Uptake of micronutrients in severely HLB-affected grapefruit trees grown on Florida Indian River flatwood soils

Citrus greening (HLB, huanglongbing) is caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus and vectored by the Asian citrus psyllid. To combat HLB-induced nutrient uptake issues, growers use higher fertilizer application rates to prolong the producing life of affected trees. However, current nutrient recommendations in Florida were created in pre-HLB times and must be adjusted. Therefore, the objective of this study was to compare granular and liquid fertilization to identify best application methods for uptake of micronutrients into both belowground and aboveground components of HLB-affected grapefruit grown on flatwood soils. A large-scale field study consisting of 6-year-old HLB-affected "Ruby Red" grapefruit trees grafted on "sour orange" rootstock was conducted in the Florida Indian River District. Eight different treatments consisting of four liquid and four granular fertilizers with 1x, 2x, and 4x the current University of Florida, Institute of Food and Agricultural Sciences recommended rates of boron, zinc, manganese, and iron were applied three times a year (granular), biweekly (liquid), or weekly (liquid), for 3 years. Analyses of nutrients in soil, root, and leaf were conducted every 6 months throughout the study. Higher rates of boron, zinc, and manganese were observed in the soil of plots treated with liquid fertilizers with 2x and 4x the micronutrient recommendations. Root and leaf micronutrient analysis revealed significant differences in boron, zinc, manganese, and iron among treatments which varied between seasons. This study demonstrated that ground-applied micronutrient overdoses and application methods impacted nutrient uptake, however, no patterns with soil concentrations and tree uptake were established.