Eager to Plant? What to Know About Soybean Seedling Diseases

RALEIGH, N.C. — As planting season approaches, it’s important to be mindful of environmental conditions that may favor the development of soybean seedling diseases—and to be prepared with strategies for early identification and management. Soilborne fungi and fungal-like pathogens can take advantage of vulnerable seedlings, leading to stand loss and, in more severe cases, substantial yield reductions. These opportunistic pathogens tend to strike early, especially in wet conditions or when seedlings are already stressed by issues like waterlogged soils, insect or nematode damage, or herbicide injury.

Common soybean seedling diseases include Fusarium root rot (Fusarium spp.), Rhizoctonia seedling blight (Rhizoctonia solani), Phytophthora root rot (Phytophthoraspp.), and Pythium seedling blight (Pythium spp.). While diseased seedlings are often visibly affected, accurately diagnosing the specific cause can be challenging. Multiple pathogens may be present at same time resulting as a disease complex, and their symptoms frequently overlap, making it difficult to distinguish among them without further investigation.

Fusarium root rot is often associated with stressful growing conditions resulting in both pre- and post-emergence damping off, leading to delayed emergence and stunting of infected seedlings. Root rot symptoms typically appear as dark brown discoloration, with lesions extending through the vascular tissue of the hypocotyls, often accompanied with missing lateral roots. Above ground symptoms include chlorosis (yellowing) of cotyledons, and wilting occurring as patches in the field.

Rhizoctonia seedling blight favors warm and moist, but not saturated soils. This disease can cause both pre- and post-emergence damping off with symptoms displaying reddish-brown, sunken lesions that girdle the hypocotyl and lower stem at the soil line, typically appearing in patches or portions of entire rows within the field.

Phytophthora root rot commonly occurs in warm and saturated conditions, typically after heavy rainfalls resulting in standing water. Symptoms of infected plants include mushy, water-soaked stems, wilting, and stunting commonly occurring in low-lying, wet-natured patches within the field.

Pythium seedling blight prefers cool, wet conditions that result in rotten, mushy seeds with poorly developed roots. Above ground symptoms include water-soaked lesions on the hypocotyl and/or cotyledons that often occur in low-lying areas and poorly drained soils.

While seedling diseases can be tricky to manage, there are ways to mitigate risk and protect yield. Planting soybeans at optimal seeding rates, planting depths, and during environmental conditions that favor quick establishment of stands will reduce plant stress, in turn reducing the risk of disease. Crop rotation may be helpful but is not always effective due to the wide host range associated with Fusarium and Rhizoctonia species, and the persistence of Phytophthora and Pythium species in the soil. Fungicidal seed treatments can be effective, but one treatment may not protect against every seedling disease. Early diagnosis and documentation of field history can assist with decisions associated with seed treatments, such as modes of action against historically known disease presence.

To find more information on scouting for seedling diseases, seed treatments to use, and general information on soybean seedling diseases visit the following links:
Scouting for Seedling Diseases – Crop Protection Network
Fungicide Efficacy for Control of Soybean Seedling Diseases – Crop Protection Network
Soybean Seedling Diseases – NC State Extension Fact Sheet

This post was co-authored by Shannon Holliday, a M.S. student in the Department of Entomology and Plant Pathology, advised by LeAnn Lux.

–Dr. LeAnn Lux, N.C. State University