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Highlights

An equation was developed for estimating the needle penetration force required for trunk injection.

Abstract. Targeted precise management of intravascular diseases such as citrus Huanglongbing (HLB) involves the application of therapeutics via trunk injection. Although trunk injection has proven more effective than foliar spray and soil drenching, multiple studies have highlighted limitations for large-scale commercial tree crop applications because of the manual process involved, emphasizing the need for an automated injection system. The design of a trunk injection mechanism to efficiently dispense aqueous therapeutic materials to the trunk of HLB-affected citrus trees requires the characterization of the hydraulic properties of the trunk and the determination of the penetration force of a dispensing needle. This study developed an equation for estimating penetration force at maximum depth based on the needle diameter, depth of penetration, and specific gravity of the wood tissue, and presents a method for estimating injection pump pressure. Trunk hydraulic conductance was measured using a hydraulic conductance flow meter, and needle insertion and removal force were measured using a force gauge in an Instron universal testing machine. The average maximum force was 2517, 3739, and 5387 N for dowel pins 4.76, 6.35, and 7.93 mm in diameter at a 12 mm penetration depth. Average specific conductivity was 2.45, 0.34, and 0.15 kgs⁻¹m⁻¹MPa⁻¹for trunk segments 0–12.7, 12.7–26.7, and 26.7–40.6 cm above the ground, with a corresponding estimated pump pressure of 0.14, 0.99, and 2.31 MPa, respectively.