Diversity and pathogenicity of species of Botrytis, Cladosporium, Neopestalotiopsis and Pestalotiopsis causing flower diseases of macadamia in Australia

Flower diseases threaten macadamia production, but there is a limited understanding of their aetiology. This study was undertaken to establish the ecology of the causal agents in commercial orchards in Australia. Raceme samples at different developmental stages were collected from 42 commercial macadamia orchards during the 2018-2020 flowering seasons. Morphological and molecular phylogenetic analyses of the isolates obtained from the samples with symptoms revealed that 16 fungal species, including two Botrytis spp., six Cladosporium spp., six Neopestalotiopsis spp. and two Pestalotiopsis spp., were associated with the flower blights. Twelve of the fungal species were identified as new records in macadamia. A comparison of the fungal genera showed that Neopestalotiopsis spp. (48%) and Cladosporium spp. (47%) were the most prevalent in the majority of macadamia production regions and were obtained from all four raceme developmental stages. The genus Botrytis was more commonly distributed (16%) in the production areas where humidity was high and was mostly isolated from the mature developmental raceme stages. Based on in planta infection assays under field conditions, the fungal pathogens varied in aggressiveness on macadamia racemes, with B. cinerea, B. macadamiae, C. cladosporioides, C. devikae, N. drenthii, N. macadamiae, N. maddoxii, N. vheenae, P. macadamiae and P. telopeae as the most aggressive and C. angulosum and C. proteacearum as the least aggressive. This study improves our understanding of the aetiology and distribution of the causal agents of flower blight complex in macadamia.