Seed treatments are frequently used to manage Phytophthora seed rot and damping-off in soybean. Metalaxyl (Allegiance-FL) and mefenoxam (ApronXL) are the only compounds used to treat soybean seed for control of Phytophthora sojae. Metalaxyl has been used for many years in several formulations, and mefenoxam is a similar chemical with the same site of action that was introduced in the past 5 years for soybean seed treatment.
A number of other Phytophthora species have developed insensitivity to these two compounds, most notably, the late blight pathogen of potato and tomato, P. infestans. Questions have been raised recently concerning the possibility of resistance to metalaxyl or mefenoxam in Phytophthora sojae. To address this question, plant pathologists in the north-central region have initiated studies funded by the North Central Soybean Research Program to determine whether P. sojae is losing sensitivity to these compounds.
Isolates of P. sojae were collected from multiple locations in Illinois, Ohio, and Ontario, and were tested in the laboratory for sensitivity to meta-laxyl and mefenoxam. The research is ongoing, but the preliminary results suggest that P. sojae is highly sensitive to these compounds in all of these areas. In Illinois, 34 isolates from 20 different counties were tested, and all were sensitive to 1 µg/ml of metalaxyl and mefenoxam. In Ohio, 33 isolates from different locations were tested, and all were sensitive to 5 µg/ml of metalaxyl. In Ontario, 40 isolates of P. sojae from six counties in eastern Ontario have been evaluated on 5 µg/ml of mefenoxam, and all were sensitive at that concentration. Additional isolates from this region will be evaluated during the summer and fall. These preliminary results are very promising and indicate that metalaxyl and mefenoxam continue to be effective compounds for control of early-season infection of soybean seed and seedlings by Phytophthora sojae over a wide geographic area in the north-central region.--Dean Malvick, Anne Dorrance (Ohio State University, email@example.com), Terry Anderson (AAFC/Harrow, Ontario, firstname.lastname@example.org), and Albert Tenuta (OMAF/Ridgetown, Ontario, albert.tenuta@ omaf.gov.on.ca)