‘Ray Ruby’ Grapefruit Affected by Huanglongbing I. Planting Density and Soil Nutrient Management

Abstract

Huanglongbing (HLB), or citrus greening disease, affects practically all fruit-bearing trees in commercial citrus orchards in Florida with no cure identified yet. High-density plantings and enhanced nutritional programs such as application of controlled-release fertilizer (CRF) with higher micronutrient levels can mitigate disease symptoms and extend the tree life span of sweet oranges (Citrus sinensis). The objective of this study was to evaluate the effects of tree planting density and application of CRF blends differing in N to K ratio and micronutrient content on grapefruit (Citrus paradisi) plant health, canopy volume, fruit yield, and fruit quality in an HLB-affected orchard. A study was conducted in Florida for two growing seasons (2017–18 and 2018–19) to evaluate the response of ‘Ray Ruby’ grapefruit on Kuharske citrange (Citrus sinensis × Poncirus trifoliata) to three planting densities (300, 440, and 975 trees per ha) and two CRF blends [12 nitrogen (N)–1.31 phosphorus (P)–7.47 potassium (K) and 16N–1.31P–16.6K] with different nutrient sources and composition. According to quantitative real-time polymerase chain reaction testing, all sampled trees tested positive for Candidatus Liberibacter asiaticus, the pathogen associated with HLB. Trees planted at 975 trees per ha had 33% lower canopy volume per tree but 160% greater fruit yield per hectare and 190% higher yield of solids compared with 300 trees per ha. Fruit produced in high-density planting (975 trees per ha) was 18% more acidic with higher soluble solid compared with low-density planting (300 trees per ha). The use of a CRF blend with higher amounts of micronutrients along with lower K increased canopy volume in both seasons and resulted in 24% and 29% reduction in fruit yield per hectare and yield of solids, respectively, in 2017–18. Our results indicate that high-density plantings increase fruit yield per area, and regardless of the N to K ratio, the use of CRF blends supplemented with micronutrients may not increase fruit yield in HLB-affected grapefruit.

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).…”

Field Performance of ‘Hamlin’ Orange Trees Grown on Various Rootstocks in Huanglongbing-endemic Conditions

“…Hence, yield efficiency (yield per unit canopy volume) is a more suitable variable for determining productivity of a mature orchard. In two recent studies of 'Ray Ruby' grapefruit, high-density planting resulted in increased yield per hectare, increased fruit TSS contents, and decreased canopy volume (Phuyal et al, 2020). This demonstrates that the use of high-yield efficient rootstocks and optimized planting density can increase productivity and minimize cost per unit production.…”

Understanding Citrus Greening Disease and Its Possible Management Strategies in Nepal

“…Frequent application of small doses of fertilizers (in leaves or roots) enabling regular supply of nutrients are more effective in reducing potential nutrient leaching. Fertigation and controlled-release fertilizers are also effective nutrient delivery methods (Phuyal et al, 2020;Morgan et al, 2016).…”