Dothistroma needle blight, caused by the fungus Dothistroma septosporum, is a major pest of pine plantations in the Southern Hemisphere, where both the host and the pathogen have been introduced. In northern temperate forests where the pest and host trees are native, damage levels have historically been low; however, Dothistroma is currently causing extensive defoliation and mortality in plantations of lodgepole pine in northwestern British Columbia, Canada. The severity of the disease is such that mature lodgepole pine trees in the area are succumbing, which is an unprecedented occurrence. This raises the question of whether climate change might enable the spread of the disease by surpassing an environmental threshold that has previously restricted the pathogen's development in northern temperate regions. Establishing a causal relationship between climate change and local biological trends is usually difficult, but we found a clear mechanistic relationship between an observed climate trend and the host–pathogen interaction. A local increase in summer precipitation, not climate warming, appears to be responsible. We examine whether the recently observed climate change trend exceeds natural fluctuations in the local climate.