Diversity in virulence and metalaxyl-m sensitivity of Phytophthora rubi isolates has implications for raspberry root rot and wilting complex management

Abstract Raspberry root rot and wilting complex (RRWC), primarily caused by Phytophthora rubi, is the most important disease for raspberry producers in Canada. There is currently little to no information on phenotypic diversity or sensitivity to metalaxyl-m of P. rubi isolates affecting raspberries in Canada. This study was conducted to understand the diversity in virulence and sensitivity to metalaxyl-m of P. rubi populations collected from diverse raspberry cultivars, fields, and locations during 2018–2020. All isolates of P. rubi (n = 24) were pathogenic on ‘Chemainus’ raspberry and showed significant variation (P < 0.05) in days to first foliar symptom development (DFSD) and progress in wilting index, as indicated by relative areas under disease progress curves (rAUDPC). Principal component analyses identified that root rot index, rAUDPC, DFSD, and percentage reduction in root length and weight were important variables in assessing pathogen virulence. Metalaxyl-m sensitivity was assessed using a radial growth assay and effective concentrations that inhibited 50% of mycelial growth (EC50) were determined for 44 isolates. The EC50 values ranged from 0.035–4.71 µg mL−1 with a mean value of 0.793 µg mL−1. About 61% of the isolates were very sensitive (EC50 values <0.20 µg mL−1), 25% of isolates were considered sensitive (EC50 values of 0.21–1.18 µg mL−1), and 14% of isolates showed reduced sensitivity to metalaxyl-m (EC50 value range of 1.19–4.71 µg mL−1). Knowledge of pathogen phenotypic diversity will be useful for raspberry breeders in screening genotypes against RRWC and the results of the fungicide sensitivity study will be useful in developing management strategies for RRWC.