Poria Incrassata: House-Eating Fungus

The solution to this and all mold/fungi problems in residential and commercial properties is to stop the moisture intrusion.

Remove one of the three necessary components for life, and mold/fungi will not appear, thrive, and flourish.

In the event fungi does appear treatment with http://www.pestproducts.com/decayfungi.htm or similar products after the water has been stopped is the common curative.

Decay fungi grow on and in wood, and destroy the wood substances as they grow. Generally, conditions that are favorable for subterranean termites are also favorable for decay fungi, and vice versa. Temperature affects the activity of both termites and decay fungi and thereby affects the rapidity of their destructive action. Subterranean termites and decay fungi both require an abundance of moisture, but termites can obtain their moisture in galleries situated in moist soil and can then feed on dry wood, whereas most decay fungi cannot colonize and decay wood which is below the fiber saturation point, a moisture content of about 30% (Amburgey, 1972). However, some fungi can transport their own water from damp soil.

Poria incrassata

This is the most destructive fungus. It becomes established in wood on or under damp soil, such as in foundation forms left alongside a foundation or under a porch slab. It can transport its own water from the moist soil to normally dry wood. Both coniferous and broadleaf species of wood are attacked, but the fungus is generally confined to structural timbers in buildings or to stored lumber. Poles, posts, ties, or slash in the forest are rarely attacked. The fungus occurs throughout most of the United States, but particularly in the southern and Pacific Coast states (Humphrey, 1923; Boyce, 1961).

Description. Water is conducted from damp ground through strands of mycelium comprising the rhizomorph, which is rootlike in appearance (figure 102, bottom). The rhizomorph varies in color from white to brown to brownish black, depending on its age, and varies from 3 mm to as much as 5 or 8 cm in thickness. The water-conducting strands can extend for 30 ft (9 m) or more across the surface of concrete, brick, or other impenetrable material and upward to the second or third floor of a building, spreading out into flattened veins embedded in sheets of mycelium. Over this entire area, the wood is moistened by the fungus and made susceptible to decay, even though it would otherwise be too dry.

The wood is moist while the fungus is alive, and in a cross section of infested timber, progress of the fungus is indicated by a dark stain (figure 102, top). The common name, "dry rot," presumably derives from the appearance of the wood after the fungus is dead. The severely decayed wood is brown, and breaks up into irregular chunks resulting from shrinkage cracks both across and in the direction of the wood fiber (figure 102, bottom). Sheets of white mycelium can often be seen in the cracks. Owing to the extraction of cellulose by the fungal hyphae, the wood becomes very light in weight and loses its structural strength. The dry material has a distinct odor where the more succulent fruiting bodies occur, reminding one investigator of "drying slippery-elm bark" or of certain of the fleshy Hydnums (fungi) when they are dried out (Humphrey, 1923). The fruiting bodies can reactivate the infection if the wood becomes damp again.

Paperlike, fanshaped sheets of mycelium that appear on wood in damp places are often the first indications of decay. Masses of mycelium may protrude along the bottoms of baseboards or where timbers are joined. Leathery, crustlike sporophores are produced annually. They vary in color, from orange through olivaceous to deep purplish black, depending on their age and location, and all these colors may be represented in order from the margin to the center of a single fructification (Boyce, 1961).

Type of Wood Attacked and Rate of Destruction

It happens that the rough frame lumber most commonly attacked by dry-rot fungus is coniferous, but Humphrey (1923) pointed out that the fungus is an omnivorous saprophyte that can attack and destroy almost any of the commercial woods of the United States, even such reputedly highly durable ones as cedar, cypress, juniper, sequoia (redwood), catalpa, greenheart, black locust, sassafras, white oak, black walnut, and black cherry, all of which are severely attacked and sometimes completely destroyed.

Rapidity of Destruction

Another important characteristic of Poria incrassata is the rapidity with which it can destroy structures. A frame house in Florida suffered severe loss in 3 years, and in 5 years about 25% of the house was visibly affected. A 3-year-old house in Kentucky suffered severely from decay of subfloor timbers, flooring, and casings. In another house in Louisiana, much of the front wall and 750 sq ft (69 sq m) of flooring had to be replaced within 3 years after construction. In a store in that state, heart cypress paneling laid over a brick wall lasted only 6 months (Humphrey, 1923).

Other Decay Fungi

The same kind of decay as produced by Poria incrassata is produced by the teardrop fungus, Merulius lacrymans, which occasionally causes much damage to structural timbers in the northern part of the United States and in Canada. The 2 rots cannot be distinguished unless sporophores are present. In Europe, where P. incrassata does not occur, M. lacrymans is the "dry-rot fungus," and is very important.

