Resistance to Phytophthora cinnamomi in Castanea spp. is under moderately high genetic control mainly because of additive genetic variance

Susceptibility to Phytophthora cinnamomi is one of the main traits in Castanea sativa breeding programs. An inoculation experiment using 25 control pollinated families, with seedlings of these families cloned by cuttings, was conducted by soil infestation with one P. cinnamomi isolate. Forty-seven days after inoculation, foliar and root collar symptoms and root necrosis were recorded. The data were analyzed using a model based on restricted maximum pseudolikelihood methods of the GLIMMIX procedure to estimate the additive, dominance and epistatic components of the genetic variance, as well as the narrow sense heritability and the breeding values. At the end of the experiment, the percentages of dead plants ranged from 4% to 56% in C. sativa, and 18% to 20% in backcrosses to C. sativa, with much lower percentages in the F1 hybrids (C. crenata × C. sativa). Foliar symptoms were proportional to mortality, affecting 28% of the plants, but root collar lesions and root necrosis were more prevalent, affecting 65% and 84% of the plants, respectively. The proportions of genetic to phenotypic variance, 0.50–0.63, and the estimated values of narrow-sense heritability, 0.30–0.46, indicate that resistance to P. cinnamomi is under moderate to moderately high genetic control caused mainly by additive genetic variance. A high number of backcrosses to C. sativa showed good breeding values for resistance to P. cinnamomi.