Rachis tip dieback: a phenology-driven, multi-pathogen disease of macadamia racemes

Abstract

Rachis tip dieback (RTD) is an emerging disease of macadamia racemes associated with the flower blight complex. Affected racemes exhibit necrosis progressing from the distal to proximal end of the rachis, causing curvature and greyish black discolouration of florets, ultimately reducing flower formation. This study characterises the aetiology and epidemiology of RTD. Field surveys were conducted on 20 commercial macadamia orchards in Australia at flowering seasons in 2023 and 2024. From over 300 raceme samples, 20 fungal genera were identified using multi-locus DNA sequencing. Species of Neopestalotiopsis (27.5%), Epicoccum (20.4%), and Diaporthe (19.1%) were the most prevalent. Pathogenicity assays confirmed that N. maddoxii, N. macadamiae, and E. italicum caused severe RTD symptoms at the raceme early elongation stage, supporting RTD as a distinct but interconnected component of the flower blight complex. To resolve infection dynamics, single- and mixedspecies inocula containing flower blight pathogens (Botrytis cinerea, Cladosporium angustisporum, N. macadamiae and N. maddoxii) and E. italicum were used to inoculate racemes at different developmental stages. The results revealed that RTD severity reduced with mixed-species inoculations compared to the mean observed across single-species. Disease severity declined when racemes were infected at more advanced maturation phases, and infection occurred exclusively at the distal rachis. These findings demonstrate that RTD incidence is governed by host phenology and multiple fungal pathogens. This understanding provides a basis for predicting disease risk and optimising the timing of management in the macadamia industry.