Interaction of Juglans species with Phytophthora citricola

Due to commercial interest in cultivating English walnut cultivars on their own roots rather than on typical rootstocks, we evaluated relative susceptibility to Phytophthora citricola on own-rooted cultivars Chandler and Vina and seedling rootstocks Northern California Black (NCB) and Paradox hybrid. In March 2003, dormant 1-year-old rootings of Vina and Chandler (originally from tissue culture) and comparable seedlings of NCB and Paradox hybrid were dug from a nursery and transplanted into 2-liter pots (one plant per pot) in a greenhouse. On May 15 and 28 (Experiments I and 2, respectively) 10 plants per walnut selection were inoculated with P citricola (90 ml of V8 juice-vermiculite-oat substrate colonized by the pathogen was placed in the soil of each pot), and 10 plants per selection received sterile substrate as a control. All of the plants were flooded for 48 hours at biweekly intervals to facilitate disease development. Two to three months after inoculating, the plant root systems were washed free from soil to determine disease incidence and severity. There was no significant interaction between experiment and the treatment factors. The combined means for percent of crown length rotted by R citricola on NCB, Paradox, Vina and Chandler was 89, 30, 59, and 82, respectively; whereas that on noninoculated plants was 0 to 5. Percent of crown circumference rotted by P citricola averaged 93, 24, 65, and 83 on NCB, Paradox, Vina and Chandler, respectively; but that on the controls was 0 to 6. For the crown rot variables, interaction between inoculation treatment and walnut selection was highly significant (P=0.0001 to 0.0005). The results suggest that under conditions highly conducive to disease, own-rooted 'Chandler' and 'Vina' and Northern California Black rootstock are highly susceptible to P citricola, whereas Paradox offers marginal tolerance to the pathogen.