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 of 19 strains was compared by inoculating five poplar clones and one willow clone in two sets of tests performed during June and September, respectively. Canker severity was measured 2 yr later by determining the lengths and the girdling index of cankers. Analysis of variance indicated 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. Race 3 was totally avirulent on the clone Italica, although the other races gave relatively small differences in canker severity. Strains belonging to races 1, 2, 3, and 5 were isolated in continental Europe (Belgium, France, and the Netherlands), whereas race 4, more virulent to Populus trichocarpa clones, was isolated in Britain or close to the Belgian coast.