A major gene for resistance to Melampsora medusae f sp deltoidae in a hybrid poplar pedigree

A three-generation Populus trichocarpa x P. deltoides (TxD) hybrid poplar pedigree, comprising backcross and F-2 progenies, was used to investigate the genetic control of resistance to an isolate of Melampsora medusae f.sp. deltoidae. Necrotic flecking and rust severity were evaluated in two successive years in different field locations, in three growth-room experiments, and in a leaf-disk assay in the laboratory. Necrotic flecking in the field and growth-room experiments was found to be governed by a single, dominant gene inherited from the I! trichocarpa parent. This was shown in both the Mendelian analysis of the full pedigree and a genome map-based analysis of qualitative and quantitative trait loci (QTL) focused in the F-2 population. Nonparametric and QTL analyses of rust severity in the field and growth-room experiments indicated that the single gene for necrotic flecking, Mmd1, played a major role in resistance to M. medusae f.sp. deltoidae in the Pacific Northwest. Mmd1 was mapped to linkage group Q, approximately 5 centimorgan (cM) from a restriction fragment length polymorphism (RFLP) marker, P222. In contrast, expression of necrotic flecking in the leaf-disk assay was low, did not indicate a role for necrotic flecking in resistance, and did not support a simple genetic interpretation.