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As the climate continues to change, gaps in our understanding of how the altered environment will affect forest hosts and their pathogens widen. In some areas pathogens thought to be present for centuries are changing their behaviour. Dothistroma needle blight caused by the fungus Dothistroma septosporum in northwest British Columbia (BC), Canada, is a good example. In this area both the pathogen and the host, lodgepole pine (Pinus contorta var. latifolia), are considered native species, but here Dothistroma has been responsible for killing mature host trees, which is unprecedented. A plausible link between warmer, wetter summers and directional climate change has been suggested as the primary driver. Those environmental conditions appear to be affecting the host/pathogen relationship for other diseases in the neighbouring central interior of BC including comandra blister rust (Cronartium comandrae). Disrupted host/pathogen relationships tend to favour the short-lived more adaptable pathogens rather than their long-lived hosts. These changes in forest health have not been well accounted for in fields of forest science that have been built on stability and predictability.