Relative susceptibility of macadamia rootstocks to fungal stem diseases in nurseries

Macadamia cultivation in tropical and subtropical regions is threatened by various pathogens, affecting different parts of the tree. Currently, there is a lack of information about the prevalence of bacterial and fungal pathogens and whether rootstock cultivars influence the occurrence of stem diseases in macadamia nurseries. We examined over 2000 grafted macadamia plants from commercial nurseries in Australia to identify the pathogens associated with diseased plants using multi‐loci DNA sequencing. as well as detached and in planta pathogenicity assays on two major rootstock cultivars (HAES 695 and H2) to confirm the causal pathogens of stem diseases. The results showed that Diaporthe litchicola and Diaporthe australiana were the most dominant and aggressive species, causing Phomopsis graft dieback, in all five nurseries examined. Stem canker (5%–8%) was caused by five Botryosphaeriaceae (three Lasiodiplodia and two Neofusicoccum species) and was more common and severe on H2 (6.0%) than HAES 695 (4.5%) and Daddow (4.0%) rootstocks. Gall canker caused by Nectria pseudotrichia occurred at low frequencies (<3%) in the nursery plants, regardless of the rootstock cultivars. Crown gall caused by Agrobacterium tumefaciens was observed in a few plants (<1%) only on H2 rootstock plants. This study showed that HAES 695 macadamia rootstock is more tolerant to the stem diseases than H2.