Resistance of Castanea Clones to Phytophthora Cinnamomi: Testing and Genetic Control

The resistance of chestnut clones to Phytophthora cinnamomi was evaluated by a soil inoculation experiment under controlled environmental conditions, as well as by excised and intact stem tests. One-year-old plants of fifty different clones were inoculated with two isolates of Phytophthora cinnamomi and evaluated fourteen weeks after inoculation. There were significant differences among clones for all root and collar rot variables. There were significant differences for isolates of P. cinnamomi but only for the collar rot variables. A total of 84% of plants grown in infested soil showed symptoms of root rot but only 50% of the plants with root rot, showed also had collar rot. The roots of resistant clones were able to confine the colonization, in roots and from roots to collar. Percentage circumference of collar rot was the best indicator or descriptor of sensitivity, a 50% of clones were resistant or highly resistant clones, with respectively less than 20% and than 10% circumference of collar rot. Percentage of survival of plants is not sufficient to indicate resistance to the pathogen, as mortality may be affected by environmental conditions or by other pathogens. The clonal heritability of collar rot variables ranged between 0.54 and 0.71. The plants grown on inoculated soil showed a reduction in growth. The phenotypic and genotypic correlations between soil infestation characteristics and the length of necrosis in both intact and excised stem tests were positive and highly significant and indicated similar resistance rankings of the clones.