Phenotypic and Genetic Characterization of Novel Strains of Xanthomonas axonopodis pv. citri Which Induce Atypical Symptoms on Citrus Leaves in Taiwan

A total of 46 strains of Xanthomonas axonopodis pv. citri isolated from Taiwan were examined for their pathogenicity on leaves of Mexican lime (Citrus aurantifolia), grapefruit (C. paradisi), Liucheng (C. sinensis) and lemon (C. limon). These strains were grouped into three types (A, A(superscript f) and A(superscript r)) based on symptoms induced on leaves of the four citrus species. There were 43 (93.5%) strains belonging to type A which induced typical erumpent canker lesions with watersoaked margin on leaves of all four citrus species. Whereas strains in the other two types A(superscript f) and A(superscript r) caused atypical symptoms on citrus leaves. Strain XW47 in type A(superscript f) which induced typical erumpent canker lesions with watersoaked margin on Mexican lime, but induced flat necrotic lesions with watersoaked margin on grapefruit, Liucheng, and lemon; Strains XW16 and XW121 in type A(superscript r) induced restricted and raised corky lesions with no watersoaked margin on leaves of all the four citrus species. Based on physiological, biochemical and genetic characterizations including NaCl tolerance, hydrolysis of gelatin, oxidation of carbon sources, polymerase chain reactions with primers specific to X. axonopodis pv. citri (Xac), rep-PCR and DNA sequence of leucine-responsive regulatory protein (lrp), strains in types A(superscript f) and A(superscript r) were identified as Xac. These two types of atypical symptoms-inducing Xac strains could be differentiated from two atypical Xac strains A(superscript *) and A(superscript w) isolated from southwest Asia and Florida by lrp sequence assay and amplified DNA profiles of PCR with primer pairs pthAP7/pthAR2, 2/3, 4/7 or ERIC1R/ERIC2. They appeared to be novel strains of Xac. We designated these two types of atypical symptoms-inducing strains as Xac-A(superscript f) type and Xac-A(superscript r) type, respectively.