2018 ASHS Annual Conference

Sweet oranges (Citrus sinensis) are impacted by huanglongbing (HLB), a disease associated with Candidatus Liberibacter asiaticus. The disease is threatening the citrus industry, with devastating effects on fruit production. Higher plant density can increase fruit yield per area under high HLB pressure, maximizing income and extending grove survival until a definite cure is found. This study evaluated the effect of tree planting density and irrigation systems on fruit yield and quality. ‘Valencia’ orange on ‘Kuharske’ citrange (C. sinensis × Poncirus trifoliata) trees were planted in Sept/2013 (2,995 trees in 1.61 ha). We tested three treatments: standard tree spacing (3.8×7 m, 357 trees/ha) + dry granular fertilizer + microsprinkler irrigation (one emitter per tree; microsprinkler 50 green nozzle, 16.7 GPH at 20 psi) (Bowsmith, Exeter, CA), high density staggered ([2.7×1.5×0.9 m]×6.1 m, 953 trees/ha) + fertigation + microsprinkler irrigation (one emitter per two trees), and high density staggered + fertigation + drip irrigation (two lines per row; Emitterline 0.58 GPH at 10 psi, 12-inch spacing) (Jain Irrigation), in a complete randomized block design with eight replications. A 16N-2.2P-16.2K dry granular fertilizer was applied twice a year at 200 kg N/ha. We scouted monthly for citrus pests and diseases. Foliar nutrients, insecticides and fungicides were sprayed following citrus standard practices. HLB incidence increased over time, reaching 87.5% in March/2017 (p<0.0001). Trunk diameter was 8% higher (p=0.0004) and canopy volume 29% higher (p=0.0003) on standard tree spacing compared to other treatments (p<0.001). Leaf nutrient concentrations were influenced by plant density, and differed among years (p<0.05). Fruit diameter and total number of fruit were measured in the last two seasons, and were not different even after hurricane Irma (p>0.05). However, there was a 38% reduction in yield in 2017/2018, with high density staggered treatments yielding on average 68 boxes of 40 kg per hectare compared to 25 boxes on standard tree spacing (p<0.0001). Soluble solids and ratio decreased from 2016/2017 to 2017/2018 (p<0.0001), while acidity increased 29% (p<0.0001). Fruit weight and diameter, juice weight and volume were higher in 2017/2018 (p<0.0001) without any treatment differences (p>0.05). Fruit from both high-density treatments were sweeter than single row, low-density treatment (p=0.0009). Staggered, higher plant density resulted in higher fruit yield and quality, producing thrice more soluble solids per area (p<0.0001), representing an important advantage to the juice processing industry. However, labor cost and long-term effects still need to be determined for commercial recommendation.