Extension Plant Pathologist
The "Dreaded" Root Rots Of Soybeans
Finally, much of Minnesota is experiencing more than one warm, sunny day in a row. The soybeans, and other crops, were stuck in a slow growth mode for the last month due to the rainy, overcast conditions. But now the crops should start developing more rapidly. Fortunately, Pythium root rot is no longer a concern since it likes cool wet soils and preys on newly germinated soybean seeds and small seedling. However, Fusarium, Phytophthora, and Rhizoctonia root rots are still a threat to soybeans. The warm conditions contributing to the rapid growth of soybeans may actually result in more noticeable root rot symptoms. Now would be a good time to dig seedling in wet, poorly drained areas of fields to look for root lesions caused by the fungal pathogens. Lesions on roots that appear red, reddish brown, or brown are most likely caused by Fusarium or Phytophthora (see Figure 1). Reddish brown lesions that occur on the stem near the ground level are probably caused by Rhizoctonia (see Figure 2). Watch stands in those areas for stand loss. There is not much you can do this year about the diseases. It is probably too late to benefit from a replant if stand loss has occurred. It would be worth while to identify what root rot diseases predominate in your fields and plan for managing them in the future.