The effect of citrus planting orientation on the gradient and incidence of huanglongbing

Successful primary spread of huanglongbing (HLB) within the citrus orchard occurs by the dispersal of the Asian citrus psyllid (ACP; Diaphorina citri) carrying the HLB bacterium “Candidatus Liberibacter asiaticus” (CLas) from external areas to blocks located in the orchard periphery. Higher ACP population is usually concentrated in a strip of up to 200 m from the orchard's edge. Consequently, a higher number of HLB‐symptomatic trees is also detected in this edge strip. Control strategies should be intensified in this strip to prevent further ACP dispersal and therefore to avoid more infected trees in inner blocks. The effect of the citrus planting direction, either parallel or perpendicular to the orchard's edge, was evaluated by comparing the HLB gradient and incidence in blocks located at the orchard's edge by the analysis of satellite images. Six areas with two neighbouring blocks (one planted in parallel and the other in a perpendicular direction) were evaluated in two periods. The exponential decay model was fitted to the HLB incidence at each distance in each block. Parameters from the model (a and b parameters) were similar between blocks planted in parallel (0.60 and 0.018 per metre, on average) and perpendicular (0.75 and 0.016 per metre, on average) planting directions. The total HLB incidence in blocks with a perpendicular planting direction (±30.3%) was higher than in blocks with a parallel planting direction (±22.8%). Overall, planting parallel to the orchard's edge is beneficial in reducing HLB incidence in orchards. This strategy can be integrated into HLB management programmes. We compared the HLB gradient and incidence in blocks with planting orientation in parallel (PA) and perpendicular (PE) to the orchard's edge, revealing that PA resulted in a lower total HLB incidence in the area than PE.