Efficacy Evaluation of Different Copper-Based Bactericides Against Citrus Canker and Candidate Role of Citrus COPT Genes in Response to Xcc

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is commonly managed with copper-based formulations, yet the performance of individual products may differ between laboratory and plant assays. We compared 16 commercial copper-containing formulations in plate-diffusion and root-drench assays, and inhibition zones varied among products. Thiosen Copper, however, produced no visible inhibition zone on agar but suppressed Xcc growth after root application. In sweet orange, this suppression appeared earlier than with Bordeaux Mixture at the same nominal total Cu concentrations. Thiosen Copper raised the leaf chlorophyll index without a significant increase in total leaf Cu, whereas Bordeaux Mixture led to greater leaf Cu accumulation. In separate experiments with Poncirus trifoliata, the two formulations produced different growth and tissue Cu responses, and root treatment altered PtCOPT transcript levels. Transient overexpression of CsCOPT4 in sweet orange leaves was accompanied by lower Xcc populations. These results reveal formulation-specific responses and nominate CsCOPT4 as a candidate for further study.