Root rot and wilting complex of red raspberry: Pathogen characterization, identifying host resistance and sustainable management options

Root rot and wilting complex (RRWC) of red raspberry, caused by Phytophthora rubi and potentially other species of Phytophthora, is an economically important disease in British Columbia (BC) and in raspberry producing regions globally. In this study, multiplex targeted-sequencing methods were used to identify Phytophthora species causing RRWC, various screening methods were developed against RRWC, and virulence diversity and metalaxyl-m sensitivity of P. rubi were evaluated, in addition to different registered and experimental fungicides to help identify sustainable management strategies. Among 128 isolates recovered during 2018 to 2020 from diverse raspberry growing areas in BC, P. rubi was identified as the main species associated with RRWC using multiplex targeted-sequencing and was predominantly isolated from raspberry cultivars 'Chemainus', 'Rudi' and 'Meeker'. In addition, P. gonapodyides was identified as a causal agent of RRWC in BC and isolated from moderately resistant cultivar 'Cascade Bounty', indicating that the current sources of resistance to RRWC may not be adequate against all species of Phytophthora. Several intact plant root inoculation (IPRI) assays, detached stem assays and an intact plant stem inoculation assay were compared in greenhouse by inoculating a virulent isolate of P. rubi in two commercial cultivars - 'Chemainus' (susceptible to RRWC) and 'Cascade Bounty' (moderately resistant to RRWC). Three IPRI assays (MSR, MST, and VOP) produced reliable symptoms in both cultivars. Among the detached stem assays, a node inoculation method performed better than other methods. Twenty-four and 44 isolates of P. rubi were evaluated for their virulence and metalaxyl-m sensitivity. All isolates of P. rubi were pathogenic on 'Chemainus' raspberry. About 14% of isolates showed reduced sensitivity to metalaxyl-m. This study also evaluated different registered and new Oomycota fungicides in the field over three years. Experimental products (tank mix of oxathiapiprolin and metalaxyl-m) alone or in combination with cyazofamid provided the effective control of RRWC and these experimental products could potentially be registered against RRWC.