Citrus greening disease (Phytophthora nicotianae), also known as Huanglongbing (HLB), is a systemic disease caused by a bacterium that is vectored by a psyllid. Huanglongbing infection impacts citrus roots by its restructuring of the root bacterial community. The motivation of this study was to evaluate the diagnostic utility of employing high-frequency ground-penetrating radar (GPR) to survey orange tree [Citrus sinensis (L.) Osbeck] lateral structural roots. Of particular interest was GPR’s ability to discriminate the roots originating from neighboring trees, since these trees may be in a dissimilar stage of HLB infection. Tests groves are situated on the Ridge and Flatwood soils of Florida. The sandy soil and shallow root systems are considered ideal operating parameters for shallow GPR 3-D investigations.
A 2,600-MHz antenna was found to effectively target and discriminate the larger structural roots originating from the target tree from those of its neighbors. The antenna reflections penetrated to 40-cm beneath the ground surface. On both landscapes, the larger 1st and 2nd order structural roots extended laterally outward within a root-zone layer approximately 10–30 cm beneath the surface. The roots increased in depth with their propagation outward. Smaller 2nd order and higher lateral roots were found in an upper layer that ranged approximately 5–5 cm beneath the surface. Those smaller roots provisioning near-surface fibrous roots were found concentrated above the larger structural roots. The roots imaged in the Ridge soils ran slightly deeper in both upper and lower root zones than the roots found in the Flatwood soils. The range of lateral rooting depths for both layers is slightly greater in the Ridge soils.