Variability and inheritance in macadamia progenies to Phytophthora cinnamomi and P. multivora the causal agents of root rot and stem canker

Phytophthora species cause root rot and stem canker in macadamia, resulting in yield reduction and tree death. Differences in disease severity between trees in commercial orchards may be attributed to genetic factors and stochastic variation. Here we examined variability in half-sib, open-pollinated macadamia seedling progenies and their maternal parents to Phytophthora stem, root and leaf infections. Open-pollinated seeds were obtained from trees of five commercial macadamia cultivars. Leaves were obtained from the same trees from which the seeds were collected. Leaf, stem and roots were inoculated separately with P. multivora and P. cinnamomi using in vivo and in planta assays. Disease severity parameters were obtained from the pathogen-inoculated plants and compared with the control plants. Both Phytophthora species caused varied levels of leaf lesion, stem canker and root rot in the macadamia cultivars. Root infections by P. cinnamomi were more severe than P. multivora, whereas, stem infections by both Phytophthora species were similar. A moderate to high narrow-sense heritability (h2) were estimated for root efficiency loss by P. cinnamomi (0.60–0.76) and P. multivora (0.36–0.69), which were higher than the combined h2 estimates for both Phytophthora species. The differential interaction of the disease severity parameters for P. multivora and P. cinnamomi suggests variation in their resistance mechanism(s) in macadamia.