Angular leaf spot of Caucasian alder incited by a pathovar of Xanthomonas arboricola

Caucasian alder (Alnus subcordata) is among the major constituents of the Hyrcanian forests extending all along the southern coast of the Caspian Sea in northern Iran. An angular leaf spot with a yellowish chlorotic halo has attained a widespread occurrence throughout the forest and suburban areas of the northern provinces including Guilan, Mazandaran and Golestan. Strains of a Xanthomonas sp. were consistently isolated from the symptomatic leaves of A. subcordata. Colonies of the isolates were yellow circular, convex and mucoid on nutrient agar containing glucose or sucrose. In multilocus sequence analysis using the genes gyrB, rpoD, dnaK, gltA, fyuA and gapA, the representative isolates occupied a clade shared by the known pathovars of Xanthomonas arboricola. The isolates were relatively heterogeneous phenotypically, but more so in their rep-PCR fingerprints. Pathogenicity of several isolates was confirmed on seedlings of A. subcordata and Alnus rhombifolia, whereas Alnus cordata, Alnus glutinosa and Alnus crispa appeared not to be susceptible. Based on these characteristics, the isolates causing angular leaf spots of Caucasian alder appear to represent a novel pathovar of X. arboricola.