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Lodgepole pine dwarf mistletoe (Arceuthobium americanum Nutt. ex Engelm.) is one of the most damaging pathogens of jack pine (Pinus banksiana Lamb.) in western Canada. Jack pine forests in the colder, more northerly areas, however, are free of dwarf mistletoe, suggesting that the pathogen is limited by low temperature. The effect of extreme cold temperatures on germination rates of overwintering dwarf mistletoe seeds and survival of dwarf mistletoe germinants was evaluated. Germinative ability of overwintering seeds increased with increasing temperatures between -39 and –35 °C, regardless of seed source. Exposure period also strongly influenced germination rates. Exposure to temperatures near –38, –46, or –53 °C for 96 or 144 h was almost always lethal. At –37 °C, germination was greater after 48 h than after 96 h, although it was still significantly lower than in the controls. Temperatures down to –6 °C in late spring did not reduce germinant survival. Overall, these results may explain the absence of dwarf mistletoe from northern areas commonly exposed to periods in winter with minimum temperatures below about –40 °C. These areas are potentially at risk from the pathogen if the climate of Canada’s northern interior continues to warm as it has over the last several decades.
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