Distribution of copper resistance gene variants of Xanthomonas citri subsp. citri and Xanthomonas euvesicatoria pv. perforans

In Taiwan, numerous crops are threatened by Xanthomonas diseases such as citrus bacterial canker caused by X. citri subsp. citri and tomato bacterial spot mainly caused by X. euvesicatoria pv. perforans. Foliar sprays of copper-based bactericides have been frequently used for control of plant bacterial diseases. However, in Taiwan not much attention was paid on copper-resistant (CuR) Xanthomonas spp. and their impact on disease control efficacy of copper-based bactericides. In this study, CuR Xanthomonas isolates were collected from citrus and tomato in Taiwan. Compared with the pronounced effect on the copper sensitive isolate, spraying of copper hydroxide at the recommended rate of 0.5 kg/ha could not protect tomato plants against bacterial spot caused by the CuR isolate. Phylogenetic analysis of concatenated copper resistance genes, copL, copA, and copB, indicate that the Taiwanese CuR isolates belong to the worldwide clade. In addition to the three previously reported variants of the copB gene, analysis of complete copB sequences from xanthomonads associated with citrus and solanaceous hosts revealed the other three variants of copB and their global distribution. Copper-resistant Xanthomonas isolates from Taiwan have the two unreported variants of copB genes which differ from the other three previously reported types in the sizes and structures. The information provided here reveals the necessity to develop and include alternative measures rather than relying on foliar sprays of copper bactericides for sustainable control of tomato bacterial spot in Taiwan.