Soybean Seedling Rots and Blights

Soybean seedling infected with Rhizoctonia damping-off, blight, and rot
Soybean seedling infected with Rhizoctonia damping-off, blight, and rot. Photo courtesy of Howard F. Schwartz. Licensed under CC BY 3.0.
Soybean seed rots and blights can have big implications on a farming operation, causing seeds to rot in the ground. The lack of emergence is at times bad enough to necessitate replanting. It is most damaging in wet, compact soil or in areas where seedlings are damaged. There are four main types that are grouped together since they are so difficult to tell apart. In general, diseased seedlings are weaker and low in vigor. All four diseases survive in infected plant residue or soil.
Fusarium is most likely to occur in temperatures less than 60 degrees F. It tends to attack seeds and seedlings, causing light to dark brown lesions throughout the root system. The roots will likely appear shrunken. It can also attack tap roots, promoting adventitious root growth close to the surface of the soil. Infected plants are generally stunted and spindly, with black or brown discolored roots.
Rhizoctonia is more likely to occur in soils that are warmer (70 to 80 degrees F. It can attack seeds or un-emerged seedlings, causing them to decay and die. Seedlings and older plants that become infected have a firm, rusty-brown decay (or lesion) on the root or lower stem that appear to be sunken in and dry. Some infections result in girdling of the plant, quickly followed by death.
Rhizoctonia Lesions on Soybean
Rhizoctonia lesions on soybean plants. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Phytophthora is more likely to occur in soils that are warmer (70 to 80 degrees F). It causes pre and post emergence damping off. The symptoms are soft, tan-brown, rotted tissue and rotting of the roots. If the seedling is at the V1 stage, infected stems will be soft and bruised, while the leaves will turn yellow. Most often premature death is a result. These symptoms are almost indistinguishable from that of Pythium.
Pythium is most likely to occur in temperatures less than 60 degrees F. It is very difficult to tell apart from Phytophthora, and often it cannot be told apart without taking samples into the lab. The main visible symptom is soft, brownish, rotting tissue. It is most common in seed and seedling damage but can impact established plants at their roots, stunting growth. Areas affected tend to be those with high residue and compact soil.
These diseases are very common in wet and compact soils, often occuring after a period of rainy weather preceded by planting. No-till fields are especially susceptible. Due to such small pathogen resistance in seedlings, the infection can be fatal. If there are enough such cases, a replant will be necessary.
Before beginning a managment program, ensure that the right diagnosis has been made. This can be done through lab testing. Primarily, it remains important to plant good-quality seed in well-drained and fairly loose soil. It is best to delay planting until soil reaches a higher temperature and loses most of its moisture. In general, crop rotation has proved marginally effective.
In the case that a replant is necessary, it is important to diagnose the specific disease and then apply the appropriate fungicide to the soil before replanting. Products containing mefenoxam or metalayl are known to be effective against both Pythium and Phytophthora. Products with fludioxonil or strobilurin are known to be effective against Fusarium and Rhizoctonia, which are true fungi. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.