Presence of defeated qualitative resistance genes frequently has major impact on quantitative resistance to Melampsora larici-populina leaf rust in P.xinteramericana hybrid poplars

Qualitative resistance to Melampsora larici populina leaf rust inherited from North American species Populus deltoides did not allow for durable control of this pathogen in interspecific hybrid cultivars. Despite significant levels of strain-specificity, quantitative resistance would exert lower selection pressures on the pathogen populations, and hence could be more durable. Previous studies restricted to a large P. x interamericana (i.e., P. deltoides x Populus trichocarpa) F, family revealed that the presence of R(1), a segregating defeated qualitative resistance gene inherited from P. deltoides, had major beneficial effects on quantitative resistance. The present study was based on 14 F(1) families from a 4x5 P. deltoides x P. trichocarpa factorial mating design where at least four defeated qualitative resistances segregate 1:1. Even though quantitative resistance assessments were conducted in the laboratory with a M. laricipopulina strain able to overcome these qualitative resistances, their presence had a significant effect on the mean level and on the genetic variability for quantitative resistance. One unprecedented result is the identification of a defeated qualitative resistance which presence is associated with lower levels of quantitative resistance. Possible inferences on the nature of the genetic relationship between both resistance types are discussed.