Identifying an Elite Panel of Apple Rootstock Germplasm with Contrasting Root Resistance to Pythium ultimum

Abstract

Apple replants disease (ARD), incited by a soil borne pathogen complex, is a major obstacle to establishing an economically viable apple orchard at replant sites. The predominant control method is pre-plant chemical fumigation of orchard soil, which is expensive and comes with environmental and regulatory concerns. To maximize the exploitation of host resistance for ARD management, high quality resistance phenotypes in apple roots to ARD pathogen infection are required for elucidating the underlying resistance mechanisms. In this study, root resistance responses to Pythium ultimum infection were systematically evaluated among the ‘Ottawa 3’ × ‘Robusta 5’ (O3R5) F1 progeny. Tissue culture-based micro propagation was employed to generate genetically-defined and age-equivalent apple plants for repeated infection assays. A wide range of plant survival rates were observed, with fewer than 30% for the susceptible genotypes and over 80% for the resistant ones. The levels of root and shoot biomass reduction among the surviving plants varied substantially between the most resistant genotypes and the most susceptible genotypes. Contrasting necrosis patterns were demonstrated along the infected roots between resistant and susceptible genotypes using a novel glass-box pot for continuous microscopic observation. Swift necrosis occurred across the entire root system within 24 hours for the susceptible genotypes; in sharp contrast, evidently deterred root necrosis was observed for the resistant genotypes. A well-defined boundary separating healthy and necrotic root tissues were often accompanied with the infected roots of resistant genotypes, while the profuse growth of P. ultimum hyphae was specifically associated with the infected roots of susceptible genotypes. The results from this study represented the first comprehensive and detailed effort undertaken to define the genotype-specific resistance responses in apple roots as they are challenged by a soil-borne pathogen.