings are encountering. Careful scouting and diagnosis are often required to identify the cause of a problem.
Scouting and Diagnosis: Scouting fields to identify when, where, and which diseases occur can assist in managing them in future crops and in understanding efficacy of seed treatments. Timely scouting is important because seedling diseases often develop rapidly and seedlings can rot quickly.
Seedling diseases can begin soon after the seed is planted and can continue for several weeks. One clue is reduced emergence and other clues are seedling death, yellowing, or stunting. The problems may start when soil is very wet, although they may be delayed a week or more after wet conditions occurred. Infected plants often show symptoms that can be caused by multiple pathogens or other stress, and cannot be accurately diagnosed without laboratory testing. For example, tan/brown, soft-rot symptoms on roots and stems caused by Pythium and Phytophthora are very similar. Fresh, intact plants without extensive rotting are needed for diagnosis by the University of Minnesota Plant Disease Clinic (pdc.umn.edu/) or other diagnostic laboratories.
Overview of Seedling Diseases and Pathogens. The pathogens that cause seedling diseases of soybean and corn are widespread and persistent in field soils across Minnesota. 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. The most common and damaging pathogens of soybean seedlings appear to be Pythium, Phytophthora, Rhizoctonia, and Fusarium.
The wet and flooded soils across the state after the recent heavy rains are especially favorable for the soilborne, moisture-loving pathogens Pythium and Phytophthora. Pythium can damage soybean or corn seedlings. Phytophthora can damage soybean seedlings or start infections in the spring that may kill soybean plants later in the summer. There are only one or two species of Phytophthora that infect soybean in Minnesota, but there are over 20 species of Pythium that infect soybean. We are studying these diverse Pythium species to learn more about which ones are most damaging and how to manage them. Seed treatments can be effective for some Pythium and Phytophthora problems. Resistant soybean varieties are usually effective for managing Phytophthora, although in some fields the resistance may not be effective.
Rhizoctonia and Fusarium seedling diseases are also widespread problems. Warm and moist soils are also favorable for Rhizoctonia root and stem rot. This can be another extremely damaging problem to soybean and possibly to corn (as well as to other crops). In a field study in Rosemount, MN in 2014, the conditions in some soybean plots were favorable for Rhizoctonia root rot and over 70% of the seedlings were killed by June 16. Some seed treatments can be very effective for managing Rhizoctonia infection of soybean seedlings.
Fusarium is a very common seedling and root rot group of pathogens in Minnesota that can cause also significant damage under favorable conditions. As with Pythium, there are many species of Fusarium (over 10) in Minnesota that can cause root rot on soybean and corn. Seed treatments have not generally been very effective in our studies against Fusarium root rot of soybean, but new treatments under development may be more effective. Additional information on soybean seedling diseases can be found at Minnesota Crop Diseases web site (http://www.extension.umn.edu/agriculture/crop-diseases/).