Prevalence, identity and seasonal variation of leaf diseases in Australian macadamia nurseries

Macadamia is an Australian native tree grown for its edible kernel in tropical and subtropical regions worldwide. However, there is limited knowledge about the fungal leaf pathogens that can infect macadamia nursery plants and potentially reduce productivity. To address this gap, we surveyed major commercial nurseries in Australia to identify leaf diseases affecting macadamia plants in different growing seasons. We identified four main leaf diseases: yellow halo leaf blight, brown leaf blight, anthracnose, and black leaf blight, each caused by multiple fungal species. Leaf anthracnose was the most prevalent disease observed in all the nurseries across the four seasons. From mature leaves with anthracnose symptoms, we isolated eight Colletotrichum spp., but only C. siamense and C. gloeosporioides sensu lato were found to cause infection in pathogenicity assays. In addition, we isolated eleven Neopestalotiopsis and two Pestalotiopsis spp. from yellow halo leaf blight lesions and found that N. clavispora and N. maddoxii were capable of causing disease. We also determined that the A. alternata and A. tenuissima in the Alternaria alternata species complex caused black leaf blight only in non-suberised new leaf flush. For brown leaf blight, Neofusicoccum parvum and Ne. luteum were isolated from symptomatic leaves, both of which caused similar levels of disease. Our findings indicate that several pathogen species are responsible for at least four leaf diseases in macadamia nurseries, suggesting a potential productivity risk when affected plants are planted in the field.