DIFFERENTIAL HOST-PATHOGEN INTERACTIONS AMONG CLONES OF POPLAR AND STRAINS OF XANTHOMONAS-POPULI PV POPULI

To verify the occurrence of physiologic races in Xanthomonas populi pv . populi, the causal agent of oozing canker of poplar, the virulence o f 19 strains was compared by inoculating five poplar clones and one wi llow clone in two sets of tests performed during June and September, r espectively. Canker severity was measured 2 yr later by determining th e lengths and the girdling index of cankers. Analysis of variance indi cated a significant poplar clone-strain interaction and no significant clone-strain-inoculation date interaction for most strains, using the two variables. Five putative physiologic races were characterized. Ra ce 3 was totally avirulent on the clone Italica, although the other ra ces gave relatively small differences in canker severity. Strains belo nging to races 1, 2, 3, and 5 were isolated in continental Europe (Bel gium, France, and the Netherlands), whereas race 4, more virulent to P opulus trichocarpa clones, was isolated in Britain or close to the Bel gian coast.