Our main goal was to find the reasons for inconsistent responses of HLB-affected citrus to Enhanced Nutrition (EN) programs and to develop feasible and economical remedies that can consistently replicate successful HLB mitigation with ENs in Florida groves. The two research objectives are summarized below.Research Objective 1: Establish nutrient sufficiency guidelines for leaf tissues of HLB-affected trees Using a survey data collection method of quarterly sampling in multiple groves of three Florida citrus production regions (Ridge, Indian River and Southwest), we assembled a large database of leaf, soil, and tree characteristics from a wide variety of HLB-symptomatic trees. Trees were classified into 5 HLB-severity classes and also grouped into “responding” or “non-responding” classes. Data were analyzed with ANOVA, DRIS, segmented regression, Cate-Nelson partitioning, and artificial neural networks in order to evaluate every possible dimension and interaction to elucidate linkages between measurements and tree performance or HLB severity. Using both segmented regression and Cate-Nelson methods with DRIS allowed us to identify new critical threshold concentrations (CTCs) for leaf nutrients of HLB-affected trees that can immediately be used to make tentative recommendations that supercede the somewhat aged CTCs published in UF/IFAS SL253 and elsewhere. There is definitely a need to update the IFAS guidelines for interpretation of sweet orange tree leaf nutrient analysis and we now have proof. When making decisions pertaining to the nutrient needs of HLB infected trees, it is important to realize that healthy trees and diseased trees behave differently. The evidence gathered in this objective shows a real need to modify these guidelines to reflect the increased need for all 11 nutrients, specifically potassium, magnesium, boron, zinc, manganese, iron and copper. With a larger database of responding and non-responding tree values these values could be improved. The results of this research were presented at grower events: Citrus Nutrition Day, Bartow; Citrus School, Arcadia; Citrus Institute, Avon Park, and published in the April 2019 Citrus Industry magazine. Research Objective 2: Determine soil conditions that favor root hair and VAM proliferation in HLB-affected treesUsing solid-phase tricalcium phosphate (TCP) as the sole source of phosphorus in a liquid culture experiment with Carrizo trees, we were able to limit the amount of available phosphorus in solution (< 1 ppm) and thereby stimulate the development of abundant root hairs by a factor of nearly two compared to a standard control with about 10 ppm soluble phosphorus. After exposing the experiment to HLB-carrying psyllids for two months, the TCP-treated trees had the lowest CLas titer, and we continued to see and measure healthy root hair development (now a 32x difference in root hair abundance, post-HLB) on the TCP treated tank solution plants compared to the control. The control tree's roots were nearly devoid of root hairs after trees became HLB positive, while the TCP-treated trees only suffered a minor reduction in root hair density. We conclude that these differences in root hair abundance could significantly improve tree health if the results could be replicated with soil-applied and incorporated TCP in the field. A paper on this research was presented at the FSHS conference in June.