QUANTITATIVE TRAIT LOCI CONFERRING RESISTANCE IN HYBRID POPLAR TO SEPTORIA-POPULICOLA, THE CAUSE OF LEAF-SPOT

Leaf spot of native black cottonwood, Populus trichocarpa, and its hyb rids, is caused by Septoria populicola in the Pacific Northwest. F-1 c lones of the most common poplar hybrid in the region, P. trichocarpa X Populus deltoides, are typically intermediate in disease phenotype be tween their susceptible P. trichocarpa and resistant P. deltoides pare nts. To uncover the genetic basis of the resistance of these hybrids t o S. populicola, a three-generation, P. trichocarpa X P. deltoides ped igree was evaluated for leaf spot by determining the percentage of spo tted leaves in a randomized, replicated planting during the 4th and 5t h years of tree growth. A genome map-based analysis of quantitative tr ait loci (QTL) revealed that a two-QTL model explained 68.3% and 61.2% , and 71.9% and 70.3%, of phenotypic and genetic variance, respectivel y, in the F-2 generation over the 2 years. One QTL conditioning resist ance was significant (threshold value for the log(10) of the odds rati o = 2.9) both years, while the second QTL was different in map positio n each year. All three QTLs were dominant, inherited from the resistan t, non-native P. deltoides parent, and located on linkage groups A, M, and X. The segregation pattern of resistance to S. populicola in the pedigree coupled with the genome map-based analysis suggests that domi nant alleles at two to three resistance loci complement each other to produce a highly resistant phenotype.