Incidence and Distribution of Conifer Broom Rusts in Colorado

Surveys to determine the incidence and distribution of broom rust diseases caused by Chrysomyxa arotostaphyli on Engelmann spruce and Melampsorilla aaryophyllacearum on white fir and subalpine fir were conducted on 41 sites within six National Forests (Gunnison, Uncompahgre, San Juan, Rio Grance, White River, and Arapaho) in Colorado in 1978. On over 20,000 trees surveyed (and 79 square miles), average disease incidence was 4.2% and 2.3% for spruce and fir, respectively. Incidence varied widely among sites and tree age classes sampled. Rust incidence was lower on seedlings and saplings than on pole- and sawtimber-size trees. Spruce broom rust was common on most sites; highest incidence was 28% on pole-size trees of the Pine Ranger District, San Juan National Forest. Fir broom rust was common on white fir on the San Juan and Rio Grande National Forests, but rare on all sampled subalpine fir. More brooms were found on dead spruce and fir than on live trees. Almost half the brooms on diseased spruce and about one-fifth of the brooms on infected fir were within one foot of the bole. Most dead broomed spruce and fir trees had associated bole cankers. Trees with brooms near the bole and trees with bole cankers are affected by the disease more than trees with a few small branch brooms. Only a small percentage of broomed spruce and fir trees had spike tops or broken tops. Cutting guidelines for infested stands are presented.

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