Resistance to Species of Phytophthora Identified among Clones of Juglans microcarpa × J. regia

Abstract

Species of Phytophthora are serious soilborne pathogens of persian (english) walnut, causing crown and root rot and associated production losses worldwide. To facilitate the development of improved walnut rootstocks, we examined resistance of 48 diverse clones and seedlings of Juglans species to P. cinnamomi and P. citricola. Plants were micropropagated, acclimatized to a greenhouse environment, and then exposed to the pathogens in artificially infested potting soil mix. Inoculated plants, as well as noninoculated controls, were subjected to soil flooding for 48 hours every 2 weeks to facilitate infection by the pathogens. Two to 3 months after inoculation, resistance to the pathogens was assessed according to the severity of crown and root rot. Clonal hybrids of J. californica × J. regia were highly susceptible to the pathogens (means 52% to 76% root crown length rotted), while several clones of J. microcarpa × J. regia were significantly less susceptible (means 8% to 79% crown length rotted). Among clones of other parentages tested, including: J. microcarpa, (J. californica × J. nigra) × J. regia, J. hindsii × J. regia, (J. hindsii × J. regia) × J. regia, [(J. major × J. hindsii) × J. nigra] × J. regia, and J. nigra × J. regia, responses varied, but tended to be intermediate. When ‘Serr’ scions were budded or grafted on J. microcarpa × J. regia clone ‘RX1’ or Paradox (J. hindsii × J. regia) seedling rootstocks in a commercial orchard infested with P. cinnamomi, all trees on ‘RX1’ remained healthy, whereas only 49% of those on Paradox survived. Thus, useful resistance to Phytophthora is available among J. microcarpa × J. regia hybrids and is evident in ‘RX1’ rootstock.