Recent developments in Phytophthora fragariae var. rubi research at the Scottish Crop Research Institute

Phytophthora fragariae var, rubi (PFVR) root rot continues to be one of the most serious diseases of raspberry in Scotland. Planting clean nursery stock helps avoided root rot, but if present, raised bed production, Recoil fungicide (mancozeb + oxadixyl) and resistant genotypes offer control. Success in SCRI's Rubus idaeus breeding programme relies on a thorough understanding of the nature and heritability of host resistance and the virulence of the pathogen population and this paper reports on recent advances in these areas. Race specificity in PFVR is partly defined by the differential response of EM 5605/12 (susceptible to races 1 and 2 and resistant to race 3). Out of 53 cultivars screened against single isolates of the three races, none responded similarly to EM 5605/12; most were equally susceptible to races 1 and 3; 13 had differential responses between races 1 and 3, although none were as definitive as EM 5605/12; and only one cultivar was completely susceptible to race 2. The rarity of differential responses similar to EM 5605/12 and the continuous variation in aggressiveness within races 1 and 3 casts doubt on the differentiation of race 1 from 3 and further investigation is ongoing. The variation among isolates also highlights the importance of selecting representative samples when screening for PFVR resistance. Glasshouse evaluation of progeny from the hybridisation of resistant and susceptible parents produced a range of responses from susceptible to resistant suggesting that there is a quantitative component to PFVR resistance. Although field resistance is the ultimate objective in the SCRI breeding programme, glasshouse progeny testing has proved useful for the identification of families with resistance and evaluating the value of new sources of resistance as parents. These glasshouse procedures have been simplified through inoculation with agar plugs of actively growing mycelium instead of zoospore suspensions.