The huanglongbing (HLB) associated pathogen Candidatus Liberibacter asiaticus (CLas) inhabits the phloem of infected citrus trees, which impedes the efficacy of foliar-applied chemicals for management of the disease. Trunk injection is an alternate method that allows the targeted delivery of agrochemicals into the vasculature, resulting in their systematic distribution throughout the tree. This study aims to determine the large-scale efficacy of trunk injection of oxytetracycline (OTC) on citrus tree health, fruit quality, and yield in existing rootstock trials in a commercial production site in Florida. The study compares 9-year-old Valencia sweet orange (Citrus sinensis) trees grafted on six different rootstocks in a large field trial in Polk County, Florida. The experimental design is a complete randomized block design, with three injection treatments: 1) no injection (control), 2) injection into the scion trunk, and 3) injection into the rootstock trunk. The injections were performed in May 2023, and injected trees received a commercial formulation of OTC at a rate of 1.1 g per tree using FlexInject injectors. Data collection included HLB status, tree size, yield, fruit/juice quality, fibrous root density, and leaf macro- and micronutrient content once a year, wound phenotype every six months, and bacterial titers before injection and 3 days, 1 week, and 1 month after injection. OTC injections into the scion or the rootstock trunk improved tree health, yield, fruit/juice quality, and the leaf Ca content. Overall, injections increased fruit yield by 24% and juice brix by 9%, increasing the revenue by 36% on a per acre basis. Injection into the scion trunk was slightly superior in terms of tree growth and fibrous root density, while injection into the rootstock trunk reduced the wound size, bark cracking, and HLB symptoms. CLas titers were reduced regardless of the injection site one month after injections. In conclusion, our results suggest that OTC injections have the potential for short-term management of HLB to help with the recovery of the Florida citrus industry in the face of endemic disease conditions. The trial will continue to assess longer-term effects.
