Botryosphaeriaceae causing branch dieback and tree death of macadamia in Australia

Incidence of branch dieback of macadamia with characteristic symptoms including dark necrotic lesions on the wood and blackening of the vascular tissue is increasing in commercial macadamia orchards in the south-eastern production regions in Australia. In many cases, disease progresses from the branch to the main trunk resulting in total tree death and reduced orchard productivity. Previously, only Botryosphaeria ribis was associated with branch dieback of macadamia, however, recent observations suggest other species in the Botryosphaeriaceae may be involved. This study aimed to identify the causal agents of branch dieback of macadamia and examine their pathogenicity on a main macadamia cultivar (HAES 246). Thirty-four representative Botryosphaeriaceae isolates, obtained from over 150 samples of branch dieback symptoms on macadamia trees, were identified using the partial sequence of the internal transcribed spacer of the rDNA and partial sequences of β-tubulin and elongation factor gene regions. Six species in the Botryosphaeriaceae were identified. Lasiodiplodia pseudotheobromae (n = 18; 53%) was the most prevalent species, followed by Neofusiccocum parvum (n = 5; 14%) , L. iraniensis (n = 4; 12%), N. luteum and L. theobromae (n = 3; 9% each) and N. australe (n = 1; 3%). Using an in planta assay, pathogenicity tests showed that all six species caused dieback and necrotic lesion symptoms on macadamia plants. Inoculated plants died within 4 weeks of inoculation, showing the characteristic symptoms (blackening of the wood tissue). Inoculation trials revealed differences in aggressiveness among the six species.