1. Please state project objectives and what work was done this quarter to address them:The overall goal of the project is to develop fertilization strategies to best match nutrient supply and demand, and develop recommendations for optimal nutrient application timing as compared to a simple constant supply, which will improve fruit yield, quality, and reduce fruit drop. A)Objective 1) Test if a reduced N-P-K nutrient supply in the fall is safe for sustaining HLB-affected citrus, and whether it can improve fruit quality to facilitate earlier maturity / harvesting and reduce fruit drop:In this quarter, fertilization treatments were applied (2nd and 3rd application) in the third year of the experiment. Routine leaf sampling was done in May and processed for nutrient analysis. Nutrient data from leaf sampling in March 2024 is described below, and leaf data from May 2024 samples is still pending. Fruit diameter measurements were started with fruit tagging, and dropped fruits counts began after the physiological drop for the 2024 season, and then will be continued to measure every 2 weeks until harvesting. Destructive root sampling in the Hamlin experiment was done in January and scanned using a flatbed scanner to estimate total root length and volume (presented below). Additionally, total root biomass per unit soil area was also calculated. Fruit growth and drop: These measurements were started after the physiological drop in May. Eight fruits were tagged in each experimental unit to measure the fruit growth recording over time, measured at every 2-week interval until November. Similarly, fruit drop counting from each plot was started after removing physiological drop fruits. Fruit dropped in both Hamlin and Valencia varieties were less in June than May, and fertilizer treatments had no significant effect on fruit drops to date. Fruit drop counting data will be used to calculate the cumulative fruit drop per tree and fruit drop rate. Leaf nutrient status in March 2024: Leaf sampling was done one month after the first 2024 season fertilization in the experiment. Most of the macronutrients and micronutrients in leaf samples were in the optimum concentration range in the Hamlin trial except Mn and Zn, which were in the low range. The status of leaf nutrients was not significantly different between fertilizer treatments. This might be due to an almost equal proportion of fertilizer applications in February and efficient nutrient uptake by the Hamlin trees during leaf flushing and bloom growth stages. However, leaf nutrients N, K, Zn, and Fe were deficient in the Valencia trees. Contrasting results might be due to the later ripening stages of the 2023 crop for the Valencia variety, where internal physiology controls nutrient uptake and distribution. Root dynamics: Root growth measurements for the 2023 season root samples were calculated using Rhizovision® root analysis software, and the total root biomass at two soil depth intervals (topsoil 0-20 cm and subsoil 20-40 cm) was measured. Total root length, total root volume, and root dry biomass per unit soil volume of fibrous (<2mm size) and coarse (>2 mm size) roots were calculated.Total root length, volume, and biomass were significantly higher in topsoil than in subsoil. The measured parameters of fibrous roots were significantly higher than those of coarse roots in surface topsoil except total length of fibrous roots which was higher in subsoil. In contrast, root biomass and volume of coarse roots were measured higher in subsoil (20-40 cm). There were no significant differences in root parameters in response to sigmoidal fertilizer management compared to the control linear fertilizer management treatments in 2023. However, overall root parameters were higher in treatment 3 (100% sigmoidal supply with 25% fall application on August) in subsoil. Objective 2) Develop an optimized, practical fertilizer timing management profile to boost fruit quality and reduce fruit drop for HLB-affected citrus based in part on the sigmoidal nutrient demand curve defined by four physiological growth phases (0=bloom/fruit set; 1=cell division; 2=cell enlargement; 3=maturation):Fruit growth measurement and drop counting of year 3 were started in June and will be reported in the next quarter. Initial data on fruit drop showed no significant difference in fertilizer management treatments. Leaf nutrient status during March was also not significantly different from fertilizer management treatments; however, suboptimal or deficient leaf nutrients N, K, Fe, and Zn were recorded in Valencia. Sigmoidal fertilizer management treatments were not significantly affecting root parameters like total root volume, root length, and root biomass compared to the linear fertilizer management (control) treatments. B) New developments: NoneC) Issues: None2. Please state what work is anticipated for next quarter:Routine lysimeter sampling, leaf sampling, processing and analysis will be ongoing, as will tree size, root growth and fruit drop and growth measurement.Plan for July to September 2024 Fertilization treatment applications (4th or 5th application) Routine leaf sampling every 2-month interval (July 30, September 30) Root scanning via Minirhizotron Trunk size measurement Routine lysimeter leachate collection Schedule Aerobatics flight for tree health, vigor, canopy volume, and area estimation 3. Please state budget status (underspend or overspend, and why):Spending rate is slightly below the target by about 15%. This is considered within the normal deviations of budgeting. The departure of a TEAMS employee from our lab has caused unanticipated reduction in salary expenses. We will request a 6-month no cost extension in September in order to complete the collection and analysis of the third year field data (Hamlin harvest in December, Valencia in February / March 2025).