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 hybrids, is caused by Septoria populicola in the Pacific Northwest. F-1 clones of the most common poplar hybrid in the region, P. trichocarpa X Populus deltoides, are typically intermediate in disease phenotype between their susceptible P. trichocarpa and resistant P. deltoides parents. To uncover the genetic basis of the resistance of these hybrids to S. populicola, a three-generation, P. trichocarpa X P. deltoides pedigree was evaluated for leaf spot by determining the percentage of spotted leaves in a randomized, replicated planting during the 4th and 5th years of tree growth. A genome map-based analysis of quantitative trait 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, respectively, in the F-2 generation over the 2 years. One QTL conditioning resistance was significant (threshold value for the log(10) of the odds ratio = 2.9) both years, while the second QTL was different in map position each year. All three QTLs were dominant, inherited from the resistant, 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 dominant alleles at two to three resistance loci complement each other to produce a highly resistant phenotype.