The principal hosts of Leucostoma canker, which is caused by Leucostoma kunzei, are the Colorado blue spruce and its varieties. The disease also occurs on Norway spruce, Koster's blue spruce, white spruce, Douglas fir and other spruces planted as ornamentals. The most commonly attacked trees are 15 to 25 years old and 20 to 40 feet high. The tree seldom dies outright because only a few limbs are killed each year, but the progression of dieback ruins the symmetry of the tree and reduces its aesthetic value.
Cankers are inconspicuous, with little or no bark deformation.
The fungus grows throughout the inner bark, causing the death of the portion beyond the canker. A heavy pitch flow is characteristic of the disease. The pitch is a clear amber color when freshly exuded, but later it hardens to form a crusty whitish coat over the cankers. Lower, healthy branches may sometimes become covered with resin exuded from infected branches above. Needles on infected branches turn grayish-green, then brown, and may drop immediately or persist on the branch for a year.