Disease management strategy for macadamia quick decline

Trees infected with macadamia quick decline (MQD) exhibit excessive sap bleeding from the trunk, frass from ambrosia beetle feeding, orange fruiting bodies of the fungus Nectria rugulosa and yellowing and browning of the leaves within the tree canopy. MQD threatens commercial and residential production of macadamia (Macadamia integrifolia) in Hawaii. To identify strategies to manage the disease and prolong macadamia production, fungicide and application measures were evaluated. Fosphite^® was examined in vitro and inhibited radial growth of Phytophthora tropicalis on fungicide-amended 10% V8 juice agar. A leaf bioassay method was developed to test for age effect, cultivar susceptibility and in vitro disease suppression. Results indicated that cultivar, leaf age and location of inoculation affected lesion size. In vitro applications of Fosphite^® reduced lesion sizes by ~70%. In field studies, a soil drench application of Fosphite^® was compared with passive and active injection methods. Results indicate that drenches and/or tree injections with phosphorous acid fungicides can arrest the progression of the MQD symptoms in infected trees, eliminate visible symptoms and prolong their productive life, in some instances for as much as 1000 additional days.