Poria Fungus Makes A Comeback

This article appeared in the February 1999 issue of PCT Magazine.

Poria (Meruloporia incrassata), a water-transporting member of the brown rot group of fungi, is fast becoming the most talked about wood-destroying organism in California. Long recognized along the Gulf Coast as a serious problem in houses with damp crawl spaces, poria is also a threat to homes with basements and slab foundations. Its distinctive liquid-filled tubes called rhizomorphs "bleed" when they are broken.

A SIMILAR M.O. Poria attacks structures almost exactly like a colony of subterranean termites. Dr. W. Wayne Wilcox, a researcher at the U.C. Forest Products Laboratory, Richmond, Calif., believes an inspector should have the same mind set whether he/she is looking for termites or looking for poria.

"These organisms come from different kingdoms, but their relationship to the structure is the same," he says. "Both ‘nest’ in the soil and reach out to the wood through cold joints and over various non-wood elements of construction. Termites build mud tubes to feed on the wood and the fungus pumps water up through its tubes to accomplish the same result." From the standpoint of damage, Wilcox adds, the most useful comparison is between poria and the Formosan subterranean termite.

A THREAT TO STRUCTURES. Like Formosan termite infestations, poria infections can destroy entire structures within a few years. When a poria infection is misdiagnosed as ordinary decay and repairs are made without breaking the connection to the soil, the whole house can be destroyed in as little as four months. The media have picked up on this ability to wreak economic ruin on home-owners and there have been numerous television and newspaper reports focusing on poria so far this year, especially in Southern California.

Luis De La Cruz, a termite inspector from California, received more than 1,000 calls recently after he described poria infections on a local television program. When he went out to confirm these suspected cases he was surprised to find that most of the homeowners had made the correct diagnosis.

PREVENTION. According to De La Cruz, poria infections in Southern California are almost always associated with infested roots or root fragments in the soil. This infected root material may have been left on the site when it was cleared for construction or it may have been brought in at a later date with fill or soil associated with landscape work. Spores are not believed to be a major source of new infections and the fungus does not appear to transfer in green lumber.

Like subterranean termite colonies, poria infections require moisture. And like subterranean termite problems, poria can be prevented by avoiding wood to soil contact, insuring adequate ventilation in crawl spaces by using high-quality pressure-treated lumber around potential entry points and by correcting the same problems with grade, irrigation systems, downspouts and other water sources that would contribute to termite problems.

According to Wilcox, most structures can be protected from poria by keeping the bare foundation visible, by providing adequate drainage and ventilation under the structure and by scheduling periodic inspections for rhizomorphs. De La Cruz says this is why early detection is important and why WDO inspectors who work in high-risk areas need to be familiar with this problem.

GROWING IMPORTANCE. Several factors are resulting in the growing incidence of poria problems in the pest control industry including: (1) the number of susceptible wooden structures is increasing; (2) more consumers are becoming aware of the problem and its economic importance; (3) the fungus and its damage are easy to recognize; (4) more inspectors are being trained to recognize it; and (5) pentachlorophenol and other chemicals are no longer available to control it.

Despite its economic importance, however, relatively little research has been done on poria and its control. PCOs who want more information should read Arthur F. Verrall’s classic paper, "Poria Incrassata Rot: Prevention and Control in Buildings," U.S.D.A. Technical Bulletin #1385, February 1968. Training seminars with updates on poria inspection and control are also available from the Pest Control Operators of California and other organizations.