First author: Department of Forest Resources, University of Idaho, Moscow 83844; and second author: USDA Forest Service, North Central Research Station, 1561 Lindig Ave., St. Paul, MN 55108
Stem canker, caused by Septoria musiva, is the most serious disease limiting intensive hybrid poplar culture in eastern North America. Populus deltoides (D) is itself resistant, but the susceptibility of western black cottonwood, P. trichocarpa (T) is apparently dominant in the F1 generation. To test a hypothesis of recessive inheritance of canker resistance, a three-generation T × D pedigree was deployed in the field in Iowa and Minnesota. In both sites and in keeping with expectations, P. trichocarpa and its F1 hybrids were susceptible to canker, whereas the P. deltoides parent was resistant. In Iowa, 10 of 70 F2 (TD × TD) individuals were free of canker, suggesting that a single recessive gene might control resistance. In the third year in Minnesota, more resistant individuals than expected were seen in the F2 generation and in TD × D and T × TD backcross progenies due to disease escape. By the fifth year, this was no longer an issue, but winter injury may have eliminated many clones. Qualitatively, however, evidence for recessive inheritance of resistance was still obtained. The only canker-free clones were in the TD × D backcross and the F2 generation, the two progenies in which they were expected. However, conclusive evidence that recessive canker resistance is conferred by a single gene was not obtained in this field study.