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Seedling Diseases in Soybeans

06/26/2018
Seedling Diseases in Soybeans
  • Several common soybean seedling diseases have shown up this season.
  • There are soybean seedling diseases for almost all environments.
  • Diagnosis of seedling diseases can be difficult.
  • Soybean fungicide seed treatments don’t always last long enough to completely protect against seedling diseases.

Common Soybean Seedling Diseases and Environmental Requirements

Seed, seedling, and stem diseases have been reported from widespread soybean fields. The most common diseases being reported include Phytophthora, Pythium, Rhizoctonia, and Fusarium. In general, most seed and seedling diseases of soybeans are favored by wet soil conditions. However, infection by these pathogens is also promoted when soybeans are under stress and/or growing slowly.

Phytophthora and Pythium are Oomycetes, fungus-like organisms. These pathogens require wet soils, or at least abundant free moisture in the soil. Oomycetes, like Phytophthora and Pythium, produce a swimming spore that moves to, and infects, the soybean roots and hypocotyl. Phytophthora infection usually occurs with warm, wet soil conditions. Pythium infection usually occurs with cool, wet soil conditions, although there are multiple species of Pythium, each having slightly different requirements for infection.

Rhizoctonia solani usually requires warm, wet soil conditions for infection but there are different strains of this pathogen. Some can infect under slightly drier soil conditions. Rhizoctonia can also continue development in the soybean plant after soils dry out.

There are multiple species of Fusarium that can attack soybeans. Conditions favoring infection are variable. Some species infect best under cool, wet conditions, while others are favored under warm and dry conditions.

Most of the common seed, seedling, and stem diseases of soybeans are present in every field. Continuous cropping of soybeans allows a buildup of these pathogens. Some of these diseases can be transported on dirty seed. Reduced tillage may contribute to the amount of disease inoculum present on crop residue in a field, but this association has not been well established.

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Disease Symptoms

It can be very difficult to identify these seed, seedling, and stem diseases of soybeans in the field, but we can often narrow the diagnosis.

Both Phytophthora and Pythium can cause seed rot, seedling damping off and taproot plus stem infection. Both initially cause a mushy tan or brown rot of root and stem tissue. Larger plants may experience loss of lateral roots with both diseases. With soybean plants that survive seedling Phytophthora infection, the taproot and lower stem often develops a brown discoloration. Active Pythium infection usually ceases as soils warm and dry out, while Phytophthora can continue to affect soybean plants throughout the growing season.

Typical Rhizoctonia symptoms in seedling soybeans includes rusty-brown lesions on the lower stem and taproot. These lesions may be sunken into the stem or taproot and may be on one side of the plant, or may girdle the stem, killing the plant. With soybean plants that have Rhizoctonia infection beneath the soil line, adventitious roots sometimes emerge above the infected area.

There are no truly typical symptoms for Fusarium infection of seed, seedling soybeans and soybean stems because of the large number of Fusarium species that may be at work on the plant. Fusarium often causes light-to-dark brown lesions on the soybean taproot and lower stem. This sometimes takes on more of a “dry rot” appearance. Lateral roots may decay and disappear, and the infection may penetrate into the pith of the taproot and stem. As with Rhizoctonia, adventitious roots sometimes emerge above infected tissue.

injury 2.jpg

Soybean Seed, Seedling and Stem Disease Management

General recommendations for management of seed, seedling and stem diseases of soybeans include the following:

  • Practice crop rotation.
  • Avoid soil compaction.
  • Plant clean, certified seed.
  • Plant soybeans into a warm, well-drained seedbed.
  • Use seed treatments.
  • For Phytophthora, select resistant seed varieties.

Seed treatments should be selected that have proven efficacy against diseases that are common in your area. Where Phytophthora or Pythium are a threat, select seed treatments effective against Oomycete diseases. Remember that seed treatments usually provide protection against soybean diseases for about 2 weeks, depending on product and dosage. Diseases can still attack soybeans when protection from seed treatments begins to fade.


Image 1:  Phytophthora Injury Conditions

Image 2:  Rhizoctonia Symptoms on Soybean Root

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