Occurrence and Significance of Dry Rot Fungus, Serpula lacrymans

Serpula lacrymans, the dry rot fungus, is more commonly found in old water damaged buildings than in nature. It does not seem to thrive at temperatures above 25 ° C. The natural habitat of Serpula lacrymans and a related fungus, Serpula himantioides is believed to be conifers wood in the western mountains of the Himalayas (See References For Dry Rot Fungus).

Detection of Dry Rot Fungus

In indoor environment, the dry rot fungus can attack any wood structure. Its mycelium can also penetrate the masonry and attack the wood located on the other side of thick walls. Serpula lacrymans is rarely considered during routine indoor air quality inspections. In the laboratory, the dry rot fungus is very rarely recovered from routine indoor air or surface samples and it could very easily go undetected. Although Serpula lacrymans is never considered during routine inspections the growth of this fungus on building materials and consequent damages are quite visible and easily recognized.

Species of Serpula

There are five different species of Serpula. However, the only species reported both as a contaminant of the indoor environment and as a potential health hazard is Serpula lacrymans. The aggressive form of Serpula lacrymans is believed to have originated from Asia, but some equally aggressive genotypes have been reported in Europe, North America and South America and Oceania (See References For Dry Rot Fungus). Studies have shown that the genetic base for Serpula lacrymans is very narrow and it’s completely different from Meruliporia incrassata. In the literature, Meruliporia incrassata is said to be the most prevalent in North America (See References For Dry Rot Fungus).

Dispersal of Serpula lacrymans

The mode of dispersal for Serpula lacrymans can be vegetative by asexual spores or mycelia, or sexual by basidiospores. Basidiospores are the main agent of dispersal. The dry-rot fungus also is capable of vegetative local dispersal by producing mycelial strands with the potential to grow
several meters across inorganic materials in search of additional organic materials. Dryness resistant spores of Serpula lacrymans can remain viable for up to 20 years  (See References For Dry Rot Fungus).

Growth requirements for Serpula lacrymans

Serpula lacrymans has an optimum growth temperature range of between 19 to 21 °C. It has low tolerance for higher temperatures, which probably explains its absence from the tropics and regions where summer temperatures are high. In buildings, Serpula lacrymans can attack wood containing very low water content, for instance 20 to 25%  (See References For Dry Rot Fungus). Under optimal conditions it can grow as fast as 80 mm per day. Compared with most other wood decay fungi, spores of Serpula lacrymans require only a small amount of free water to germinate and grow. For example, for most wood fungi, the minimum water activity for spore germination is 0.9993 whereas for Serpula lacrymans the minimum water activity is 0.970 (13). Note that the minimum water activity for a microorganism could be high or low depending on factors such as temperature, food source, pH., etc.

Serpula destroys healthy wood due to its unique ability to transport water over long distances through mycelial structure called rhizomorphs. In this way, it is able to wet the timber and then feed on the cellulose.

Growth of Serpula lacrymans on building materials

In buildings, Serpula lacrymans produces pancake-like fruit bodies, 2–20 mm thick and can reach up to 1 m in diameter. The fruit bodies (figure 1 A) release into the air billions of yellow, brown or reddish basidiospores (figure 1 B). Once the spores germinate, mycelium penetrates the wood and later the flat, fleshy fruiting bodies develop on the wood surface. The life cycle continues as countless basidiospores are produced and released into the air. This large quantity of airborne spores settles and covers large areas and is visible to the naked eye as brown or red dust.

The concentrations of spores present in indoor air of affected buildings may be very high. In one study, concentrations of up to 360 000 spores per cubic meter of air could be measured in a contaminated basement  (See References For Dry Rot Fungus). It has been estimated that the spore release by a 1 meter square fruit body can reach 3 x 109 spores per hour.

The mycelium of Serpula lacrymans changes color depending on its physiological state: the young mycelium is white. It then becomes brown or reddish brown at maturity (See References For Dry Rot Fungus). 


References For Dry Rot Fungus