First Report in Thailand of Xanthomonas axonopodis pv. citri-A* Causing Citrus Canker on Lime

Asiatic citrus canker, caused by Xanthomonas axonopodis pv. citri (Xac-A), is a bacterial disease of economic importance in tropical and subtropical citrus-producing areas. Xac-A can cause severe infection in a wide range of citrus species and induces erumpent, callus-like lesions with a water-soaked margin. Severe attacks cause premature fruit drop and twig dieback. It has consequently been submitted to eradication efforts and international regulations. Recently, a group of strains with a host range restricted to Mexican lime (Citrus aurantifolia), but not infecting grapefruit (C. paradisi) known to be very susceptible to Xac-A, was described in different areas of southwest Asia, including Saudi Arabia, Oman, Iran, and India (3). Phenotypic and genetic similarities with Xac-A designated it as a variant called X. axonopodis pv. citri-A*. A variant with a similar restricted host range, designated X. axonopodis pv. citri-A^w, was also detected in Florida and likely originated from India (2). Amplified fragment length polymorphism (AFLP) analysis of 26 isolates from Thailand and additional reference isolates from Xac-A, -A*, -A^w, and X. axonopodis pv. aurantifolii (2,3) using SacI/MspI and four primer pairs (unlabeled MspI + 1 [A, C, T, or G] primers and 5'-labeled SacI + C primer for the selective amplification step), separated the Thai isolates into two distinct groups. A group composed of 20 isolates from different citrus species, including five isolates from lime, was closely related to strains with a large host range and previously identified as Xac-A. A second group, solely composed of six isolates from lime, was genetically related to Xac-A*. All Thai isolates were collected before 1991. Isolates genetically closed to Xac-A* originated from the central and northern provinces, whereas isolates related to Xac-A originated from all sampled provinces. On the basis of AFLP, no Thai isolate was related to X. axonopodis pv. aurantifolii. A specific X. axonopodis pv. citri nested-PCR assay (1) produced the expected fragments for all Thai isolates. Mexican lime leaves inoculated with Thai isolates of Xac-A and Xac-A* using a detached leaf assay (3) showed typical canker symptoms 1 week after inoculation. When inoculated to grapefruit or sweet orange, the Thai isolates genetically related to Xac-A* by AFLP analysis did not induce any canker symptoms, while isolates related to Xac-A produced canker symptoms on these two citrus species. In Thailand, Xac-A* induced severe symptoms on lime trees, including extensive defoliation and numerous twig cankers that often developed as diebacks. The Xac-A* variant appears epidemiologically important on lime, consistent with previous reports from southwest Asia. The detection of Xac-A* in Thailand makes it necessary to evaluate its geographic distribution in southeast Asia. References: (1) J. S. Hartung et al. Phytopathology 86:95, 1996. (2) X. A. Sun et al. Plant Dis. 88:1179, 2004. (3) C. Vernière et al. Eur. J. Plant Pathol. 104:477, 1998.