Strategies for controlling macadamia quick decline

Quick decline of macadamia (Macadamia integrifolia) trees continues to be a serious problem in Hawaii. Initial signs and symptoms include bleeding, the presence of Ambrosia beetles and orange fruiting bodies of Nectria sp. followed by yellowing and browning of leaves within the tree canopy. Isolations from diseased branches have yielded Phytophthora capsici. A zoospore suspension of P. capsici was injected into branches of cultivar "HAES 344" to incite macadamia quick decline (MQD) signs and symptoms. Two branches showed for the first time MQD bleeding, powder posts and Nectria in an inoculation proving Koch's postulates and also indicating that P. capsici may be the primary causal agent. Control methods for MQD with the fungicide Fosphite were investigated on the cultivar "HAES 333". Soil drenching proved ineffective and was abandoned for a trunk injection method for delivering the fungicide. Initially, a passive trunk injection method was utilized. A 10 ml syringe containing concentrated Fosphite was used to dribble the fungicide into a 1%" deep hole that was drilled at a downward angle into the trunk of the tree. Unfortunately, only 25% to 50% of the label rate was applied. A more efficient and reproducible method utilized a pressurized injection system (Arborjet Tree I.V. System). With this system, the entire amount of fungicide was delivered into 12 trees at the label rate. The Fosphite control vs. no control field was monitored every two weeks for signs of MQD. MQD trees with no control died within an average of 205 days or 6.8 months after first signs of infection. Fosphite-treated trees have survived thus far for an average of 614 days. With treatment, trees to date have lived an additional 409 days. The trunk injection method may provide the macadamia industry a well needed control method for MQD.