GENETIC DIVERSITY AMONG XANTHOMONAS-CAMPESTRIS STRAINS PATHOGENIC FORSMALL GRAINS

A collection of 51 Xanthomonas campestris strains from throughout the world was studied to detect and assess genetic diversity among pathoge ns of small grains. Isolates from barley, bread wheat, bromegrass, can ary grass, cassava, maize, orchard grass, rice, rough-stalked meadow g rass, rye, timothy, and triticale were analyzed by pathogenicity tests on bread wheat cv. Alondra and barley cv. Corona, indirect immunofluo rescence, and restriction fragment length polymorphism (RFLP). Three p robes were used for the RFLP analysis. They were an acetylaminofluoren e-labelled 16S+23S rRNA probe from Escherichia coli and two P-32-label led restriction fragments from either plasmidic (pBSF2) or chromosomal (pBS8) DNA of X. campestris pv. manihotis. Strains clustered in 9 and 20 groups with the rRNA probe and the pBSF2 DNA probe, respectively. Strains of X. campestris pv. graminis, X. campestris pv. phleipratensi s, and X. campestris pv. poae are shown to be related but are also dis tinguishable by RFLP patterns, serology, and pathogenicity on bread wh eat. Strains pathogenic only for barley and not for wheat grouped toge ther. Another group is temporarily designated deviant X, campestris pv . undulosa. These South American isolates from bread wheat did not rea ct by indirect immunofluorescence and produced atypical lesions in pat hogenicity tests. The results stress the need to perform pathogenicity tests before strains are named at the pathovar level. The importance of the different probes used for epidemiological studies or phylogenet ic studies of closely related strains is underlined.