Spatial pattern and the effects of climatic factors on husk spot disease in macadamia

Abstract

The relative importance of Pseudocercospora macadamiae conidia produced from the subepidermal stromata of diseased husk and the effect of climatic factors on husk spot epidemics were assessed on managed (fungicide-protected) and unmanaged macadamia trees. The spatial pattern of diseased fruit in the tree canopy was quantified using the binary form of Taylor’s power law on 54 trees for 4 years. The a and b parameters of the power law showed year-to-year variations in the unmanaged epidemic. This was attributed to differences in climatic conditions and the amount of early infection in the tree canopy. Overall, diseased fruit aggregated, heterogeneity was in excess of the levels expected for a random pattern (a >1; b >1), and the degree of aggregation increased with the incidence mean. The rate of increase in disease incidence was significantly lower in the managed rather than the unmanaged epidemic. However, the rates of increase of disease severity were comparable, suggesting that other factors contribute to abscission of diseased fruit. The effect of each climatic factor examined on disease severity was <50%. The number of rainy days (R ² = 74%) was the single-most important factor influencing disease incidence. The results indicate that P. macadamiae conidia are mostly rain-splash dispersed, thus proximity of new fruit to the source of inoculum (diseased husk) significantly influences disease incidence. Practical implications for effective husk spot control are discussed.