The fungal disease Cytospora canker is common on many species of trees in Wyoming including aspen, cottonwood, poplar, elm, willow, birch, maple, mountain ash, linden, honeylocust, ash, sycamore, mulberry, oak, apple, stone fruits and spruce.
    Cytospora invades trees through wounds and is transmitted by splashing water, wind, insects or many other methods.  Cytospora cannot be controlled with fungicides but its progression can be slowed by removal of infected branches.  Prune 4 to 6 inches ahead of the canker, cutting back into un-infected tissue.  Disinfect tools between cuts to avoid spreading the disease.
    To protect trees from infection, protect trees from stresses and follow proper watering and fertilizing practices.