Economic Thresholds for Phytophthora Foot Rot of Citrus Using Estimates of Value of Tree Loss and Chemical Control Costs

Abstract

Economic thresholds (ET) for Phytophthora foot rot of young citrus trees planted at three tree spacing densities were calculated as the number of trees/acre/year that must be lost to offset the costs of fungicide treatment. Thresholds were based on the present value of lost trees during the first four years of orchard establishment and the cost of annual and cumulative annual fungicide applications of fosetyl Al or metalaxyl. Economic thresholds for the first two years after orchard establishment ranged from 1.5% to 2.6% and 0.6% to 2.4% annual tree losses/acre to offset treatment costs of metalaxyl and fosetyl Al, respectively. Tree losses/acre had to exceed 2.2% to 3.9% and 1.9% to 5.2% to offset treatment costs of metalaxyl and fosetyl Al, respectively, in yr 3 and 4. Routine treatment programs for foot rot were not recommended because annual economic tree losses to foot rot rarely exceed fungicide treatment costs on a cumulative annual basis. The adaptibility of ET determination to a perennial cropping system where pest population densities and pest aggressiveness can be substituted for with a spatial unit (tree or whole plant) is discussed.