‘Ray Ruby’ Grapefruit Affected by Huanglongbing II. Planting Density, Soil, and Foliar Nutrient Management

Abstract

Since the arrival of Huanglongbing (HLB) disease in Florida, several management approaches, including modification of orchard architecture design and nutritional therapy, have been explored. High-density plantings anticipate early economic returns from HLB-affected orchards. With no cure available for HLB, balanced nutrient application through soil and foliar spraying can mitigate the disease. A 2-year study was conducted to investigate the effects of three grapefruit (Citrus paradisi) planting densities [single-row (300 and 440 trees per ha), and double-row high-density (975 trees per ha)], two controlled-release fertilizer (CRF) blends, and foliar-applied micronutrients (FAM) (a blend of B, Mn, and Zn at 0, 1.5, 3, and 6 times the recommended rates) on grapefruit growth and fruit yield, physiological parameters, and foliar nutrient concentrations in an HLB-affected orchard. All the trees tested positive for HLB based on real-time quantitative polymerase chain reaction (qPCR) test. The highest planting density resulted in the lowest trunk diameter and canopy volume. Despite lower yield per tree in 2019–20, 975 trees per ha planting induced the greatest fruit and solid yields per ha. Also, the fruit produced from 975 trees per ha planting tended to be acidic with the deposition of more soluble solids. Use of CRF with higher micronutrients increased canopy volume with the expense of reduced fruit number in 2019–20. FAM did not affect cycle threshold (Ct) value and tree growth parameters. Fruit yield, photosynthesis rate, and stomatal conductance (gS) decreased, and all leaf nutrient concentrations except B increased in 2019–20 with all FAM rates tested. In conclusion, our study showed that high-density planting optimizes yield under HLB-endemic conditions. In addition, supplemental soil and foliar micronutrient application do not enhance yield of HLB-affected trees over a 2-year timeframe, warranting further research for confirmation of results.

Nitrogen, Calcium, and Magnesium Inconsistently Affect Tree Growth, Fruit Yield, and Juice Quality of Huanglongbing-affected Orange Trees

“…The high planting density used in this study (1111 trees/ha) might have masked the effects of plant nutrition on vegetative growth variables, as higher densities promote higher competition among nutrients, water, space, and light, which may affect vegetative growth (Weil and Brady, 2017). This is supported by the findings of Phuyal et al (2020aPhuyal et al ( , 2020b who observed lower canopy volume and trunk diameter with a high planting density (975 trees/ha) when compared with lower planting densities of (300 and 440 trees/ha) of 'Ray Ruby' grapefruit on Kuharske citrange. However, they found greater fruit yield per hectare with the highest planting density, with no significant yield difference among fertilizer treatments.…”

“…In contrast, Atta (2019) did find that the 280 kgÁha À1 N rate significantly increased canopy volume of 'Hamlin' on Swingle rootstock, indicating that scion and rootstock selections can affect fertilization experiments of HLB-affected trees. Similarly, Phuyal et al (2020aPhuyal et al ( , 2020b found that higher N, Mg, and micronutrient inputs increased canopy volume of 'Ray Ruby' grapefruit on Kuharske citrange, although this occurred at the expense of fruit yield.…”

“…Nutrition plays an important role in plant defense mechanisms against multiple pests and diseases, as well-fertilized plants are less susceptible than nutrient-deficient ones (Garc ıa-Mina, 2012;Spann and Schumann, 2010). Recent evidence suggests that HLB symptoms in citrus can be reduced with a balanced nutritional program, including improved irrigation, fertigation, and nutrient management (Atta, 2019;Atta et al, 2020;Handique et al, 2012;Kadyampakeni et al, 2016;Morgan et al, 2016;Phuyal et al, 2020aPhuyal et al, , 2020bRouse et al, 2012;Uthman et al, 2020bUthman et al, , 2020aZambon et al, 2019). Thus, until commercially available resistant citrus varieties are developed, limiting the effects of HLB by focusing on nutrient management can help keep commercial growers in production (Alvarez et al, 2016;Spann and Schumann, 2010).…”

Impact of Ground Applied Micronutrients on Root Growth and Fruit Yield of Severely Huanglongbing-Affected Grapefruit Trees

“…[26]. However, a study by Phuyal et al [27] found that supplemental soil and foliar micronutrient applications with 0, 1.5×, 3×, and 6× the recommended UF/IFAS rates of B, Mn, Fe, and Zn did not improve yield or tree growth of HLB-affected 'Ray Ruby' grapefruit trees on 'Kuharske' rootstock in the IRD. Another study found that ground fertilizer applications of B with 0, 2×, and 4× the current UF/IFAS recommendation of B had no consistent impact on tree canopy for 9-year-old 'Vernia' sweet orange trees on Rough Lemon rootstock [28].…”

An Overview of the Mechanisms Against “Candidatus Liberibacter asiaticus”: Virulence Targets, Citrus Defenses, and Microbiome

“…For several years, it has been reported that the application of enhanced nutritional products can extend the vigor of HLB-affected citrus and trigger citrus defense against C Las ( Spann and Schumann, 2009 ; Shen et al, 2013 ; da Silva et al, 2020 ; Shahzad et al, 2020 ; Dong et al, 2021 ). Although nutrient treatments have no effect on reducing C Las titer and cannot enhance yield of HLB-affected citrus in the field ( Gottwald et al, 2012 ; da Silva et al, 2020 ; Phuyal et al, 2020 ), the application of macro-and micronutrients have been adopted worldwide as they induce host defenses and help maintain production of HLB-affected trees. For instance, a field study in Florida showed application of phosphorus (P) oxyanion solutions to HLB-affected citrus mitigated disease symptom severity during a 3-year field trial ( Zhao et al, 2013 ).…”