Trunk Fungus on Birch

Fungal Contamination

• A variety of heart rot fungi infest birch trees. Wind-borne fungal spores from growths on previously infected trees may travel several miles to infect new ones. They enter birches through damage from ice and snow, animal browsing, fire or improper pruning. Mushroom spores infect trees for only a few days each year. Conks, however, produce billions of spores each day. They expose birches to contamination for months at a time.

Birch Defenses

• Intact birch bark keeps all heart rot fungi at bay. When spores breach damaged bark, the tree's topmost layer of inner tissue, or sapwood, releases chemicals. These kill some of the tree's own cells while simultaneously slowing the fungi's spread. The sapwood's cambium, a layer separating the inner bark and wood, walls off the bark wound from the xylem network that carries the tree’s water and nutrients. The tree’s already-dead heartwood contains fungus-resistant chemicals but in much smaller amounts than those found in most tree species. This defensive flaw allows some of the invading spores to establish and spread within the tree.

Types of Rot Fungi

• White and brown rot fungi attack birches. The texture of an infected tree's heartwood indicates its attacker's type. White rot fungi, including oyster mushrooms (Pleurotus ostreatus), turkey tails (Trametes spp.) and artist's conk (Ganoderma applanatum) leave the heartwood white to yellowish, with a stringy or spongy, moist feeling. Dry, powdery heartwood indicates an infection of sulfur fungus (Laetiporus sulphurus) or conk-producing birch polypore (Pitoporus betulinus).

Trunk Fungus Appearance

• Sulfur fungus, with orange-yellow conks that measure up to 1 foot across, is easy to identify. Birch polypore's short-stemmed, white to brown conks surface in summer, age to black and persist all winter long. Both growths indicate late-stage heart rot. The oyster mushroom fungus produces clusters of flat, white mushrooms between 2 and 8 inches across. Colored and hairy turkey tail fungi produce clumps of flat-topped mushrooms. Up to 10 inches wide, they have concentrically patterned circles in white or shades of brown.

Prevention

• Heart rot most often affects older birches. Once established, it resists management. Correctly pruning younger birches to the desired shape eliminates the need for large limb removal -- and large wounds -- on older trees. Pruning young trees outside the ridge of bark on their branch crotches encourages cambium formation and faster healing.