Phosphite and metalaxyl rejuvenate macadamia trees in decline caused by Phytophthora cinnamomi

•Phosphite application at root flushing periods control Phytophthora cinnamomi in macadamia.•Foliar and trunk applications of phosphite rejuvenate diseased macadamia trees.•Soil drench with metalaxyl controls tree decline in macadamia.•Phytotoxicity of phosphite on macadamia leaves occurred at relatively low rates.

Phytophthora cinnamomi is a significant pathogen of macadamia but little information on disease management exists. Through a series of glasshouse and field trials using different rates and modes of application of phosphite, this study provides data and information on effective application rates of phosphite for the control of macadamia tree decline caused by P. cinnamomi. We also determined the efficacy of bark spray application of phosphite and soil drench application of metalaxyl on recovery of declining macadamia trees. Overall, our results indicate that regardless of the method of application, biannual applications of phosphite during autumn and spring root flushes in macadamia are effective control options for tree decline. Phosphite applied as trunk injection resulted in relatively higher detection of phosphonic acid levels in the plant tissues compared to other methods of application, and the phytotoxic effect was also more severe than foliar spray application. Foliar and trunk spray application of phosphite or soil drench with metalaxyl-M resulted in consistent (R2 > 0.98) rejuvenation of diseased trees over 3 years compared to untreated control trees that continued to decline (y = 1 + 0.22ln(x); R² = 0.96). Phytotoxicity of phosphite on macadamia leaves occurred at relatively lower rates than indicated for other tree crops. The sensitivity of macadamia to phosphite relative to other tree crops is discussed.