Pathogenicity, Fungicide Resistance, and Genetic Variability of Phytophthora rubi Isolates from Raspberry (Rubus idaeus) in the Western United States.

Root rot of raspberry (Rubus idaeus), thought to be primarily caused by Phytophthora rubi, is an economically important disease in the western United States. The objectives of this study were to determine which Phytophthora species are involved in root rot, examine the efficacy of different isolation methods (cane, root, and root/soil baiting with young raspberry plants), and determine if pathogenicity, fungicide resistance, and/or genetic variation exists among P. rubi isolates collected from raspberry fields in Washington, Oregon, and California. Of 275 samples, direct isolation from cane material resulted in a greater number of P. rubi isolates (39%), whereas root/soil baiting yielded the least (11%). Sequencing of the internal transcribed spacer region of 210 of the total 597 collected Phytophthora isolates showed that all but one isolate (identified as P. bisheria) were P. rubi. Results of the pathogenicity and fungicide resistance to mefenoxam comparing 14 total isolates from Washington, Oregon, and California showed that isolates were similarly virulent against red raspberry and the EC₅₀ frequency distributions showed no significant difference. These results, combined with amplified fragment length polymorphism results show that P. rubi isolates from Washington, Oregon, and California represent one large mixed population. This work provides novel insights into the isolation and biology of P. rubi in western U.S. raspberry production systems.