Symptoms

PRR can cause damage throughout the year and can cause seed rots, pre- and post-emergence damping off of seedlings, and stem rot of plants at various growth stages. Damping off may cause a light brown, soft rot to develop on roots or the hypocotyl when the seedlings emerge.

Root and stem rot appears later in the season, and severity is directly related to the level of genetic resistance in the plant. Colonization begins in the roots and moves up the plant. Reddish-brown to black lesions start just below the soil line and may extend several nodes up the stem (Figure 1). The lesion typically encircles the stem and always starts at the plant base. As colonization continues, the roots and stems turn brown and chlorotic leaves wilt, but the petioles remain attached. Cultivars with high levels of partial resistance do not get the stem rot phase, but the plants may be stunted.

Environmental Conditions

Spore germination begins when soil temperatures reach 60°F with high soil moisture. The optimum soil temperature for disease development is 77 to 86°F. This disease is most common in low-lying areas of a field, in poorly drained or compacted soil, and in soils with high clay content. It also may occur in well-drained fields during growing seasons with excessive moisture availability.

Management

Cultural Management: PRR is more severe in poorly drained or flooded areas. Any management practice that improves soil drainage, such as tillage or placing drain tiles, may reduce the incidence and severity of PRR. Because P. sojae oospores can survive in soil for long periods of time, rotation is not an effective management option but will reduce the amount of inoculum, compared with continuous soybeans.

Chemical Control: Seed treatment fungicides can be used for management of early season seed rot and damping off caused by P. sojae. The compounds labeled for control are ethaboxam, metalaxyl, and mefenoxam. Mefenoxam is one of the chemical compounds that has been isolated from metalaxyl and is the most active part of metalaxyl products. As a result of isolating the active component, products containing mefenoxam are typically used at half the active ingredient rate of metalaxyl products.

A list of products containing these compounds and their recommended rate of application is provided in Table I. Generally, all varieties of soybean grown in problem fields should be treated since conditions favoring P. sojae also favor Pythium spp., which commonly also causes seedling problems in Nebraska. While all products listed in Table I have activity against P. sojae, the higher rate is needed for control in most fields with a history of PRR.

Genetic Resistance

The use of resistant varieties is the most effective way to manage Phytophthora root and stem rot of soybean. Genetic resistance in the host is expressed in terms of Rps genes. The race-specific genes confer complete resistance to a specific race of P. sojae. The Rps gene differentials used in race testing are denoted as Rps 1a, 1b, 1c, 1d, 1k, 3a, 6, 7. The pathogen exists in races or biotypes that interact with these genes. A race is identified by its interaction with eight of the 14 known Rps genes. In a resistant reaction, the plant survives infection; susceptible varieties are killed when infection occurs (Table II). Race-specific resistance is effective in the early stages of germination.

Soybean varieties are marketed on the basis of their genetic makeup in relation to the predominant P. sojae races in a given area. The predominant races in Nebraska are 3 and 33. The most widely available resistance genes in the Midwest are 1c and 1k, commonly referred to as “c” or “k” in seed company literature. Gene 1c protects against races 1, 3, 8, 9, 13, 23, 28, 41, and 44, and gene 1k protects against races 1, 3, 4, 5, 8, 9, 13, 14, 18, 23, 43, and 44. Gene 3a is the only gene that protects against 99 percent of the races that occur in Nebraska. Currently, 89 percent of the maturity group 2 and group 3 soybean varieties marketed in Nebraska contain some resistance to P. sojae. Resistance within these maturity groups is listed in Table III. Growers should consult local seed company representatives to request varieties with different Rps genes than those marketed in a specific area.

Table II. A list of the races of Phytophthora sojae identified in Nebraska through 2007 and the soybean plant’s Rps gene reaction to these races (R=Resistant and s=susceptible).

Table III. Number of varieties with Phytophthora sojae resistance genes available in Nebraska for maturity groups two and three in 2015 seed catalogs¹.

1. Based on a survey of Asgrow®, Channel®, Hoegemeyer™, Latham®, Mycogen®, NK®, Pioneer®, and Producers® 2015 seed catalogs.

The other parameter on which soybean varieties are rated for P. sojae is partial resistance (also called field resistance or tolerance). Soybean varieties with high levels of partial resistance may become infected with P. sojae, but symptoms are less severe compared with highly susceptible varieties. In field research trials conducted in Nebraska, high levels of partial resistance performed as well as varieties with resistance genes and partial resistance.

For fields with a P. sojae biotype that is aggressive against the resistance genes available in commercial varieties, this is the only choice for management with genetics. When possible, a combination of good partial resistance and an Rps gene is recommended. Partial resistance alone will not be as effective during early growth stages or under high disease pressure.