Development of effective method for pathogenicity and screening of Phytophthora spp. on red raspberry seedlings

Root rot and wilting complex (RRWC), primarily caused by several Phytophthora rubi, is an economically important disease of red raspberry in British Columbia. Reliable, rapid, and efficient screening methods are lacking for screening raspberry genotypes in raspberrybreeding programmes. The objective of this study was to compare intact plant root inoculation (IPRI) and detached tissue assays to develop rapid and efficient screening technique for conducting pathological trials and screening raspberry genotypes to RRWC. We compared four IPRI assays, two detached tissue assays and one intact plant stem inoculation (IPSI) assay. A virulent isolate of P. rubi was inoculated in cultivars ‘Chemainus’ and ‘Cascade Bounty’. Experiments were arranged in completely randomized design with three replications in each experiment. For IPRI assays, days to first symptoms development (DFSD), plant wilt progress, and root assessment were recorded. For detached tissue and IPSI assays, DFSD and lesion size were assessed. In susceptible cultivar ‘Chemainus’, all four IPRI assays were found effective assays. DFSD and plant wilt progress were similar among assays. However, variation was observed among four assays in resistant cultivar ‘Cascade Bounty’. Among the detached stem assays, node inoculation method performed better than other methods. Detached stem assays are useful for rapid pathogenicity test of P. rubi, whereas WPRI assays are better for screening germplasms for disease resistance.