A considerable amount of what has been said about P. incrassata pertains also to M. lacrymans, including the rhizomorph and its water-conducting capacity. The specific name lacrymans, which means "weeping," refers to the characteristic the fungus shows in active growth under damp conditions, when innumerable globules of water are seen, "which sparkle in the light of a torch like a large number of teardrops" (Hickin 1963c).

The mycelium of the teardrop fungus grows rapidly, and is snowy white, but where the edge of an expanse of mycelium comes into contact with drier air or is exposed to light, it becomes bright yellow (Hickin, 1963c). When the mycelium of the closely related Poria incrassata grows through the cracks in the floor into a lighted room, the mycelium turns orange in color, and this color change has been likewise observed in partially lighted basements (Humphrey, 1923)

The prodigality of nature is indicated by the estimate that 1 sq m of a sporophore of M. lacrymans will produce over 50 million spores per minute over a period of many days, and that other wood-decay fungi have been found to be even more fruitful.

Many species of decay fungi attack buildings. In New York state, Poria incrassata is not known to occur, but in an extensive survey, 11 other species of Poria were found attacking wooded buildings, resulting in 22.8% of the incidence of decay. In the same family of fungi (Polyporaceae) Lenzites saepiaria made up 12.4% of the total and, in the family Hydnaceae, Odontia spathulata was found with about the same frequency (12.8%,) (Silverborg, 1953). The latter species produces a white, stringy rot, unlike the brown, cubical type associated with the other common building-decay fungi. In the foregoing investigation, Lenzites trabea was found in 4.6% of the instances of decay. This fungus came prominently to the attention of entomologists in the early 1960's, when an extract from L. trabea rotted wood was found to be an attractant to termites (Esenther et al., 1961). It was later discovered that this extract induced trail-following by the eastern subterranean termite, Reticulitermes flavipes, similar to that induced by a trail-marking pheromone secreted by the sternal gland of this insect (Smythe and Coppel, 1966a). Trail-marking substances isolated and purified from R. viriginicus and from wood infested with L. trabea were found to be identical (Matsumura et al., 1969).

Molds and Stains

Wood may be attacked by molds, which are superficial fungi, generally green, yellowish green, red, or black, depending on the species. Molds do not damage wood and can be easily scraped off, but they indicate that conditions are right for the growth of decay-producing fungi and that corrective measures should be taken.

Wood is also affected by sap-stain or blue-stain fungi. They penetrate more deeply than molds, but do not significantly weaken the wood. The fungi are usually bluish, and cannot be scraped off wood like molds. They do no damage unless they occur where they may be considered to be unsightly, but like the molds, they indicate that conditions are favorable for the growth of the more destructive decay fungi (Anonymous, 1963).

Control of Decay Fungi

The same measures and precautions during and after construction that decrease the susceptibility of subarea structural timbers to subterranean termites also pertain to the control of decay fungi. They include not only the measures practiced to eliminate excessive moisture, but also the use of pressure-treated lumber and the removal of all wood scraps or other cellulose debris that might provide food for fungi or termites.

All that is required for the control of a waterconducting decay fungus such as Poria incrassata is to break the connection between the source of water and the infected structural wood member. The source of water should be eliminated, if possible, and the water-conducting rhizomorphs should be removed. The wood will then dry out and the fungus will die. Replacement of the infected wood member is required only if the wood is too weak to support its load. Verrall (1968) advised against dependence on surface applications of wood preservatives, or on such supposedly decay-resistant woods as redwood, as controls against fungi that are "water-conductors."

Wood used in construction should not have a moisture content of more than 20%. Depending on the atmospheric conditions in the area in which it is dried, air-dried wood may have a moisture content between 6 and 24%, whereas kiln-dried wood has a content of from 6 to 12% Wood that is not pressure-treated with a wood preservative should not be allowed to contact the soil. The caulking around windows, doors, bathtubs, and sinks should be maintained to prevent accumulation of water in walls. Paint decreases penetration of water into wood except at joints between the wood members (Amburgey, 1972).

In many homes, condensation of moisture within insulated walls and floors favors wood decay. Warm, moist air passing into walls comes in contact with wood members with temperatures below the dew point of the air, resulting in deposit of excess water. Measures that may be taken to reduce or prevent condensation vary in different geographical regions, and are discussed in chapter 10, under "Sources of Moisture in Buildings."

Does it cause a house to smell? I know of a house that when I go inside for 15 minutes the smell jumps on me, it's in my hair and clothes.

It's in everything there, like tupperwear, furniture, papers.