Submitted to: Joint Meeting of the American Phytopathological Society and Society of Nematology
Publication Type: Abstract Only
Publication Acceptance Date: 8/1/2007
Publication Date: N/A
Technical Abstract: Phytophthora crown and root rots cause serious losses to commercial walnut production worldwide. Horticulturally acceptable walnut rootstocks resistant to Phytophthora spp. and other soilborne pathogens are needed. From 1997–99, more than 75 Paradox seed families (hybrids among black walnut species and English walnut [Juglans regia]) were evaluated for resistance to P. citricola (Pcit), an aggressive and prevalent pathogen on California walnuts. The trials, conducted in a greenhouse, subjected plants in pots of soil with and without Pcit to repeated episodes of soil flooding conducive to infection by the pathogen but not harmful otherwise. Seedlings selected for putative resistance to Pcit, tolerance to other pathogens, or parentages of interest were clonally multiplied, rooted, and included in repeated evaluations of resistance to Pcit during 2001–06. Among 13 clones screened in two or more repeated trials, two (RX1, J. microcarpa × regia; and VX211, J. hindsii × regia) consistently expressed moderate resistance to Pcit. All other selections, which included complex and simple hybrids among black walnut species (J. major, hindsii, nigra, and californica) expressed moderate to high degrees of susceptibility to the pathogen in one or more trials. Clones RX1 and VX211 are promising rootstocks for walnut and will be released for commercial use.