2017 ASHS Annual Conference

Grapefruit (Citrus paradisi) is severely impacted by huanglongbing (HLB), a disease associated with Candidatus Liberibacter asiaticus. The disease poses a threat to the Indian River fresh market industry due to the negative effects on fruit production. Higher plant density can increase fruit yield per area under high HLB pressure, maximizing income and extending grove survival until research provides a solution for the disease. This study evaluated the effect of different tree spacing on fruit yield and quality. We tested three plant densities: standard tree spacing (4.6 × 7 m, 293 trees/ha), high density on single row (3 × 7 m, 429 trees/ha) and high density staggered ([2.7 × 1.5 × 0.9 m] × 6.1 m, 970 trees/ha). The experimental design was a randomized complete block design with eight replications. ‘Ray Ruby’ grapefruit on ‘Kuharske’ citrange (C. sinensis × Poncirus trifoliate) rootstock was planted in Sept/2013 (total 1,196 trees/2.13 ha). Irrigation was applied on-demand using 40.5 LPH @ 20 psi micro jet (one jet every two plants). A 16N-2.2P-8.3K dry granular fertilizer was applied twice a year at 180 kg N/ha. Foliar nutrients, insecticides and fungicides were sprayed following citrus standard practices. We scouted for psyllids, leaf minors and other citrus pests monthly. HLB incidence increased over time, reaching 96% in March/2017 (p<0.0001). Trunk diameter and canopy volume increased over time, and were 20% and 33% smaller, respectively, on high density staggered compared to other treatments (p<0.001). Leaf macro and micronutrient concentrations were influenced by plant density treatment and sampling date (p<0.05). Fruit diameter was 2% smaller (p=0.0191) and total number of fruit was 52% higher in 2016 than 2015 (p=0.0219). There was no difference in yield between seasons but high density staggered yielded 116 boxes with 38.5 kg per ha compared to 46 boxes on standard tree spacing, while high density on single row yielded 79 boxes (p=0.0014). Fruit quality parameters were measured in 2016 only. Soluble solids concentration, acidity and solids/ha were 7.6%, 10% and 140% higher, respectively, on high density staggered compared to the other treatments (p<0.01). Fruit weight and diameter, juice weight and volume and ratio were not significant (p>0.05). Higher plant density resulted in higher fruit yield and quality, representing an important advance to the grapefruit production system. However, labor cost and effect on plant growth over time still need to be determined for commercial recommendation.