The effect of RUS, a major quantitative resistance (QR) factor inherited from Populus trichocarpa, was assessed in two Populus deltoides · P. trichocarpa F1 progenies against eight isolates of Melampsora larici-populina, the causal agent of Eurasian poplar rust. Six isolates were identified on which RUS had no significant effect. The first RUS-defeating isolate identified suggested a pre-existing potential to overcome RUS in the pathogen’s populations. The P. deltoides genetic background made no difference either to the ability of a given isolate to overcome RUS or to the relative and absolute aggressiveness of the isolates. This study illustrates how extreme the isolate-specificity ofQR can be. It also yields insights into the relationship between size of uredinia and sporulation rate, discussing the epidemiological significance of spore production permm2 uredinium.