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What causes rachis tip dieback in macadamias?
Quick find: this post is based on the published summary (abstract) of the study, not the full paper.
Here is the puzzle. Botrytis blight, Cladosporium blight, Pestalotiopsis blight and Phytophthora blight are distinct diseases of macadamia flowers, and rachis tip dieback (RTD) is part of the symptoms of all of them. Yet some racemes show flower blight without RTD, while others have RTD but keep healthy flowers, and RTD has also been linked to physiological and environmental stresses. A team from the Centre for Horticultural Science at the Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, went looking for its cause in 20 commercial macadamia orchards in Australia (Xu et al., 2026).
What this means for your grow
The authors conclude that RTD has a biotic cause and is part of the flower blight disease complex in macadamia. They found that several fungal species can infect the same racemes and occur together in the same tree showing RTD. The published summary reports no treatment or control trial, so this is not something to act on at home yet; it is background for commercial macadamia growers and researchers working on flower blight.
What did they test and find?
- The survey: RTD in 20 commercial macadamia orchards in Australia, over the flowering seasons of 2023 and 2024.
- The samples: 307 raceme samples with RTD and 54 asymptomatic samples. DNA sequencing identified 20 fungal genera.
- The most common fungi in RTD samples: Neopestalotiopsis (27.5%), Epicoccum (20.4%), Diaporthe (19.1%), Cladosporium (7.8%), Alternaria (5.6%) and Fusarium (5.6%).
- The main culprits: pathogenicity assays with representative isolates and flower blight pathogens showed that N. maddoxii, N. macadamiae and E. italicum were the primary pathogens associated with RTD.
- Known blight pathogens can do it too: established flower blight pathogens were capable of inducing RTD symptoms.
- New to macadamia: species of Diaporthe and Epicoccum are reported in the study as novel pathogens of macadamia racemes.
- Mixed infections: the high diversity of fungal species showed that multiple species can infect the same racemes and co-occur in the same tree expressing RTD symptoms.
What could it mean?
In the authors' words, the study revealed that RTD has a biotic cause and is part of the flower blight disease complex in macadamia. The full paper discusses how RTD contributes to yield losses and a potential tool for predicting flower blight severity to support management decisions, but the abstract does not give those details. For now it is a result for commercial macadamia growers and researchers, not a tested method to try at home.
What were the limitations?
The published abstract does not state the study's limitations.
Trial details
- Who: Centre for Horticultural Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Brisbane, QLD, Australia (the authors' affiliations).
- Where: 20 commercial macadamia orchards in Australia.
- When: the flowering seasons of 2023 and 2024.
- Crop: macadamia, sampled as racemes.
- Samples: 307 raceme samples with RTD and 54 asymptomatic samples.
- Methods: fungi identified by DNA sequencing; pathogenicity assays with representative isolates and flower blight pathogens.
- Results: 20 fungal genera identified. Most frequently isolated from RTD samples: Neopestalotiopsis 27.5%, Epicoccum 20.4%, Diaporthe 19.1%, Cladosporium 7.8%, Alternaria 5.6%, Fusarium 5.6%. Primary pathogens associated with RTD: N. maddoxii, N. macadamiae and E. italicum.
- Funding: Hort Innovation (award MC21001) and a University of Queensland Research Training Scholarship, as listed in the paper's Crossref record.
- Publication: Plant Disease 110(10), pages 4390-4400, October 2026 (first published 30 September 2026, according to Europe PMC), https://doi.org/10.1094/pdis-02-26-0303-re.
- Not reported in the published abstract: the states or regions of the orchards, the cultivars, tree age, orchard management and spraying, weather during flowering, how the pathogenicity assays were set up and their statistics, how common RTD was across the orchards, the yield-loss findings, and conflicts of interest.
Links
- The study: Etiology of Rachis Tip Dieback of Macadamia Flowers in Australia
- The institution: Centre for Horticultural Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland
References
- Xu, X., Rincón-Flórez, V. A., Carvalhais, L. C., & Akinsanmi, O. A. (2026). Etiology of rachis tip dieback of macadamia flowers in Australia. Plant Disease, 110(10), 4390-4400. https://doi.org/10.1094/pdis-02-26-0303-re