Soybean seedling diseases are one of the most important causes of reduced stand establishment and can result in economic losses. 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 exhibiting significant damage.

Seedling diseases are most common when wet weather proceeds and follows planting in the spring. 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.

The following are the three most common seedling diseases of soybean in Alabama:

• Pythium seedling blight
• Rhizoctonia seedling blight
• Fusarium root rot

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. Symptoms of Pythium blight are most common 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. 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

Many species of the fungus Fusarium live in the soil and can infect soybean. Infected plants may be 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.

Determining Infection and Avoiding Disease

Diagnosing seedling blights is difficult because plants can be infected with multiple pathogens and symptoms can be confused with symptoms caused by herbicide damage. 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.

Crop rotation may not effectively manage seedling diseases because these fungal pathogens can survive in infested soil for a long period of time and many are capable of infecting other agronomic crops. Seedling diseases may be more prevalent in no-till or reduced tillage systems since these soils typically warm up slower in the spring and retain more moisture. In these systems, 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.