A Greenhouse Technique for Assessing Phytophthora Root Rot Resistance in Glycine max and G. soja.

New sources of soybean (Glycine max) resistance to Phytophthora sojae are needed to provide effective resistance because of the rapidly changing race patterns of P. sojae in fields. The objectives of our study were to develop a method to screen Glycine soja for resistance to P. sojae and then use this methodology to screen G. soja lines for resistance to P. sojae races 1, 3, and 20. An agar plug-inoculation method, in which a 3-mm-diameter mycelial plug of the fungus was placed mycelium side down on cotyledons of 10-day-old soybean seedlings, was directly compared with the traditional hypocotyl inoculation method. There was no significant difference between the hypocotyl- and plug-inoculation methods when tested on four soybean differential lines using three P. sojae races. The plug-inoculation method then was used to screen 430 G. soja accessions for resistance to P. sojae race 3. Nine G. soja accessions were retested with races 1, 3, and 20. Of the 430 G. soja accessions tested, 22 accessions had survival rates higher than 75% and nine had rates higher than 90% against race 3. Additionally, five of the nine accessions that were tested again had greater than 60% survival against races 1, 3, and 20. These results suggest that the plug-inoculation method can be used as an alternative to the hypocotyl-inoculation method. Potential sources of new P. sojae resistance and/or tolerance may be present in G. soja, but additional genetic research is needed to determine if these sources are different from sources currently found in G. max.