Soybean seedling diseases are often the cause of reduced stands and can cause significant economic losses on an annual basis. Diseased seedlings are often less vigorous and severe stand loss may require replanting of affected areas. So far, this spring, the persistent wet weather we have been experiencing has created ideal conditions for the onset of early-season soybean seedling diseases. Diagnosing seedling diseases in the field can be difficult since the symptoms of various seedling blights are similar, and seedlings often decompose quickly. Since it’s too wet to plant, now is a great time to scout your existing emerged fields for stand issues.
Below are common diseases we can find in Knox County.
Fusarium Root Rot
Many Fusarium species reside in the soil and can infect soybean. Infected plants may be stunted and spindly with brown or black lesions on the roots and poorly developed root systems.
In severe cases, seedlings may die before emerging. Species of Fusarium can infect plants under a wide variety of environmental conditions. Fusarium root rot is often associated with stressed plants.
Rhizoctonia Seedling Blight
Is caused by the fungus Rhizoctonia solani. The characteristic symptom of Rhizoctonia seedling blight is a reddish-brown lesion on the lower stem or hypocotyl, usually at the soil level. Lesions on the diseased stem appear sunken (canker-like) and dry, and can girdle the hypocotyl.
This disease can occur over a wide range of soil conditions. Affected plants typically appear in patches or in rows in a field. Temperature and moisture requirements for infection and disease progression vary, although this disease tends to be common in warm and moist, but not saturated, soil. Stressed seedlings may be more vulnerable.
Phytophthora Root and Stem Rot
Is caused by the oomycetes Phytophthora sojae and P. sansomeana. Infected plants appear alone or in patches. The stems of Phytophthora-infected seedlings are typically mushy and water-soaked. Overall, infected seedlings will be wilted and stunted. These symptoms resemble those of many other seedling disease pathogens. Phytophthora sojae can also cause a stem rot of mature plants, characterized by chocolate brown stem lesions. Phytophthora root rot occurs across many environments, but is most common in warm (>60°F) and wet conditions.
Pythium Seedling Blight
Another group of oomycetes, contains multiple species that can cause soybean seedling blight and seed rot. Species of Pythium cause symptoms similar to Phytophthora species.
Pythium seedling blight symptoms include rotten, mushy seeds or seedlings with poorly developed roots. Water-soaked lesions may be present on the hypocotyl or cotyledons.
Pythium seedling blight can occur across a range of temperatures, but high soil moisture increases the likelihood of disease and disease severity. Consequently, symptoms are most severe in poorly drained soils and areas prone to flooding. Recent research indicates that many Pythium species prevalent with temperatures of 50-60°F or 85-95°F.