Sources of Resistance to Phytophthora cinnamomi in Juglans spp. for Potential Rootstocks

A diverse collection of Juglans species and hybrids with potential to serve as walnut rootstocks was evaluated to identify resistance to Phytophthora cinnamomi , a destructive pathogen affecting commercial production of Persian walnuts worldwide. A total of 35 Juglans genotypes, comprising hybrids and Juglans microcarpa , plus three Juglans regia genotypes as comparison, were inoculated during two seasons (spring and autumn) for 3 years, with two isolates of P. cinnamomi . Inoculations were carried out on excised shoots of the different genotypes by replacing a bark disk with a P. cinnamomi culture plug. After incubation, necrosis length caused by the pathogen was measured. Data were collected and statistically analyzed with generalized linear mixed models. This work pointed out a seasonal influence on some Juglans genotype response to P. cinnamomi: both hybrids and J . microcarpa groups were significantly less sensitive in autumn compared with spring ( P = 0.0006), thus this condition must be considered when selecting Juglans for resistance to this pathogen. Three genotypes of J. regia , known for its susceptibility to P. cinnamomi , were used as comparison. Results show good levels of resistance to P. cinnamomi in J. microcarpa genotypes, confirming literature results. Among J. microcarpa genotypes, the Jmi03 is quite promising for its in vitro propagation. A number of Juglans hybrids, H5/18, 7/28,8/29, 10/43, and 6/22, showed significantly higher levels of resistance to P. cinnamomi , compared with susceptible J. regia genotypes. Evaluation, in naturally infected fields, of clonal genotypes and seedling-progenies of resistant genotypes, grafted with commercial walnut varieties, is currently under way and will provide additional information for successful usage.