HLB is known to make citrus roots more susceptible to Phytophthora root rot. It also reduces the efficacy of chemical management of Phytophthora root rot, creating a difficult management scenario. Current Phytophthora management recommendations are based on pre-HLB work done in the 1980s. These three conditions raise the question of whether yield improvement from Phytophthora management is enough to pay for the management costs themselves. The goal of this project is to develop new soil propagule density managment thresholds and recommendations for chemical management of Phytophthora root rot based on ecomonic analysis of yield responses in different soil conditions. Objective 1) Determine if labelled Phytophthora management maintains efficacy in the field on HLB-affected trees for reducing fibrous root loss and improving yield.No further phytophthora counts have been taken from the southwest Florida site this quarter. However, yield data from Valencia was taken in February. In the preliminary data, there were no significant differences among the treatments but there was also a tremendous amount of variation within the treatments. The phosphite alternated with Presidio treatment had less than half the variation among plots that the other treatments. Differential variation among the treatments was also observed for Acid and brix but not as strongly for the ratio. This means that further analysis will be needed to reduce the amount of variation among the treatments. From this preliminary analysis of the sugars, there was no significant difference among treatment quality measures for the Valencia.We initiated a second location for two new trials, early oranges and Valencia in the Wachula area. The sites have a history of phytophthora. The preliminary counts from the blocks varied between 40 and 128. We have taken the per plot data after laying out the trials but counts are not complete. It is expected that treatments will start in the first week of June. We plan to have all the treatments of the first set of trials with the additon of Orondis Ultra. Objective 2) Determine benefit-cost thresholds for Phytophthora treatment on HLB-affected treesI have spoken to Ariel Singerman and determined what data he needs to proceed and I preparing it to send to him for the 2021 season.