Source of Pseudocercospora macadamiae inoculum in macadamia trees and its use for characterising husk spot susceptibility in the field

Abstract

Pseudocercospora macadamiae causes husk spot of macadamia. Husk spot control would be improved by knowledge of the persistence of the pathogen in orchards between seasons, and by characterising macadamia cultivars for susceptibility to infection, premature fruit abscission, and spatial escape from inoculum sources. Field observation of seven macadamia cultivars found a significant exponential relationship between husk spot incidence and the prevalence of fruit pericarp that failed to abscise (sticktights) in the canopy. Viable conidia of P. macadamiae were readily (>100 conidia per fruit) produced on sticktights for 30 months. The incidence of husk spot was up to four times greater in trees with sticktights than trees without sticktights. Assessment of the propensity to form sticktights, husk spot incidence and fruit abscission after the insertion/removal of sticktights, demonstrated differences in macadamia cultivars; cv. A16 was susceptible to infection and formation of sticktights, but tolerant of premature abscission; cv. 246 was susceptible to infection, tolerant of premature abscission, and spatially escapes P. macadamiae inoculum due to a lack of sticktights; cv. A38 was susceptible to infection, premature abscission, and sticktight formation. Our findings suggest that disease could be reduced by removing sticktights from trees, and that fungicide inputs could be reduced in trees with naturally low numbers of sticktights. In addition we show that sticktights can be used as an inoculum source for field assays to better characterise macadamia cultivars for better informed cultivar selection, and as a tool in breeding programs to screen for germplasm with high levels of resistance.