Diagnosing Soybean Seedling Diseases

Diagnosing Soybean Seedling Diseases

Soybean seedling diseases are one of the most important causes of reduced stand establishment and can cause economic losses, according to an article on CropProtectionNetwork.org. Diseased seedlings are often weak and less vigorous, and severe stand loss may require replanting. Seedling diseases are often more prevalent when wet weather follows planting. They are generally more common in compacted and poorly drained soils or in areas where seedlings have been injured.

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Diagnosing seedling diseases in the field can be difficult since the symptoms of various seedling blights are very similar. This article looks at the characteristics of the organisms that cause seedling diseases, describes injury symptoms that may appear similar to these diseases, and briefly addresses how to manage seedling diseases.

The Crop Protection Network examined the following soybean seedling diseases:

  1. Fusarium root rot
  2. Rhizoctonia seedling blight
  3. Phytophthora root rot
  4. Pythium seedling blight

Fusarium Root Rot (Fusarium spp.)

Many Fusarium species reside in the soil and can infect soybean. Infected plants may be stunted and spindly, and roots may have brown or black discoloration. Affected plants may also have poorly developed root systems.

In severe cases, seedlings may die before emerging. Fusarium species can infect plants under a wide variety of environmental conditions. Fusarium root rot is often associated with stressed plants.

Rhizoctonia Seedling Blight (Rhizoctonia solani)

Rhizoctonia seedling blight is caused by the fungus Rhizoctonia solani. The characteristic symptom of Rhizoctonia seedling blight is reddish brown lesions on the seedling’s lower stem or hypocotyl, usually at the soil level. Lesions on the diseased stem appear sunken and dry.

This disease can occur over a wide range of soil conditions. Affected plants typically appear in patches. Temperature and moisture requirements for infection vary, and stressed seedlings may be more vulnerable to infection and disease.

Phytophthora Root Rot (Phytophthora sojae)

Phytophthora root rot is caused by the oomycete Phytophthora sojae. Infected plants appear alone or in patches. P. sojae also causes a stem rot characterized by chocolate brown stem lesions, but the symptoms of the seedling phase resemble the symptoms of many other seedling diseases.

Phytophthora-infected seedling stems are soft and water-soaked. Overall, infected seedlings will be wilted and stunted. Phytophthora root rot occurs across many environments, but is most common in warm (>60°F/15°C) and wet conditions.

Pythium Seedling Blight (Pythium spp.)

Many Pythium species of oomycetes can cause soybean seedling blight. Pythium species cause symptoms similar to Phytophthora sojae, which is also an oomycete. 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 disease severity. Consequently, symptoms are most severe in poorly drained soils and areas prone to flooding.

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