A Modified Method to Screen for Partial Resistance to Phytophthora sojae in Soybean

Phytophthora root and stem rot, caused by Phytophthora sojae Kauf and Gerd, is one of the most damaging diseases of soybean [Glycine max (L.) Merr.] in the United States. Partial resistance (PR), which is defined as the relative ability of a soybean plant to survive root infection without showing severe symptoms, is an effective way to manage this disease. A modification of the layer test method used to screen for PR to P. sojae in soybean was evaluated. Instead of the P. sojae-colonized agar layer, which is used in the current greenhouse cup assay, P. sojae-infested rice (Oryza sativa L.) grains were used. In addition, a dry root weight (Drw) measurement was compared to a disease severity root rot score, which uses a 1 to 10 visual scale. The rice method was not statistically different from the layer test for the variables evaluated. Advantages of the rice method include reduced cost and the ability to screen soybean germplasm with more than one pathotype of the pathogen in a single assay. A mixture of several pathotypes of P. sojae ensures compatible interactions between isolates used and all known Rps genes, thus avoiding Rps genes that could go undetected and mask PR during screening. Although collecting and handling of roots for Drw data may require more time, it is a more objective variable, which assures precise scoring, it is not rater dependent, and less training of personnel is required.