Effect of plant age and length of growing season on the development of blister rust cankers in western white pine

The hypothesis that resistance to blister rust in western white pine increases with tree age and a short growing season was tested. For a short growing season, 18 clones (40 ramets) from mature parent trees were inoculated and placed at a high-elevation site (1000 m). Resistant and susceptible seedlings (68 and 214, respectively) were also inoculated and established at the high-elevation site. After 3 years, there were cankers on the resistant (97%) and susceptible (99%) seedlings, while none of the 40 older ramets were infected. For a longer growing season, the surviving 38 ramets (17 clones) were reinoculated and maintained at a low-elevation site (50 m) and observed for canker development. After 3 years, 11 ramets (10 clones) became infected. Eight of the cankered ramets (73%) had slow-canker-growth resistance responses. These observations are consistent with the above hypothesis. There are implications for the deployment of certain resistant stocks, as the effectiveness of resistance to disease may va...