‘Valencia’ orange (Citrus sinensis [L.] Osbeck) was grown on 17 rootstocks through seven years of age and the first four harvest seasons in a central Florida field trial severely affected by huanglongbing (HLB) disease. All trees in the trial had HLB symptoms and were shown by PCR to be infected with Candidatus Liberibacter asiaticus (Las). Large differences were noted between rootstocks for many metrics examined, including yield, fruit quality, and tree size. Highest yields in the trial were on US-942 rootstock, which was significantly more productive than trees on the common commercial rootstocks Carrizo, Kuharske, Cleopatra, and Kinkoji. Other new hybrid rootstocks also performed well in this trial strongly affected by HLB, including the rootstock US-1516, which had the second highest cumulative yield, best tree health rating, and lowest number of trees lost due to HLB damage. Comparison of tree performance in this trial with a similar trial conducted prior to the HLB epidemic, allows us to estimate that the disease resulted in a 33% reduction in yield and 21% reduction in tree growth through seven years of age. Use of a tolerant rootstock is suggested as an effective means of ameliorating crop losses to HLB

Nutrient assessment of plants, a key aspect of agricultural crop management and varietal development programs, traditionally is time demanding and labor-intensive. This study proposes a novel methodology to determine leaf nutrient concentrations of citrus trees by using unmanned aerial vehicle (UAV) multispectral imagery and artificial intelligence (AI). The study was conducted in four different citrus field trials, located in Highlands County and in Polk County, Florida, USA. In each location, trials contained either ‘Hamlin’ or ‘Valencia’ sweet orange scion grafted on more than 30 different rootstocks. Leaves were collected and analyzed in the laboratory to determine macro- and micronutrient concentration using traditional chemical methods. Spectral data from tree canopies were obtained in five different bands (red, green, blue, red edge and near-infrared wavelengths) using a UAV equipped with a multispectral camera. The estimation model was developed using a gradient boosting regression tree and evaluated using several metrics including mean absolute percentage error (MAPE), root mean square error, MAPE-coefficient of variance (CV) ratio and difference plot. This novel model determined macronutrients (nitrogen, phosphorus, potassium, magnesium, calcium and sulfur) with high precision (less than 9% and 17% average error for the ‘Hamlin’ and ‘Valencia’ trials, respectively) and micro-nutrients with moderate precision (less than 16% and 30% average error for ‘Hamlin’ and ‘Valencia’ trials, respectively). Overall, this UAV- and AI-based methodology was efficient to determine nutrient concentrations and generate nutrient maps in commercial citrus orchards and could be applied to other crop species.

Trunk injection is a targeted delivery of pesticides, insecticides, nutrients, or other plant protection materials into the stem or trunk of woody plants as an alternative to spraying or soil drenching. Trunk injection has historically been used for disease and pest management of high-value forest tree species or ornamental plants when aerial applications are problematic due to spatial problems and health-related concerns. An interest in using the injection technique for protection of agricultural crops in commercial production systems has emerged more recently, where foliar applications and soil drenches have proven ineffective or pose environmental hazards. This review provides an overview of the basic principles of trunk injection and the plant physiological implications, its current use in commercial agriculture and other plant systems, and associated risks.

Huanglongbing (HLB) is a devastating bacterial disease associated with ‘ Candidatus Liberibacter asiaticus’. The location of the pathogen within the vasculature of the tree has left growers with limited options for the effective management of the disease. Trunk injection is a crop protection technique that applies therapeutics directly into the xylem of woody tree species and allows for their systemic uptake and transport, which may provide more effective management of vascular diseases such as HLB. In this study, mature ‘Valencia’ and ‘Hamlin’ sweet orange ( Citrus sinensis) and ‘Duncan’ grapefruit ( C. paradisi) trees were injected with oxytetracycline (OTC) in the spring and/or fall to evaluate the effects of injection timing and response to injection. In addition to seasonal evaluations of tree health and bacterial titer, preharvest fruit drop, yield, and fruit quality were measured at harvest to determine the effects of OTC injection. The benefits associated with injection included a reduction in fruit drop, an increase in fruit yield and fruit size, and improvements in juice quality. However, results varied due to the timing of injection and were not consistent across all three varieties. Residue analysis at different time points after injection suggests that trunk injection effectively delivers therapeutics to mature citrus trees. This study provides fundamental information on the short-term benefits associated with trunk injection of OTC for HLB management in citrus groves. The potential for use of trunk injection at the commercial scale and the possible risks are discussed.