Tall, symmetrical fir trees (Abies spp.) are graceful pine family conifers most at home in cool or cold mountain environments. Of the United States' 12 fir species, the Douglas fir has the most commercial importance. Its strong, stable wood makes excellent lumber. All fir trees, including decay-resistant Douglas firs, are susceptible to fir tree fungus infections. Some of these fungi merely disfigure the trees. Others are deadly.
Velvet top fungus (Phaeolus schweinitzii) affects old growth forest stands of Douglas firs. Several Phytophthera fungi species are responsible for fir tree root rot. Melampsorella caryophyllacearum causes fir broom rust disease.
Velvet top fungus is nearly impossible to diagnose because its only external symptoms are flat, brown fungal caps around or on the fir tree's base in the summer and fall. Phythophthora-stricken firs have yellowing needles on slowly growing--or wilting--branches. The dead trees retain their needles. The dead roots and needles are cinnamon or black. The trees' wood may be butterscotch in color.
Fir broom rust produces chronic "brooms" of short, thick twigs on the tree's crown branches. Thicker and shorter than normal needles, the brooms' needles develop orange capsules that make them appear yellow-orange during the spring. Insect and needles drop from the tree in the autumn. Cankers--thickened tissue--become visible on the branches at the base of the brooms when the needles fall, according to the Natural Resources Canada Forestry Development website.
Velvet top fungus can infest a fir tree for as long as 300 years before its effects are obvious. During that time it decays the upper roots and weakens the lower stems. It leaves the sapwood unharmed. Root and stem damage, however, eventually topples the trees. Root rot fungus travels through the roots into a fir tree's wood, finally infecting and killing the entire plant. Fir broom rust needle loss compromises photosynthesis, slows tree growth and is occasionally fatal.
Both velvet top and root rot fungi can attack fir trees for years--or, in the case of velvet top, centuries--before causing visible damage. Fir broom rust fungus, on the other hand, produces twigs, brooms and needle loss in the spring, summer and fall immediately following infection.
Because velvet top fungus exhibits no symptoms until it has already weakened old-growth Douglas firs, no practical means of controlling the disease exists. Root rot control requires a multi-step program, including planting trees in soil known to be free of the fungus; using raised beds where soil drainage is poor; pre-planting application of a soil fungicide; and post-planting spring and fall fungicide applications.
Removing large brooms from firs infested with broom rust improves the trees' appearance, notes Brennan A. Ferguson of Ferguson Forest Pathology Consulting. It may be advisable for trees in home landscapes. Removing entire trees with brooms near their trunks is also advisable, in case the fungus has weakened their wood. Other control measures aren't necessary. Brooms left on fir trees with no landscape or commercial value provide nesting sites for birds and other wildlife.