Soybean seedling diseases can cripple the establishment of soybean stands. A cool, wet winter has set the stage for cooler soil temperatures and high moisture levels in fields. These elements could create the perfect environment for diseases to take hold, resulting in an economic loss for farmers.
Alabama Cooperative Extension System plant pathologist Ed Sikora said soybeans infected with a seedling disease are typically weak and less vigorous.
“When stand loss is severe, growers may need to replant a field or a portion of a field if it is exhibiting signs of significant damage,” Sikora said.
Factors and diagnosis
Seedling diseases are most common when wet weather precedes and follows planting in the spring. Sikora said compacted and poorly drained soils often predispose seedlings to infection.
“Differentiating seedling diseases in the field can be difficult since the symptoms of various seedling disorders are similar,” he said. “The three most common seedling diseases of soybean in Alabama are Pythium seedling blight, Rhizoctonia seedling blight and Fusarium root rot.”
Sikora said diagnosing seedling blight is difficult because plants can be infected with multiple pathogens.
“Seedling blight symptoms can be confused with symptoms caused by herbicide damage,” he said.
Pythium seedling blight
Symptoms of Pythium seedling blight include rotten, soft and 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 disease severity. Growers commonly see symptoms of this disease in poorly drained soils and areas that flood regularly.
Rhizoctonia seedling blight
Symptoms of Rhizoctonia seedling blight include reddish brown lesions on the seedling’s lower stem or hypocotyl, usually at the soil line. These lesions appear sunken and dry.
Affected plants typically appear in patches in the field. This disease can occur over a wide range of soil conditions. Temperature and moisture requirements for infection vary, but seedlings stressed by soil compaction or other factors may be more susceptible to the disease.
Fusarium root rot
Sikora said many species of the fungus Fusarium live in the soil and can infect soybean. Infected plants may appear stunted and spindly, and roots may have a brown or black discoloration. Affected plants often have a poorly developed root system.
In severe cases, seedlings may die before emerging from the ground. Fusarium can infect plants under a wide variety of environmental conditions and is often associated with stressed plants.
Sikora said crop rotation may not effectively manage seedling diseases because the fungal pathogens can survive in infested soil for a long period of time. The fungal pathogens affecting soybean seedlings are capable of infecting other agronomic crops.
Seedling diseases often show more prevalence in no-till or reduced tillage systems since these soils typically warm up slower in the spring and retain more moisture.
In these systems, Sikora said additional practices, such as fungicide seed treatment, may be required to manage seedling blights. Fungicide seed treatments vary in efficacy, and products that control Pythium do not affect Rhizoctonia and Fusarium species and vice versa.
Therefore, it is important to accurately diagnose the seedling blights present in a field and choose fungicide seed treatments accordingly.
“For an accurate diagnosis, we suggest submitting symptomatic plants to a local diagnostic lab along with relevant background information for the field to confirm the cause of the problem and to determine an appropriate management program,” Sikora said.
Alabama Cooperative Extension contributed this article.