Resistance in wild macadamia germplasm to Phytophthora cinnamomi and Phytophthora multivora

The four Macadamia species (M. integrifolia, M. tetraphylla, M. ternifolia and M. jansenii) occur naturally in the wild in fragmented habitats in Australia and there is limited information on their vulnerability to pathogens including species of the genus Phytophthora. Macadamias in commercial orchards are affected by Phytophthora cinnamomi and Phytophthora multivora causing stem canker and root rot. Wild germplasm is often regarded as sources of resistance in macadamia breeding programme. We assessed the performance of 152 trees of wild macadamia genotypes in the field using a Phytophthora disease severity rating scale and used in vivo leaf assay to examine their susceptibility to P. cinnamomi and P. multivora. Macadamia ternifolia trees showed the highest Phytophthora disease severity compared with the other species. In the in vivo trial, there were significant variations in disease severity among the genotypes within each Macadamia species. Comparison of the mean leaf lesion area of the Macadamia spp. showed that M. tetraphylla and M. jansenii were the most resistant to P. cinnamomi, whereas M. ternifolia and M. jansenii followed by M. tetraphylla had the least disease severity to P. multivora. The quantitative variations among the genotypes with strong differential effects resulted in demarcation of the wild genotypes into three resistance groups. Overall, a total of 14 M. tetraphylla and 2 M. integrifolia genotypes were identified as resistant to both P. cinnamomi and P. multivora in the in vivo assay.