Landscape - Decay in Trees

I came across a good article on decay in trees and thought I would share it with you.

Living landscape trees are subject to internal decays of branches and trunk and of buttresses and roots. Decay in living trees appears as a softening or weakening of the woody xylem tissues of the sapwood and the heartwood. These decays are caused by fungi that can decompose the structural material of the tree.

Disease progress. The disease begins when a windblown spore of a wood-decay fungus comes in contact with a tree wound and, given the right conditions (possibly involving other non-decaying microorganisms), germinates. Wounds such as those caused by lawn equipment, construction activity, pruning, or wind and ice damage are typical locations for this activity. The germinating spore produces a germ tube and then branched hyphae and mycelia which invade fiber, vessel, tracheid, and ray cells of the wood. Wood often becomes discolored. The fungal hyphae release enzymes that break down cellulose (brown rot) or cellulose and lignin (white rot), thus providing food for the fungus and loss of rigidity for the tree. Parts of the tree may break and fall, or the tree may topple over if roots are decayed. The fungus reproduces by forming a mushroom, conk, or shelf-like structure (fruiting structure in which spores are formed) directly on the limbs, trunk, butt, root flares or on roots at some distance from the base of the tree.

However, trees are not passive victims; they respond to wounding and fungal invasion. When an injury occurs, the invading wood-decay fungi encounter tree defense mechanisms. The vertical movement of the decay fungus is impeded by plugging of xylem tracheids and vessels with resins and tyloses above and below the wound. Horizontal movement of fungi is impeded in an inward direction by the already existing growth rings and in a radial direction by toxic substances produced by the ray cells. Decay is prevented from moving outward to new growth by barriers laid down by the vascular cambium after injury occurs. Of these various defenses the strongest is that formed by the new growth. Consequently, decay will not proceed into subsequent yearly growth increments unless they are re-injured.

In circumstances where wounding is minimal and the tree is quick to respond to wounding and invasion, the tree suffers some internal discoloration and little or no decay, thus retaining its structural integrity. However, where wounding is extensive and when the tree is not capable of producing a quick, effective response, the decay fungus spreads and the tree is weakened.

Wood decay fungi are usually identified by their conks. The conks of some of the common wood decay fungi important to tree health are described here. The first four listed have slits, or gills on the underside of the conk or mushroom cap, and the others have pores on the conk underside. Several of them are characterized as having a broad host range and can attack many species while other decay fungi are specialized, attacking just one or a few tree species.

Armillaria mellea, the shoe string root rot or oak root rot fungus has a wide host range including conifers and hardwoods and is very important in shade tree pathology. White patches of mycelium and shoestring-like rhizomorphs of the Armillaria fungus can be found under the bark of infected trees. Clusters of gilled, tan-colored mushrooms may develop at the base of infected trees.

Flammulina velutipes (Collybia velutipes) annually produces clusters of mushrooms with stout stalks and moist, smooth reddish orange to reddish brown caps. The cap has white gills and is one to three inches across and the dark, hairy stalk is one to three inches long. This fungus produces white rot of a wide range of tree species.

Pleurotus ostraetus, the oyster mushroom, causes a white rot of the heartwood and sapwood of many landscape trees. The fungus produces an off-white fleshy shelf-like mushroom up to eight inches across. The mushroom may have a short stout stalk and the underside of the conk has gills.

Schizophyllum commune annually forms clusters of small, leathery gray fan-shaped conks one or two inches across. The gilled cap of this fungus is attached directly to the tree without a stalk. The fungus causes white rot on a wide range of declining and dead trees.

Fomes fomentarius produces gray, woody, perennial hoof-like fruiting conks. The conks have pores on their underside and may be eight inches wide and several inches thick. The fungus causes white rot on a wide range of trees.

Ganoderma applanatum (Fomes applanatus) forms a thin shelf-like gray-brown conk up to one foot across. The underside of the conk consists of minute white pores and the surface turns brown when bruised or scratched - giving it the name artists' conk. This fungus causes a white rot of a broad range of tree species.

Ganoderma lucidum also produces a large conk, usually at the base of the infected tree. The upper surface has a reddish-brown smooth varnish-like appearance while the undersurface with pores is white to tan. This fungus also has a wide host range.

Inonotus dryadeus (Polyporus dryadeus) causes a root and butt rot particularly on oaks. The decay begins on the root system and the fungus eventually reaches the butt of the tree where it forms large, tough, irregularly shaped gray to brown brown shelves with pores at or just above the soil line. With age, they become rough and dark brown.

Laetiporus sulphureus, (Polyporus sulphureus) also called the sulfur fungus causes brown rot of the butt and heartwood of living and dead trees. This fungus produces clusters of annual bright yellow or yellow-orange conks up to a foot across. The undersides of the conks are covered with minute pores. It has a wide host range.

Phellinus robineae (Fomes rimosus), a pathogen of black locust, produces yellowish brown to gray conks with a brown pore-bearing surface. The conks are several inches to a foot across and protrude shelf-like away from the trunk of affected trees.

Stereum gausapatum, a major oak pathogen, and other related Stereum species cause white rot of many different trees. Conks appear as clusters of thin, brownish, shelf-like structures a little more than two inches across. Injured conks may leak a red fluid.

Trametes versicolor (Coriolus versicolor, Polyporus versicolor) fruiting bodies annually form dense overlapping clusters on decaying branches and trunks. The caps, one to two inches across, are leathery and colorfully arrayed with zones of white, yellow, red, brown, gray, green and bluish green. Tiny white pores can be found on the cap underside. Generally causing white rot on stressed trees, it can in some circumstances cause heart rot of wounded, but not otherwise stressed trees.

Trametes hirsuta (Coriolus hirsutus, Polyporus hirsutus), somewhat larger than T. versicolor, produces leathery gray to brownish caps without zones. This fungus also causes a white rot of many tree species.

Reprinted from "SOME OF THE CAUSES OF DECAY IN TREES" By John Hartman in the August 13, 2007 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.