Characterization of Two Major Genetic Factors Controlling Quantitative Resistance to Melampsora larici-populina Leaf Rust in Hybrid Poplars: Strain Specificity, Field Expression, Combined Effects, and Relationship with a Defeated Qualitative Resistance Gene.

ABSTRACT Two genetic factors explain a significant proportion of the variability for quantitative resistance to Melampsora larici-populina leaf rust in a Populus deltoides x P. trichocarpa F(1) progeny. One is inherited from P. deltoides and is associated with a defeated qualitative resistance gene R(1), and the other, R(US), is inherited from P. trichocarpa. To assess the potential contribution of these two factors for durable resistance breeding, 284 genotypes from this F(1) progeny were studied in laboratory experiments with three M. larici-populina strains and in a field experiment under natural inoculum pressure. Results confirmed that both factors can have strong beneficial effects in the laboratory. These effects were strain specific, thus impairing their chances for durability. However, association of both factors led to synergistic effects in most situations. In accordance with good field-laboratory relationships, especially those involving uredinia-size laboratory measurements, field effects of both resistance factors were significant. R(US) led to a significant reduction of rust colonization on the most infected leaf in the field, and its effect was significant both in the presence and the absence of R(1). In contrast, the presence of R(1) was useful in the field only when R(US) was absent. The nature of the genetic relationship between both factors remains unknown, but benefits from their association should be quantified over a longer period to evaluate potential adaptation of the pathogen.