Plant Health Initiative

Phytophthora Management

Variety selection is key to Phytophthora control. Photo credit: Craig Grau, University of Wisconsin

1. Plant resistant varieties

Variety selection is the key for managing Phytophthora stem and root rot. Many soybean varieties marketed in the North Central region have resistance genes (Rps genes). The most common Rps genes that are available include Rps1a, Rps1c, Rps1k, and Rps3a. This information is available from seed distributors and state variety evaluations.

Race-specific resistance is a complete resistance to a specific race of P. sojae, in which the fungus is unable to colonize the plant tissue.

Varieties with the same resistance genes may perform differently because of different levels of partial resistance (also called field resistance or tolerance) to Phytopthora infection. Soybean varieties with high levels of partial resistance can become infected with P. sojae but symptoms are not as severe as varieties that are highly susceptible. Partial resistance will not be as effective during early growth stages or when disease pressure is high.

For fields or regions where varieties with Rps1a, 1c, or 3a are developing Phytophthora root rot, plant a variety with a different Rps gene. For fields or regions where varieties with Rps1k are developing disease, plant a variety with Rps3a or Rps gene combination, combined with partial resistance.

2. Monitor the performance of the Rps genes

Growers should monitor the performance of the resistance package of the soybean varieties they choose. If optimum disease conditions for Phytophthora infection occur during the growing season, scout those areas of the fields to look for stem rot development. If a large number of plants with Phytophthora stem rot are found, make a note to choose varieties with a different Rps gene and higher levels of partial resistance for the next season.

The Phytophthora fungus is known to adapt to the Rps genes of soybean varieties, but it's a slow process. Careful monitoring of plant performance is all that is needed. If a large number of plants with Phytophthora stem rot are found when optimum disease conditions occur, this may indicate that a new race or pathotype has become dominant in your field.

University soybean researchers in the North Central region are actively monitoring Phytophthora races in the region in order to advise soybean seed companies of changes in Phytophthora populations. Switching or stacking Rps genes in new soybean varieties may be recommended.

3. Treat seeds

Partial resistance to P.sojae is not effective in the seed or early growth stages. Seed treatments with fungicides can provide some early protection for those fields where P. sojae has been a continuous hurdle. Effective seed treatments for Phytophthora seed and root rot have metalaxyl or mefenoxam (Metalaxyl-M) as an active ingredient.

4. Improve soil aeration, drainage and structure

Wet and waterlogged soils provide a favorable environment for many soilborne pathogens including P. sojae. Use good soil management practices. Improve soil drainage through tiling or tillage, except when tillage will compact the soil. Compact soil and poor soil structure leads to poor aeration and increased disease levels.

5. Rotate crops

Crop rotation prevents the rapid build-up of inoculum but will not eliminate the disease or eradicate Phytophthora because the oospores can survive in the soil for long periods of time. Planting soybeans year after year can increase the Phytophthora population and promote the development of new races. Under high levels of inoculum, the effectiveness of partial resistance declines.

Contributors:
Anne Dorrance, Ohio State University
Loren Giesler, University of Nebraska
Dean Malvick, University of Minnesota
Laura Sweets, University of Missouri