2019 ASHS Annual Conference

Citrus greening or Huanglongbing (HLB) disease is a serious threat to citrus production in Florida. Since a cure is still unknown, disease management is necessary to maintain trees in production. Management strategies rely on different approaches: controlling the insect vector, applying antimicrobials and enhance plant nutrition to extend tree survival. We are conducting a field trial on 5-year-old ‘Ray Ruby’ grapefruit (Citrus paradisi) on Kuharske citrange (Citrus sinensis·× Poncirus trifoliata) rootstock planted in the Flatwoods soils in Fort Pierce, FL. The objective is to understand how tree spacing, soil and foliar fertilization affect fruit yield and fruit quality on HLB-infected grapefruit trees in the Indian River citrus district. We tested three planting densities [standard spacing (300 trees/hectare), high-density single row (440 trees/hectare), and high-density double row staggered in diamond setting (975 trees/hectare)], two soil application blends of controlled-release fertilizer (CRF) (16N-1.31P-16.6K and 12N-1.31P-7.47K plus higher micronutrient rate), and four foliar rates of micronutrient (a blend of Mn, Zn, and B at 0×, 1×, 3×, and 6× IFAS recommendation). All the trees tested positive for HLB based on real-time quantitative PCR test. The preliminary results indicate the highest yield was obtained with high-density double row staggered at 170 boxes of 38.5 kilograms per hectare (P<0.001). Canopy volume was 43% higher with standard spacing than high-density double row staggered (P<0.0001). Trunk diameter was 2.3% larger on standard tree spacing than high-density double row staggered with no effect of nutrient treatment (P<0.0001). Fruit with high-density double row staggered were more acidic (average titratable acidity 1.16 mg/100 mL) than other treatments irrespective of the nutrient application (P<0.0001). Soluble solids content was 5.75% higher on high-density double row staggered trees than standard spacing (P=0.0278) with no effect of nutrient treatment. There was no effect on yield by both CRF formulations (P=0.8428) and foliar treatment (P=0.7126) in the first year. The 12N-1.31P-7.47K increased the canopy volume by 8% (P<0.0001). Canopy volume remained unaffected by foliar treatment (P=0.3782). Leaf nutrient concentration increased with the increase in foliar rate for all micronutrients, particularly Mn (P<0.0001) that exceeds the optimum range with 6× application of IFAS rate. The study shows that high-density double row staggered results in higher yield with lower canopy volume. CRF blends with higher micronutrients can contribute to greater canopy growth without increasing fruit yield. Foliar treatment of micronutrients can maintain tree health by improving nutrients status of the plant.