Phenotyping Castanea hybrids for Phytophthora cinnamomi resistance

Castanea sativa is susceptible to Phytophthora spp., a serious root pathogen causing ink disease, while C. crenata and C. mollissima show resistance to infection. Interspecific controlled crosses were established for introgression of resistance genes from the resistant species into the susceptible C. sativa, and two mapping populations were created. Phytophthora cinnamomi resistance of each progeny was evaluated by root and excised shoot inoculation tests. The number of days of survival after root inoculation was the best discriminator of resistance to P. cinnamomi while the percentage of shoots with internal lesions was the symptom most associated with survival. The lesion progression rate in the excised shoot inoculation test was strongly and negatively correlated with survival in the root inoculation test. The excised shoot inoculation test appears to be a reliable approach for screening the resistance of chestnut genotypes to P. cinnamomi. Strong genetic correlations were obtained between survival and ink disease symptoms and among symptoms, indicating that common or linked genes might influence resistance to P. cinnamomi. The most resistant genotypes selected from this study will be tested for other commercial variables, such as ease of vegetative propagation and stock–scion compatibility.