Soil-Applied Controlled Release Fertilizer (CRF) Treatments Impact the Health and Growth of HLB-Infected Trees—Results from Greenhouse and Field Experiments

Nutritional programs being used in Florida in efforts to maintain productivity from HLB-infected trees have been largely focused on foliar applications.  Based on the overall improved performance of sweet orange trees on numerous rootstocks in an extensive field trial with trees being grown with ground-applied controlled release fertilizers (CRF), we conducted a one-year greenhouse study to investigate the effects of soil-applied macro- and micro-nutrient overdoses in CRF forms on HLB-infected trees of Valencia sweet orange on the somewhat tolerant rootstock UFR-3 (a "tetrazyg" hybrid of Nova+Hirado Buntan pummelo + Cleopatra+Argentine trifoliate orange). All trees received Harrell’s Nursery CRF for base nutrition, except the liquid fertilizer control group. Micronutrient treatments were applied at a 3x concentration including the following: Tiger-Sul  Zinc 18% (18% Zn, 65% S), Tiger-Sul Iron 22% (22% Fe, 55% S), Tiger-Sul Manganese 15% (15% Mn, 65% S), Tiger-Sul "Arnolds mix" (3.85% Fe, 7.50% Mn, 5.85% Zn, 63% S), Florikote Polymer Coated Sodium Borate (8.82% B), Florikote Polymer Coated Magnesium Sulfate (13.9%), Florikote Polymer Coated Triple Super Phosphate (40% P2O5), Florikote FeSO₄ Polymer Coated Ferrous Sulfate (28% Fe, 17% S) and BioChar from Southern Yellow Pine (97%). Macaronutrient treatments were applied at a 2x concentration including: Florikote Polymer Coated Mini Ammonium Sulfate (19% N), Florikote Polymer Coated Sulfate of Potash (47% K₂O) and Florikote Polymer Coated Urea (42% N). Treatments were applied at 0 and 6 months.  After one year, trees were evaluated for overall health and appearance and growth, with special emphasis on comparisons of feeder root growth.  Leaf midrib and root nutritional analyses, and RT-PCR to determine relative Liberibacter populations were also conducted. Results from the greenhouse study indicate that overdoses of micronutrients in the TigerSul (clay prill) form increase feeder root density, and thus overall tree growth.  Manganese appears to be the most important micronutrient in this regard. The polycoated sodium borate treatment also showed improved feeder root growth and tree health. There was an apparent synergy between the TigerSul products and the Harrell’s CRF.  Acidification of the root zone by the slow release of the sulfate could be increasing the efficiency of CRF nutrient uptake by the roots—leading to improved tree health. Results from field experiments supporting this hypothesis will also be discussed.