Serpula lacrymans


Serpula lacrymans, formerly named  Merulius lacrymans or  Merula lacrymans, is known as the dry rot fungus and is by far the most dangerous and destructive of the wood destroying fungi. It can cause extensive damages by decomposing the carbohydrates of wood materials in houses {4}. Often referred to as a “building cancer” {6}, this fungus is considered as the most damaging destroyer of wood construction materials in temperate regions {5} but can also cause structural damage to masonry; however, the fungus often remains undetected until it has caused advances stages of decay {6}.


Kingdom Fungi Order Boletales
Phylum Basidiomycota Family Serpulaceae
Class Homobasidiomycetes Genus Serpula

The genus  Serpula is part of the phylum Basidiomycota, also known as the « club fungi». Basidiomycetes and Homobasidiomycetes are characterised primarily by the production of sexual spores (basidiospores) on a specialised cell called a basidium, usually in groups of four. Many fungi of this phylum have septum structures called a clamp-connection during most of the life cycle (see laboratory section). No other group of fungi has such structures.

There are five distinct named species of  Serpula registered with the UNIPROT data bank {14}. The only species reported both as an indoor contaminant and linked to health problems is  S. lacrymans.

The origin of the aggressive form of  S. lacrymans is mainland Asia, although a few aggressive genotypes have migrated worldwide to Europe, North and South America and Oceania {7}. Several studies have indicated that  S. lacrymans has a narrow genetic base {4}, being completely different from  Meruliporia incrassata, another “house eating fungus” belonging to a separate taxonomic family.


Serpula lacrymans has been found, either in the wild or in old buildings, on all five continents, Europe {18; 9}, North and South America {7}, Oceania {7} and Asia {19}. In nature, it does not appear to grow well at temperatures above 25°C {7}. 

This fungus is not very visible in the wild where it colonises and destroys tree stumps, mostly evergreens (CTBA 1996) {18}.There are no official census numbers on its prevalence in nature. Its ecological niche is never far from inhabited spaces.

Indoors, it can attack wood although it has been reported that its mycelium can penetrate masonry and infect timber on the other side of thick walls.

More details

The 'wild' habitat of  Serpula lacrymans and its close relative , S. himantioides, has been described.  The fungi were found growing on well decayed coniferous wood within the Narkanda region of the Western Himalayas at between 2800 and 3100 m above sea level.

During the same study, researchers observed that temperature, osmotic potential, initial pH and air-current influenced the average colony extension rate of 'wild' type and a selection of building isolates on 2% malt extract agar. The 'wild' and building isolates behaved as two separate cohorts; the 'wild' isolates appeared to be less affected by temperature extremes, whereas building isolates grew more rapidly under moderate microenvironmental conditions {19}.

Growth requirements

S. lacrymans has an optimal growth temperature of 19-21  °C and does not tolerate high temperatures, which probably explains its absence from the tropics and regions with high summer temperatures {7}.

Indoors, it can infect wood with very low water content, even as low as 20-25% {8} and, under the proper conditions, can spread as fast as 80 mm a day; Serpula spores require a  moisture content in timber of only 20% to geminate. Because it requires such low concentrations of available water in order to germinate and grow, it can destroy completely sound wood, reducing it to a dry and brittle state. Such destruction is accomplished by its unique ability to transport water over great distances through its mycelial structure, thus dampening sound timber and allowing it to feed from the cellulose.

Water activity :  Aw =  0.20 to 0.25  {8}

Growth on building materials or indoor environment

Within a building, this fungus frequently produces pancake-like fruiting bodies discharging billions of airborne red basidiospores (8.0-10 x 5.0-6.0 mm). Once spores have germinated and the mycelia have infested the wood structures, fruiting bodies develop as flat, fleshly surface growth, where again, immense numbers of rusty red basidiospores are produced and released in the air; this large quantity of airborne spores will deposit itself of surfaces leaving large areas covered with a red dust visible to the naked eye.

More details

Indoor air spore concentrations in old wooden or abandoned houses may be high. In one study, up to 360 000 spores per cubic meter were measured in a contaminated cellar {9};  however, in the same study, the authors mentioned that no spores were measured in the undisturbed air of an old unaffected cottage. Once matured, this fungus produces a fruiting body which spreads its microscopic spores throughout the building in order to infect other areas. 

Laboratory section

Normal laboratory precautions should be exercised in handling cultures of this species within Biosafety Level 2 practices and containment facilities.

Serpula lacrymans is rarely investigated for during regular inspections nor is it grown from air or surface samples in routine indoor environmental studies and can therefore be very easily overlooked in routine laboratory work. In any event, the growth of  Serpula lacrymans on building materials and the ensuing damages are quite visible and easily recognised in situ.

