Integrated Crop Management

Identification of soybean seedling diseases

Stand establishment problems may be increased this spring because of the higher than normal precipitation and low seed quality. I have received more disease samples or questions on damping-off up to mid-May compared with this same period the past three springs. Determining which fungi are the cause of a problem in a particular field is key to managing seedling diseases because these fungi require different management treatments. This article describes how to identify fungal diseases in soybean.

How to identify seedling diseases

In Iowa, 90 percent of seedling diseases are caused by four fungi: Pythium, Phytophthora, Rhizoctonia, and Fusarium. From year to year, Pythium and Phythophthora accout for 50-60 percent of the problems, Rhizoctonia for approximately 20-30 percent, and Fusarium for 10 percent. Other fungi cause minor problems.

Pythium damping-off is the first seedling disease to occur in a growing season because this fungus prefers cold soil temperatures. Dead seedlings may be visible on the ground with infected plants killed before the first true leaf stage. Plants often have a rotted appearance. Leaves of infected seedlings are initially gray-green and then turn brown. A few days later, the plants die. Diseased plants are easily pulled from the soil because of rotted roots.

Damping-off by Pythium/Phytophthora.

The symptoms of Phytophthora damping-off are very similar to those of Pythium damping-off. However, the Phytophthora fungus prefers warm soil (approximately 80°F). If symptoms similar to those caused by Pythium occur when soil temperatures are warm or in late-planted soybean, the disease is more likely caused by Phytophthora. Another way to determine the causal fungus is to check for stem rot. In June or early July, if weather is favorable, Phytophthora infection may continue to develop on the soybean stem, resulting in chocolate brown discoloration from the soil line up, a unique symptom of this disease.

Phytophthora stem rot.

Similar to Phytophthora damping-off, seedling blight by Rhizoctonia normally appears when the weather becomes warm. However, disease caused by Rhizoctonia exhibits different symptoms from those caused by Pythium and Phytophthora. Stem discoloration by Rhizoctonia is usually limited to the cortical layer of the main root and hypocotyl. Infected stems remain firm and dry without rot appearance typical to Pythium or Phytophthora rot. Typical symptoms are localized brown-to-reddish brown lesions on the hypocotyl at the soil line.

Seedling disease by Rhizoctonia.

What to do if you find seedling diseases

Damping-off severity generally occurs at three levels:

  1. scattered dead plants in a field but no significant stand reduction,
  2. obvious stand reduction but replanting is not needed, and
  3. severe reduction resulting in replanting.

If you decide to replant, consider seed treatment to reduce the risk of further damping-off. Select fungicides targeting the fungi that cause the seedling blight. For situations such as those mentioned in 1 and 2, you do not need to do anything this year if the disease is Pythium or Rhizoctonia damping-off. Take good disease notes for preventive measures such as seed treatment to reduce the likelihood of a disease problem in the next soybean crop.

If you have Phytophthora damping-off on a Phytophthora-resistant variety, the resistance has been defeated by the fungus. Although the disease may continue to develop into stem rot in the middle of the growing season, major damage by Phytophthora damping-off in Iowa is to seedling stages. Consider using a variety with a better resistance gene for the next soybean crop.

This article originally appeared on pages 79-80 of the IC-486(10) -- May 21, 2001 issue.

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