2017 ASHS Annual Conference

Precise irrigation and fertigation management provide a less-limiting environment to roots while minimizing over irrigation and leaching of nutrients. This concept can improve tree growth in the presence of HLB and help optimize water and nutrient use. Higher tree density can increase fruit yield per area under high HLB pressure. This study evaluated the efficiency of open hydroponics on ‘Ray Ruby’ grapefruit production under different irrigation systems and tree density. We tested a combination of rootstocks {‘Sour orange’ [C. aurantium] and ‘US897’ [Cleopatra (C. reticulata) × Flying Dragon (Poncirus trifoliata)]}, tree spacing {standard [3.8 × 7 m, 358 trees/ha] and high density staggered [(2.74 × 1.5 × 0.9 m) × 6.1 m, 953 trees/ha) (HDS)]}, fertilization [16N-2.2P-8.3K dry granular and 15N-2.6P-22.4K water-soluble fertilizer (WSF)], and irrigation systems (3.8 LPH drip and 40.5 LPH micro jet), arranged on five treatments: T1) ‘Sour Orange’ + standard spacing + dry granular fertilizer + micro jet, T2) ‘US897’ + HDS + WSF + drip, T3) ‘Sour Orange’ + HDS + WSF + drip, T4) ‘Sour Orange’ + HDS + WSF + micro jet, and T5) ‘US897’ + HDS + WSF + micro jet. The experimental design was a completely randomized with five replications. Trees were planted in Sept/2013 (total 2,769 trees/3.23 ha). Foliar nutrient, insecticide and fungicide were sprayed using standard practices. We scouted for psyllids, leaf minors and other citrus pests monthly. HLB incidence increased over time, reaching 87.5% in March/2017. T1 presented two negative reps (p<0.0001). Trunk diameter and canopy volume increased through time, and were higher on T1 and T4 compared to other treatments (p<0.001). Leaf macro and micronutrient concentrations were influenced by treatment and sampling date (p<0.05). Fruit diameter, total number of fruit and yield were 4%, 226% and 183% higher in 2016 than 2015 (p<0.0001). T1 yielded 86 boxes with 38.5 kg/ha compared to an average of 261 boxes for other treatments. T3 and T4 resulted in the highest yield (p=0.0051). Fruit quality parameters were measured in 2016 only. Fruit weight and diameter, juice weight and volume, soluble solid concentration, acidity, and total solids/ha were not significant (p>0.05). High density staggered planting resulted in higher fruit yield, irrespective of rootstock and irrigation system, 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.