Potential of Eurasian poplar rust to overcome a major quantitative resistance factor

The effect of R_US, 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 R_US had no significant effect. The first R_US‐defeating isolate identified suggested a pre‐existing potential to overcome R_US in the pathogen’s populations. The P. deltoides genetic background made no difference either to the ability of a given isolate to overcome R_US or to the relative and absolute aggressiveness of the isolates. This study illustrates how extreme the isolate‐specificity of QR can be. It also yields insights into the relationship between size of uredinia and sporulation rate, discussing the epidemiological significance of spore production per mm² uredinium.