Integrated Crop Management

Scout now for Phytophthora root rot

Phytophthora is an old pest to Iowa soybean growers, and most producers know how to effectively handle the disease caused by Phytophthora. In recent few years, the disease has been under control with no severe outbreaks, although it continues to show up in scattered areas. As the season progresses, the ISU Plant Disease Clinic is beginning to receive samples of seedlings killed by Phytophthora.

When does the disease occur?

The disease can affect soybean from the seedling stage to maturity. When Phytophthora root rot strikes early, it causes damping off or seedling blight that can result in stand reductions sometimes mistakenly attributed to environmental conditions or missed seeds during planting. In Iowa, symptoms of this disease are very noticeable after July when temperatures start to increase.

What are the symptoms of the disease?

[1] Seedlings killed by Phytophthora.
[2] A brown lesion caused by Phytophthora stem rot.

Stand reduction occurs when the disease infects at the seedling stage. Diseased seedlings have rotted roots and wilting in aboveground parts of the plant. Leaves turn gray then yellow, and remain attached to the plants. Because the Phytophthora fungus enters and colonizes on plant roots, roots of infected plants are discolored, rotten, and eventually are destroyed. Brown lesions often develop, leading from the soil line up the stem of the plant. The lesion will girdle the stem and kill the plant, and if not killed, the plant will be stunted. Infected plants often are found in circular patches in low spots of fields. Groups of infected plants also may be found scattered throughout a field.

What conditions are favorable to the disease?

Phytophthora root rot is prevalent in heavy, clay soils or soils with poor drainage during wet weather. In seasons where heavy rainfall occurs early and in poorly drained soils, the disease can become severe. Heavy, clay soils are ideal environments for the fungus. Clay contains small soil pores that quickly become filled with water and do not drain well. Periodic rainfall patterns, one week of wet weather followed by dry weather, are ideal for the development of this disease. Warm soil temperatures of 75-80° F are optimum for infection. This year's cold, dry spring should have slowed the early season development of the disease.

How does tillage affect the disease?

In a regional project, plant pathologists have found that no-till practices increase the risk of Phytophthora root rot, especially for seedling damping-off. There are two reasons for the increase:

  1. The pathogen survives better in no-till compared with conventional tillage. When infested soybean residues are left on the soil surface, the fungus is distributed mainly near the soil surface and, therefore, has a greater chance of contact with seedlings.
  2. Soil compaction in no-till results in poor drainage, which is favorable to disease development.

What should be done about this disease?

To reduce seedling damping off, reduce compaction and improve drainage. However, use of resistant varieties is the most effective way to control the disease. See the April 7 ICM newsletter (page 26) for information on how to select resistant cultivars. The fungicide metalaxyl is very effective in control of Phytophthora. This fungicide (under the trade name Apron) can be used for seed treatments to control damping-off or seedling blight. Chemical seed treatments are recommended in fields with a high risk of damping-off caused Phytophthora.

This article originally appeared on pages 122-123 of the IC-478(15) -- June 30, 1997 issue.

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