Poor stands, skips in rows, or larger areas with few soybean plants are indications of poor seed quality or that one of the pathogens discussed here was active shortly after planting. Many of these fungi also infect and colonize root tissues when soils are saturated for long periods of time, leading to yield reductions in severe cases.
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| Figure 9. Symptoms of post-emergence damping-off. Note dying seedling in center of furrow. | Figure 10. A healthy soybean seedling on left with soybean seedlings with pre-emergence damping-off. Soybean seedlings were dug from the furrow two weeks after planting. |
Phytophthora sojae and Pythium spp.
These two pathogens are water molds. They have a strict requirement for free water to form free-swimming spores (zoospores) which can then infect plants. The zoospores infect roots and colonize the plant, which results in a soft, light brown to tan discoloration. The tissue, in advanced stages, collapses, and when seedlings are pulled from the soil, there are no visible roots. There are several Pythium species capable of causing seed rot and damping-off, and they are active when soil conditions are wet and cool (55F). Some warmer temperature-loving Pythium spp. and Phytophthora sojae are more active when soil temperatures are >60F.
Rhizoctonia spp.
This pathogen survives in the soil as very small sclerotia, and infections occur across all soil moistures. This pathogen typically causes a sunken, brick-red lesion at or just below the soil surface, but seed rot and root rot can also occur.
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| Figure 11. Brick-red lesions on lower hypocotyl which are caused by Rhizoctonia solani. | Figure 12. Wilted seedling with brick-red lesion on lower hypocotyl caused by Rhizoctonia solani. |
Fusarium spp.
Several species of Fusarium have been isolated from roots of soybeans. Damage from Fusarium is usually associated with plants that have been under some form of stress. This is especially true for soybeans exposed to saturated soils or herbicide injury.
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