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

Abstract

Citrus greening (huanglongbing, HLB) is a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas) and vectored by the Asian citrus psyllid (Diaphorina citri) (ACP). No cure is yet available, and as a result, increased fertilizer applications continue to be a major management method used to prolong the productive life of affected trees. However, questions remain regarding proper fertilizer rates and in what form these nutrients should be applied to increase tree health and yield. Therefore, the goal of this study was to identify optimum micronutrient application forms and rates to increase root growth, canopy size and fruit yield as well as reduce leaf and root CLas cycle threshold (Ct) value of HLB-affected grapefruit trees (Citrus × paradisi Macfad). A large-scale field study consisting of six-year-old HLB-affected ‘Ruby Red’ grapefruit trees grafted on ‘sour orange’ (Citrus × aurantium) rootstock was conducted in the Florida Indian River District. Eight different treatments consisting of four granular and four liquid fertilizers with 1×, 2×, and 4× the current University of Florida, Institute of Food and Agricultural Sciences recommended rates of boron, zinc, manganese, and iron were applied for three times a year (granular), biweekly (liquid), or weekly (liquid), for three years. Root density, length and diameter, root, and leaf CLas Ct value, canopy volume, visual HLB symptoms, and fruit yield were measured throughout the study. Regardless of treatment, tree health declined over time, and no significant impact on severely HLB-affected grapefruit tree health was detected within the three-year time frame of the study. This was the first study to solely compare the impact of ground applied micronutrient application rates and methods on HLB-affected grapefruit tree health in Florida’s Indian River District. More time would be needed to determine the potential benefits of increased micronutrient performance of HLB-affected grapefruit trees.