Timing of copper sprays to protect mechanical wounds against infection by Xanthomonas citri subsp. citri , causal agent of citrus canker

Copper sprays are the most important measure for control of citrus canker. Mechanical wounds caused by cultural practices are entry points for the causal bacterium, Xanthomonas citri subsp. citri, and exacerbate the disease. It is not known whether additional applications would protect mechanical wounds from infection by X. citri. This study aimed to assess the predisposition of mechanical wounds to infection and the efficacy of a single copper spray applied at different times for protecting wounds against X. citri under greenhouse conditions. Access of wounds to infection was assessed by spray-inoculating a suspension of the bacterium on leaves from 0 to 7 days after wounding. Timing of a copper application was assessed by treating the wounded leaves at different points before or after bacterial inoculation and/or wounding. The incidence of wounds with citrus canker decreased over time after wounding. No lesions developed in wounds inoculated 7 days after injury. Copper sprayed at all timings reduced the incidence of citrus canker at wound sites compared to the non-treated leaves. Copper sprayed before wounding and/or inoculation showed the greatest reduction (>90%) in the incidence of citrus canker. The reduction in canker associated with wounds ranged from 91.8 to 96.1% for copper sprays applied from 24 h before to 0.5 h after wounding and inoculation, and was 67.8% when copper was applied up to 8 h after wounding and inoculation. As X. citri is splash-dispersed, we conclude that wounds need to be protected with copper sprays if rain is forecast within 7 days of a wounding event. In case no rain is expected, there is no need to protect the trees with copper. However, if trees are wounded when the canopy is wet, a copper spray should be applied before or immediately after the wounding event (but not more than 8 h later). A single appropriately timed copper spray allows the protection of wounds against citrus canker even after the tissue is injured.