Influence of soil moisture conditions on severity of Botryosphaeria branch dieback caused by Lasiodiplodia iraniensis and impact of plant growth in macadamia

Australian macadamias experience outbreaks of Botryosphaeria branch dieback. Extreme environmental conditions are thought to be responsible for the episodic outbreaks of diseases caused by Botryosphaeriaceae species in horticultural ecosystems. However, studies on the interactive effects of biotic and abiotic stresses in macadamia are unknown. Knowledge of management practices that contribute to the severity of the disease would assist in decisions for Botryosphaeria branch dieback control in commercial macadamia orchards. Using an in planta stem wound inoculation assay with Lasiodiplodia iraniensis, we investigated the association of soil moisture conditions as a factor of plant stress with the severity of Botryosphaeriaceae infection in macadamia. Sap flow rate and soil water potential were monitored in 72 macadamia potted cv. HAES 344 plants (18-month-old and 12-month-old grown in pine bark-sand mixture) that were kept under three watering treatments, based on the amount of water used per day (W-d) at the start of the trial, at W-d, 1/2 of W-d and 1/4 of W-d for 14 weeks. A significant differential effect of the watering regimes was observed on disease severity, revealing a strong (R-2 > 0.87) negative linear relationship between disease severity and sap flow rates. Necrotic lesions were significantly (P < 0.001) longer in the lowest and highest watering regimes. The average sap flow rate of 0.4 kg h(-1) was recorded in the plants under moderately watered conditions, <1/2>W-d regime, whereas 50% and 75% reductions in sap flow rates were recorded at the lowest and highest watering treatments, respectively. Our results indicate that maintaining optimal soil water potential between -50 and -80 kPa through irrigation practices may reduce the risk of Botryosphaeria branch dieback outbreaks in macadamia orchards.