Scientific Study

Australian macadamia production is threatened by a disorder known as abnormal vertical growth (AVG), for which the aetiology is unknown. AVG is characterized by vigorous upright growth and reduced lateral branching, flowering and nut set that results in over 70% yield loss annually. Six commercial macadamia orchards were surveyed in 2012 and again in 2018 to examine spatio-temporal dynamics of the epidemic. Data were subjected to point-pattern and geostatistical analyses. AVG incidence of all orchards showed better fit to beta-binomial distribution than binomial distribution. AVG incidence level of orchards varied between 5% and 47% in 2012, and 13% and 55% in 2018. AVG spread at a slow pace in all orchards, averaging at about 2% increase in disease incidence per annum. Spatial patterns of AVG were highly aggregated on both survey years by a number of tests at different scales and spread was mainly between neighbouring trees in a row or trees that were opposite to each other in different rows. Semivariograms showed large range values (ca. 15 - 120), indicating aggregation of AVG-affected trees beyond quadrat levels. Furthermore, clusters of disease were mainly at the edge of the orchard on the first survey date and the disease progressed towards the centre of the orchard over time. It is concluded that AVG is caused by an infectious agent, and based on patterns of spread, we hypothesise that spread is facilitated by root grafting or root-to-root contact. Furthermore, a vascular-limited pathogen could be involved that modulates plant hormone production.