Evaluation of in-field inoculation methods for characterisation of macadamia germplasm to husk spot incidence and severity

Abstract

Husk spot, a fungal (Pseudocercospora macadamiae) disease of macadamia pericarps, induces premature abscission in several major commercial cultivars. Breeding for resistance to husk spot is a priority of the Australian macadamia industry. Due to the large tree size of macadamia and high numbers of progeny in breeding populations, inoculating for resistance screening is laborious and time consuming. Previously utilised methods included direct applications of P. macadamiae suspensions and hanging of bags of diseased husks above developing fruit in tree canopies. In this study, both methods were modified to allow for efficient application in large-scale breeding populations and their efficacy was evaluated. Two quantities of diseased husk per bag: 'large' (75 g) and 'small' (30 g) and two concentrations of sprayed P. macadamiae suspensions: 5 x 105 propagules/mL 'stock' and 'dilute' 2.5 x 105 propagules/mL were tested across two fruiting seasons. Treatments were compared against a control (sterile water) in commercial cultivars 'A38' and 'A4'. Husk spot incidence and severity produced by small bags were significantly affected by season. A significant season effect was less common for other treatments. All four treatments infected over 50% of target fruit in each season, but the highest husk spot incidence across both seasons (≥85%) was produced from large bags. Overall, the large bags were the most reliable method for infection of target fruit. Results also demonstrate the importance of considering the effect of season when selecting husk spot inoculation methods.