A fungal pathogen is unlikely to be the cause of abnormal vertical growth syndrome in macadamia

Abnormal vertical growth (AVG) in macadamia is a syndrome that reduces flowering and alters tree architecture, resulting in significant yield losses. The epidemiological parameters suggest the cause is probably a biotic agent with the ability to modulate plant hormone production or signalling. To determine if a fungal pathogen is the cause of AVG, we compared the fungal profile of macadamia trees with or without AVG symptoms using conventional culturing and DNA metabarcoding techniques. A diverse range of fungi was isolated from leaf, root and stem samples of macadamia cultivars HAES 344 and A16. The abundance of the isolated fungi was at least three-fold higher in trees with AVG symptoms than in trees at the non-AVG site. Similar results were obtained using DNA metabarcoding. Although the fungal genus Sebacina was the most dominant in the DNA metabarcoding, whereas Diaporthe (>40%) was dominant using the culturing techniques, no unique fungal species was consistently associated with all AVG samples. Therefore, we conclude that it is unlikely that the cause of AVG in macadamia is a fungal pathogen.