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Could other root pathogens be behind macadamia decline?
Quick find: this post is based on the published summary (abstract) of the study, not the full paper.
Of 259 soil samples taken from declining macadamia trees across Australia's major production regions, 41 turned up oomycetes other than Phytophthora. In leaf and seedling tests, representative isolates of all four kinds the team identified caused necrosis.
Phytophthora species are well-documented pathogens of macadamia orchards, but the other oomycetes living around the trees had not been explored. Researchers from the University of Queensland set out to find which ones are associated with declining macadamia trees, and whether they have the potential to cause disease (Rincon-Florez et al., 2026).
What this means for your grow
If you grow macadamias, this is a reminder that Phytophthora may not be the whole story when trees decline: the authors say these other oomycetes could contribute to the macadamia root disease complex. One result stands out for anyone raising young trees: Globisporangium spinosum was isolated only from nurseries, and it caused the largest area of leaf necrosis in the tests. The published summary does not report any treatment or control being tested, so there is no new fix to try yet.
What did they test and find?
- The samples: bulk and rhizosphere samples from trees showing decline symptoms, in 73 commercial orchards and nurseries, collected over 8 years. Of the 259 samples, only 15% (41) yielded isolates other than Phytophthora.
- What turned up: ITS and COX1 sequencing identified four taxa across three genera. Pythium diclinum/lutarium made up 53.7% (22 isolates); Phytopythium vexans (10) and Phytopythium helicoides (1) together made up 26.8%; and Globisporangium spinosum made up 19.5% (8).
- Where they were found: Ph. vexans occurred in all production regions, while Ph. helicoides was found only in south-east Queensland. P. diclinum/lutarium came mostly from the southern production range, and G. spinosum came only from nurseries.
- Leaf tests: on detached leaves, G. spinosum caused the highest leaf necrotic area (47.2%), followed by P. diclinum/lutarium (40.8%), Ph. helicoides (38.1%) and Ph. vexans (33.7%).
- Seedling tests: Ph. vexans and G. spinosum caused significantly (p < 0.05) higher root infection than P. diclinum/lutarium.
What could it mean?
The authors conclude that three oomycete genera other than Phytophthora are associated with the rhizosphere and bulk soils of declining macadamia trees, which highlights their potential contribution to the macadamia root disease complex. For now this is a result for macadamia growers, nurseries and plant pathologists to be aware of. The published summary does not say how much of the decline seen in orchards these organisms cause, or how to manage them.
What were the limitations?
Limitations are not reported in the published abstract. The disease tests reported there used detached leaves and seedlings, not orchard trees.
Trial details
- Who: Centre for Horticultural Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Dutton Park, Queensland, and the School of Agriculture and Food Sustainability, The University of Queensland, St Lucia, Queensland (the authors' affiliations).
- Where: 73 commercial macadamia orchards and nurseries across Australia's major production regions. The abstract names south-east Queensland and the southern production range.
- Crop: macadamia trees showing decline symptoms.
- How long: sampling over 8 years.
- Samples: 259 bulk and rhizosphere samples; 15% (n = 41) yielded non-Phytophthora isolates.
- Identification: ITS and COX1 sequencing; four taxa across three genera.
- Tests: pathogenicity assays on detached leaves and on seedlings, using representative isolates of all four taxa.
- Results: isolates: P. diclinum/lutarium 53.7% (n = 22); Ph. vexans (n = 10) and Ph. helicoides (n = 1) together 26.8%; G. spinosum 19.5% (n = 8). Leaf necrotic area: G. spinosum 47.2%, P. diclinum/lutarium 40.8%, Ph. helicoides 38.1%, Ph. vexans 33.7%. Seedling root infection significantly higher (p < 0.05) for Ph. vexans and G. spinosum than for P. diclinum/lutarium.
- Funding: Hort Innovation (awards MC16018 and MC21001), as listed in the paper's Crossref record.
- Publication: Plant Pathology 75(4), e70265, July 2026 issue (published online 23 August 2026), https://doi.org/10.1111/ppa.70265. Licence: CC BY-NC-ND 4.0.
- Not reported in the published abstract: the years of sampling; which states and sites were sampled, beyond south-east Queensland; the decline symptoms; macadamia cultivars or rootstocks and tree ages; how the nursery trees were grown; the plants, conditions, number of isolates and replication used in the leaf and seedling tests; statistics beyond p < 0.05; conflicts of interest; and limitations.
Links
- The study: Globisporangium, Phytopythium and Pythium Species Showed Pathogenic Potential on Macadamia in Australia
- The institution: Centre for Horticultural Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland
References
- Rincon-Florez, V. A., Hsiao, C. C., Kamaruddin, R., Jeff-Ego, O. S., & Akinsanmi, O. A. (2026). Globisporangium, Phytopythium and Pythium species showed pathogenic potential on macadamia in Australia. Plant Pathology, 75(4), e70265. https://doi.org/10.1111/ppa.70265