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.During this quarter, soil samples for root density and phythophthora counts were collected from both sites. The Hamlin block was harvested. The quality and sizing data were generated in the packinghouse facility.In preliminarily reviewing the data, there were significant differences among the root densities for Hamlin but not the Valencia. The range for Hamlin was 0.096 to 0.314 roots/ml of soil and for Valencia it was 0.032 to 0.076 roots/ml soil. The treatment with the greatest number of roots was the Ridomil, Presidio, Phosphite, and Orondis regimen and it had significantly more than the UTC. There were no significant differences among the total phytophthora propagule count for the Valencia which ranged from 2.8 to 9.6 propagules/cm3 of soil. When separated for the species there was also no difference. In the Hamlin trees, there was a significant difference among treatments. The Ridomil, Presidio, Phosphite, and Orondis treatment still had no propagules and a statitically equivalent number were observed in the Phosphite alternated with Orondis tretatment. The treatment with the most propagules was Phosphite alternated with Ridomil treatment which had 56.0 propagules/cm3 of soil, significantly higher than other treatments. In the fall propagules counts at both sites, the block effect was much weaker but additional statistical analysis will still be needed to take this factor into account in a better manner. When looking at the Hamlin fruit weights, there was no treatment effect but there was a significant block effect. In looking at the pounds solid per box from each plot, there was a weakly significant effect (P < 0.1) for treatments which ranged from 5.40 to 5.02 lb solids/box. The best treatment was the Phosphite treatment and block was highly significant. The brix and acid had no signficant differences but the brix acid ratio with the highest ratio is for the UTC (16.36) and lowest was Phosphites rotated with Presidio (15.24). Objective 2) Determine benefit-cost thresholds for Phytophthora treatment on HLB-affected treesAs planned in the proposal, this objective awaits this years yield results to begin calculating benefit-cost based on a combination of change in yield from the previous year and comparison among treatments within blocks.