Sustainable control of husk spot of macadamia by cultural practices

Husk spot caused by the fungal pathogen, Pseudocercospora macadamiae, is a serious disease of macadamia in Australia. P. macadamiae conidia infect the fruit pericarp giving rise to premature fruit abscission. Husk spot shifts the fruit abscission pattern forward such that the peak in fruit abscission occurs earlier in the harvest period resulting in a higher proportion of immature fruit. The fungus survives across seasons on infected split pericarps called sticktights, which do not abscise and remain in the canopy. Large-scale field trials on cultural control measures aimed at breaking the disease cycle to reduce disease pressure and thus crop losses through the removal of sticktights were established. A combination of application of mechanical tree shakers to remove sticktights from the tree canopy and routine monitoring of kernel maturity were examined for their utility as cultural practices, in addition to or as a replacement of chemical spray applications for husk spot control. Although the efficiency of mechanical removal of sticktights varied among trees, the results showed that husk spot severity was significantly (P<0.001) reduced when sticktights were removed in both cultivars 'A16' and 'A38' trees compared to the untreated control trees. Removal of sticktights combined with routine monitoring of kernel maturity so as to schedule harvest rounds by kernel maturity rather than by calendar had a significant impact on reducing the impact of husk spot in our field trials.