Ability of Melampsora larici-populina to Overcome a Major Quantitative Resistance Gene: Rapid Screening Procedure and Frequency of the Phenomenon

Poplar leaf rust caused by Melampsora larici-populina poses a significant threat to populiculture, undermining both qualitative and quantitative resistance. This study developed and validated a rapid screening protocol to detect M. larici-populina strains capable of bypassing the Rus quantitative resistance gene in Populus. Four pathogen populations, representing contrasting selection pressures, were screened, and a collection of overcoming and non-overcoming isolates was established. Results showed that on average 50% of strains could bypass Rus, confirming the pathogen’s considerable adaptive potential. The created collections enable broader aggressiveness testing of poplar cultivars and facilitate investigations into the genetic determinants of resistance circumvention at individual and population levels.