Environmental and inoculum-source effects on resistance of Idaho F2 western white pine in British Columbia

Abstract

An Idaho orchard seed source (RTB F2) of western white pine (Pinus monticola) was confirmed as having fewer blister rust cankers (average 18%) caused by Cronartium ribicola than native stock (average 62%) in plantations in the interior of British Columbia, Canada, but not at low, coastal elevations (average 59%). Placing RTB F2 seedlings at different sites after five separate inoculation tests confirmed that more cankers developed at the coast than in the interior of British Columbia. This suggested that differences between coast and interior results were due to environmental effects, not to inoculum sources, as coastal and interior inoculum caused the same effects. Putative resistance traits in RTB F2 are needle shed and fungicidal short shoot. However, needle shed occurred only at one site, and it was associated with poor growing conditions, not genetic traits. Most seedlings classified as fungicidal short shoot 2 years after inoculation eventually developed cankers. However, after 3 or 4 years, some seedlings shed their infected needles naturally without developing cankers (a spots-only response), and others developed resistant cankers called slow canker growth. Based on published data, only 35% of the inoculated seedlings were expected to be cankered, but because needle shed did not occur, and because fungicidal-short-shoot seedlings were observed longer than in the previous study, 98% (mean value) were cankered when planted at the coast, and 74% (mean value) were cankered when planted in the interior. As an alternative to a needle-shed trait to account for the good field resistance (few cankers) of Idaho F2 stock in the British Columbia interior, it is hypothesized that good field resistance is the result of both a short growing season and natural infection at a later age, which enhances resistance responses