white pine blister rust
A rust disease of five-needle pines, originating in Asia. It is an early example of an alien invasive species. At the turn of the 20thC the disease was imported into the north-eastern USA from Europe on seedlings of eastern white pine (Pinus strobus). It has spread from the north-east to the west of the US where the relatively high humidity assists spore dispersal. The infection progresses to the stem to cause perennial cankers or swollen areas that eventually girdle and kill the shoot. Seedlings are most susceptible since the main stem is relatively likely to be infected, while mature trees are more likely to suffer die-back in the outer crown.