A MAJOR GENE FOR RESISTANCE TO MELAMPSORA-MEDUSAE F-SP DELTOIDAE IN AHYBRID POPLAR PEDIGREE

A three-generation Populus trichocarpa x P. deltoides (TxD) hybrid pop lar pedigree, comprising backcross and F-2 progenies, was used to inve stigate the genetic control of resistance to an isolate of Melampsora medusae f.sp. deltoidae. Necrotic flecking and rust severity were eval uated in two successive years in different field locations, in three g rowth-room experiments, and in a leaf-disk assay in the laboratory. Ne crotic flecking in the field and growth-room experiments was found to be governed by a single, dominant gene inherited from the I! trichocar pa parent. This was shown in both the Mendelian analysis of the full p edigree and a genome map-based analysis of qualitative and quantitativ e trait loci (QTL) focused in the F-2 population. Nonparametric and QT L analyses of rust severity in the field and growth-room experiments i ndicated that the single gene for necrotic flecking, Mmd1, played a ma jor role in resistance to M. medusae f.sp. deltoidae in the Pacific No rthwest. Mmd1 was mapped to linkage group Q, approximately 5 centimorg an (cM) from a restriction fragment length polymorphism (RFLP) marker, P222. In contrast, expression of necrotic flecking in the leaf-disk a ssay was low, did not indicate a role for necrotic flecking in resista nce, and did not support a simple genetic interpretation.