Any of several diseases of pines, caused by rust fungi (see fungus) of the genus Cronartium. Blister rust affects sapwood (see wood) and inner bark and produces external blisters from which additional spores of the fungus are released and resin oozes, forming characteristic hardened masses on the trunk. It affects pines of all ages and sizes, retarding growth and weakening stems as it spreads along the trunk, sometimes killing the tree; young trees are killed more quickly than older ones. Measures taken to fight blister rust include growing resistant varieties, destroying nearby alternative host plants, observing strict sanitation measures, and spraying with fungicides.