Abstract
White pine blister rust, caused by the non-native, invasive fungal pathogen Cronartium ribicola, is a significant cause of mortality in white pines (Pinus subgenus Strobus) in North America. Along with climate-driven range contraction, mortality from blister rust can seriously impact the abundance and distribution of the nine white pine species native to the United States and Canada. Very little evaluation of this disease in southwestern white pine (Pinus strobiformis) has been previously undertaken, but genetic resistance to the disease has been documented, including major gene resistance (MGR) conferred by a dominant R gene. Data is emerging suggesting that the species also has quantitative disease resistance (QR). Our results suggest QR occurs at low frequency, with perhaps 10% of trees having a moderate level (> 35% survival). We assessed progeny arrays from 40 P. strobiformis families (1873 seedlings), originating from three populations, inoculated with C. ribicola. Subsequently, the seedlings were assessed for signs, symptoms and resulting impact in a common garden trial over a 7.5-year period to determine the types and frequency of resistance in a portion of this species’ range. There was a high incidence of both stem symptoms and mortality in the P. strobiformis families tested, and families ranged in survival from 0 to 84.6%. Three families had > 70% survival, representing perhaps the highest documented QR to date in a North American white pine species. Approximately 29.1% of the 441 surviving seedlings showed no stem symptoms, and of the approximately 70.8% of seedlings surviving with infections only few (24 of 316) had infections of moderate to high severity. QR traits associated with improved survival were primarily related to lower severity of infection, a reduced number of stem symptoms, and an increased number of bark reactions. Despite the high overall susceptibility, the presence of QR appears to be at a frequency and level useful to forest managers involved in restoration and reforestation efforts.
Highlights
Non-native invasive pathogens and pests have had a substantial negative impact on tree species and their associated ecosystems
Seventeen parent trees selected from the Lincoln NF, the site where the first P. strobiformis trees with major gene resistance (MGR) were documented, were canker-free, suggesting putative resistance, in addition three trees with moderate-toheavy cankering were selected to serve as susceptible controls (Lincoln S)
Many tree species in North America are susceptible to nonnative pathogens and pests (National Academies of Sciences, Engineering, and Medicine, 2019)
Summary
Non-native invasive pathogens and pests have had a substantial negative impact on tree species and their associated ecosystems. Cronartium ribicola J.C. Fisch, the invasive non-native fungal pathogen responsible for the disease white pine blister rust, has caused high mortality to both economically and ecologically important white pine species since its introduction in North America in the early 20th century (Kinloch, 2003). There are eight species of Pinus subgenus Strobus, known as the white pines or five-needle pines, in the western United States (and three of these occur in Canada). All of the North American white pine species are extremely susceptible to white pine blister rust (Hoff et al, 1980; Kinloch, 2003; Sniezko et al, 2008) and all but Pinus longaeva have C. ribicola within their native range in the United States and Canada, but the disease has not yet been documented in Mexico or Central America
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