Field-testing whitebark pine resistance to white pine blister rust: A simple, effective approach to progeny testing for restoration

The endangered conifer whitebark pine (Pinus albicaulis), native to high elevation forests of western North America, is declining mainly due to the introduced pathogen Cronartium ribicola, causing the disease white pine blister rust. Low levels of genetic resistance to blister rust are present in whitebark pine populations. Traditional methods of inoculating seedlings to determine resistance to the rust are costly, labour intensive, and have limited capacity due to facilities. This presents a bottleneck in the development of resistant material for restoring stands that have been heavily infected by the rust. Here we test an alternative approach to controlled inoculations that could screen large numbers of families with lower costs and requiring fewer specialized resources. We screened 214 open-pollinated families from 44 provenances from British Columbia (BC), Washington (WA), Oregon (OR), Idaho (ID), Montana (MT), and Nevada (NV) to determine: (1) the effectiveness of natural rust inoculation from Ribes nigrum in a common garden; (2) family- and provenance-level resistance to blister rust; and (3) climate variables related to population-level rust resistance. Eighty-one of these families were previously screened by the US Forest Service in Cottage Grove, Oregon using standard artificial inoculation methods. The natural inoculation was effective, with 73 % of seedlings displaying stem symptoms of the disease, and 95 % showing rust infection. A clear relationship was found between distance from the nearest Ribes nigrum plant and severity of blister rust. Linear mixed models with spatial correlations were fitted to rust infection data using ASReml-R to estimate breeding values, heritability, and among-population differentiation (QST) for rust resistance. Population differentiation for rust resistance was moderate, with the highest resistance in provenances from the Cascade Mountains of Washington, and the most susceptible families from the BC Chilcotin and Okanagan regions. Heritability of rust resistance was moderate (0.23). The field testing was broadly consistent with classifications of families into resistant, moderate, and susceptible groups based on artificial inoculations. This method of screening could be used at a broader scale to determine families resistant to white pine blister rust without specialized testing facilities and increase the availability of resistant seedlings for restoration.