Differences between this and other decay fungi:
The water conducting fungus (Poria Incrassata) occurs mainly in the
southern states but may occur anywhere in the United States. When Poria
Incrassata attacks a building, spectacular damage often results: once well
established it can destroy large areas of flooring and walls every year
or two. Fortunately, control is relatively simple. i.e., the permanent
elimination of the water supporting its growth. Although Poria Incrassata
is relatively rare, the rapid and extensive damage it can cause makes it
desirable to understand the conditions leading to attack, the signs indicating
an attack is in progress, and methods of prevention and control of an attack.
Water conducting fungi differ from most other decay fungi in several
respects:
Large, tough rhizomorphs (water conducting strands) are formed which
conduct water from a constant source (usually the soil) to dry wood in
buildings, wetting it sufficiently to support decay. As decay proceeds,
water is conducted to dry wood adjacent to that already colonized. In this
manner, as long as the supply of water is available, water conducting fungi
can colonize and decay the wood to the entire house.
Early control recommendations called for the removal of decayed wood
and all sound wood within 2 feet (60cm) of obvious decay on the assumption
that Poria Incrassata, once established, can decay wood with metabolic
water as the sole source of moisture. However, experience clearly shows
that conducted water is necessary to maintain growth under most, if not
all, usual building conditions. Occasionally, Merulius lacrymans, which
is primarily European, occurs in the northern states. Control is the same
as for Poria Incrassata.
Poria Incrassata is more sensitive to high temperatures than most
decay fungi and is killed in moist wood at temperatures only moderately
above air temperature maxima. This explains why Poria Incrassata occurs
in the more protected parts of buildings and not in wood exposed to full
sun. Poria Incrassata can decay not only the common construction woods
but also heartwood of western red cedar, redwood, cypress, and juniper.
Therefore, when this fungus is present, heartwood of naturally decay- resistant
wood species must not be relied on as a sole means of prevention. In addition
to attacking most woods classed as naturally decay resistant, laboratory
tests show that Poria Incrassata is resistant to many copper containing
fungicides.
The practical significance of this tolerance is uncertain, but no
failures of wood treated with copper fungicides have been reported in buildings.
Therefore, there is little justification for excluding any commonly accepted
preservative in building construction, but in repairing damage by Poria
Incrassata, wood treated with non copper fungicides should be given preference.
Poria Incrassata is extremely sensitive to drying. In naturally infected
wood, it can survive only 32 days of air drying. In laboratory tests, all
artificial infections were dead in one day at 10 percent relative humidity,
in 5 days at 65 percent, and in 10 days at 90 per cent. Sensitivity to
drying, in conjunction with the need for conducted water, forms the basis
for the presently recommended simplified control measures.
Factors Favoring Attack:
use of moist, infected wood
Attacks of Poria Incrassata occur mostly in houses only a few years
old or in those with recent structural changes. This suggests that the
use of infected wood could be a method of introducing the fungus into the
buildings. As in houses, infections are rare in lumber storage yards, but
a number have been observed in yards with untreated sills in soil contact.
Infected soil, used in earth filled structures, such as porches, or landscaping
are other possible means of introduction.
Wood Soil Contracts:
The point of initial development in a house usually is untreated
wood in soil contact. The most common contracts are sills or headers in
contact with earth fills under porches and steps, untreated wood piers,
untreated sills on low concrete slabs without a subslab vapor barrier,
forms for pouring concrete left in place, and siding in soil contact because
of elevated grade levels.
Constant Water Supply:
The constant sources of moisture found in the many cases studied
in the south were: soil (78%), rain seepage (16%), moist concrete (12%),
and leaky plumbing (9%). In the rain seepage cases, faulty downspouts wetted
concrete or brickwork, which acted as reservoirs between showers; the moist
concrete cases are wet basements or low-lying slabs on wet sites where
the slabs were unprotected by vapor barriers; and the leaky plumbing cases
were mainly fine spray leaks that wetted substructures sufficiently to
support decay but were not obvious unless careful inspections were made.
In the majority of cases studied, the soil under infected buildings
was wet or moist. The fungus can maintain itself in moist soil under soil
covers and can attack covers, such as roll roofing, that contains cellulose
fibers. Thus, lack of soil drainage is an important factor in attacks The
most severe attacks have been in houses over wet, poorly ventilated crawl
spaces. But ventilation alone will not prevent attack, since severe damage
has been found in houses 18 inches (46cm) above ground on open, concrete
piers.
Extensive surveys show that most houses with factors known to be
associated with Poria attack, such as forms left under a porch, still never
are attacked. Since most houses are not attacked, it often is difficult
to induce many owners to apply the preventive measures that will avoid
damage should the fungus be present. However, any feature known to favor
attack should not be allowed to remain.
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Mark Pierce
Oregon Certified Home Inspector
Office: 800-995-4063 
Cell:503-757-7784
OCHI #345-NIBI
#143902RT-PCO #153495-WSDA WDO #61752
