Detection of viable Xanthomonas citri pv. citri, the causal agent of citrus canker in commercial fruits by isolation and PCR-based methods

Résumé

Xanthomonas citri pv. citri (Xcc), the causal agent of citrus canker, is a quarantine pest in many citrus-growing countries and is strictly regulated by the international phytosanitary programs. Citrus fruits with canker-like symptoms imported from India and Pakistan were collected from Riyadh supermarkets, Saudi Arabia. Different diagnostic detection methods, including culturing, direct PCR, BIO-PCR and bioassays were applied on the sampled citrus fruits. The direct culturing on KCB medium succeeded in detecting living cells of Xcc in 49 out of the 217 lesions observed on the sampled citrus fruits. However, only 5.55 and 24.77% from lesion washates of Pakistani and Indian fruits, respectively, caused citrus canker lesions on the inoculated grapefruit and/or Mexican lime leaves compared with 100% from pure cultures. The application of 3 diagnostic primer sets (2/3, J-pth1/2 and Xac01/Xac02) confirmed the bacterial etiology of 83 out of the 217 lesions. Based on BIO-PCR assay, 57 out of 217 lesions were Xcc positive. Significant differences were observed between direct PCR and BIO-PCR results. Overall, this study shows that chemical and physical fruit treatments prior citrus fruit exportation is not 100% efficient in killing all Xcc populations. Moreover, it is essential to integrate different diagnostic techniques to improve the accuracy of detection of quarantine bacteria and consequently prevent their introduction and dissemination.