Phytophthora damping-off of soybeans

Phytophthora root rot of soybean is a persistent and widespread problem affecting much of Iowa. We have received many reports of damping-off caused by Phytophthora sojae in the last few years, and with the warm soil conditions and wet weather in early June, we anticipate that it will be a problem again this year.

Damping-off by Pythium/Phytophthora.

Phytophthora root rot is prevalent in heavy clay and waterlogged soils. The disease is usually present in low spots or in other areas with poor drainage. Stand reduction occurs when the disease infects at the seedling stage and causes root rot and wilting of plants. A poor stand can sometimes be mistakenly attributed to herbicide damage or to other adverse environmental conditions. Large brown lesions develop leading from the soil line up the stem of the plant. These lesions eventually girdle the stem and kill the plant. Roots may become discolored and stunted and eventually die.

An Iowa State study, funded by the Iowa Soybean Promotion Board, showed that fields with conservation tillage have large populations of Phytophthora sojae. The amount of the disease was greater under conservation tillage than under conventional tillage in most soil types and especially in clay soils. By leaving debris on the soil surface instead of burying it by conventional tillage, the fungus is distributed near the surface of the soil where it will probably come in contact with seedlings. Soil compaction caused by conservation tillage also may reduce drainage and provide a favorable environment for the disease to develop.

Varietal resistance still remains the best bet to control this disease. Common resistance genes found in commercial soybean varieties sold in Iowa are shown in the table. Rps1k is one of the most widely used resistance genes because it is effective against races 1, 3, and 4, the most prevalent races of Phytophthora found in Iowa. Race 25 can overcome Rps1k, as well as other resistance genes. Although race 25 can be found in the state, it comprised less than 5 percent of races isolated in a 1996 study done by Iowa State researchers. Since that study was conducted, we have noted an increasing number of instances where the Rps1k gene has been overcome. Race 25 may be becoming more common. We still recommend planting a variety with Rps1k if a grower has problems with damping-off, although, the possibility exists of failure.

If Phytophthora causes severe stand loss due to damping-off, replanting should be undertaken with the addition of a fungicide seed treatment such as Apron. A uniform coating of fungicide on the seed coat is critical for effective control.

Table 1. Resistance gene reaction to races of Phytophthora.

Resistance gene
Race Rps1a Rps1c Rps1k
1 R R R
3 S R R
4 S S R
25 S S S

R-Resistant; S-Susceptible.

This article originally appeared on page 105 of the IC-482(15) -- June 21, 1999 issue.

Updated 06/20/1999 - 1:00pm