Variety selection is key to managing Phytophthora. Photo: Anne Dorrance, Ohio State University
1. Plant resistant varieties
Variety selection is the key for managing Phytophthora stem and root rot. There are two different types of genetic resistance available in soybean varieties.
Race-specific resistance is a complete resistance to a specific race (pathotype) of P. sojae, in which the fungus is unable to colonize the plant tissue. Many soybean varieties have resistance genes, called Rps for “resistant to Phytophthora sojae”. The most common Rps genes that are available include Rps1a, Rps1c, Rps1k, Rps3a, and Rps6 or a combination of one or more Rps genes. This information is available in seed catalogs and in state variety evaluations.
Varieties with the same resistance genes may perform differently because of different levels of partial resistance (also called field resistance or tolerance) to all pathotypes of Phytopthora. 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 the first 7-10 days until seedlings are established, 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.
Phytophthora 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 pathotype (race) has become dominant in your field.
University soybean researchers in the north-central region are actively monitoring Phytophthora pathotypes 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 until the seedling is established. Seed treatments with fungicides can provide some effective 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.
However, planting soybeans year after year can increase the Phytophthora population in the soil and promote the development of new races. Under high levels of inoculum, the effectiveness of partial resistance declines.