Colony, macroscopic morphology

The fungus grows on malt agar (MEA): with a diameter growth rate of 50 mm to 110 mm  within 2 weeks, at room temperature.

The colonies are woolly, with divergent fan-shaped hyphae at the edge; the hyphae are white to cream-colored or yellowish and become red in color with KOH. The reverse side of the colony remains slightly yellowish.

Microscopic morphology

Edge and aerial hyphae are 3-9 µm wide and are septate and branching, usually with clamp connections. The clamp connections is a wall structure connecting the producing cell to the adjacent cell, over the septum. This type of cellular connection exists only in Basidiomycetes.

Among the aerial mycelia, some hyphae are unseptate and thick-walled, 2-4.5 µm in diameter, sometimes encrusted with yellow crystals.

Mycelium strands consist of a core of narrow, interconnected hyphae, which are surrounded by thicker hyphae (description adapted from Allergopharma fact sheet {17}).

Spores (basidiospores) are oval to kidney-shaped, thick walled, yellowish to rusty brown.  

Specific metabolites

Organics compounds (including VOCs)

In vitro studies of  S. lacrymans metabolites stemming from cultures on glass slides and on pine shavings {6} resulted in the identification of 19 volatile organic compounds (VOCs).

The major volatile identified was 1-octen-3-ol, which accounted for up to 95% of the volatile profile from living mycelium growing on glass slides. The volatile profiles from S. lacrymans on pine shavings were more complex, with the monoterpenes a-pinene, limonene and 3-carene as major VOC compounds.


There are no reported mycotoxins produced by Serpula that would be deleterious to humans or animals.

S. lacrymans produces some toxic substances that are biologically active against bacterial cells  in vitro.

More details

Some metabolites isolated from  S. lacrymans may have interesting properties. 
Serpula lacrymans was found to be a potential source of thermostable antibiotic(s) presenting antibacterial properties in an  E. coli broth microdilution test {16}.

Adverse health reactions

Irritation and inflammation

No specific irritation or inflammation symptoms have been attributed to S. lacrymans.

Toxic effects (mycotoxicosis)

No human or animal mycotoxicosis has been associated with S. lacrymans

Infections and colonisations

No human or animal infection has been associated with S. lacrymans

Allergic components

There are no identified S. lacrymans allergen fractions.

Allergic reactions

In Great Britain, Frankland et al. {12} reported that among 100 allergic patients (various allergies – poly-allergic) recruited in hospital external clinic, 20 were positive (skin prick test and intradermal test) to allergenic extracts of Merula lacrymans (S. lacrymans). Ten years later, Herxheimer et al. {11} reported that among 200 people suffering from seasonal allergies, three of them (1.6%) reacted positively following bronchial challenge carried out with nebulized extracts of S. lacrymans (Merulius lacrymans in the publication).

Herxheimer et al. {10} subsequently evaluated 19 people exhibiting seasonal asthma (or an aggravation of asthmatic symptoms at the time of the pollinic season) by intradermal test and bronchial challenge. Fourteen patients had a positive reaction to S. lacrymans and to other Basidiomycetes included in the extracts; the authors concluded that the basidiospores or the fragments of these mushrooms could trigger seasonal allergies. More recently, among 105 asthmatics patients treated in an allergology ward in Poland, 24% showed a positive skin prick test to Merulius lacrymans (S. lacrymans) compared to 10% in controls.

It should be noted that cases in those studies had been already sensitized with various allergens. In that respect, a clinical response to fragments or S. lacrymans spores of can fall under a context of polysensitisation (sensitizing to multiple allergens, frequently observed in atopic subjects). Several polysensitized people are also polyallergic, expressing clinical signs in the presence of various antigens. As an example, among 2,434 polysensitized patients studied by Pham-Thi et al. {24}, 80% were sensitized to mite allergens, grass pollen (75%), trees pollen (57%), animal dander (50%) and moulds (17%); this observational, prospective and cross-sectional study showed that polyallergies are a common feature of polysensitized patients.

Hypersensitivity pneumonitis

During the seventies, an extrinsic allergic alveolitis was reported in a teacher who displayed breathlessness of insidious onset, living in a flat with extensive dry rot (formerly Merulius lacrymans). The patient had specific IgG and IgE antibodies against this fungus and serum precipitins, as well as a positive reaction (intradermal) to S. lacrymans {8}. A study conducted both on clinical data and environmental samples obtained in the homes of twelve patients diagnosed with allergic alveolitis revealed positive reaction following an exposure to a mixture of spores of moulds and S. lacrymans; a serology precipitin test showed that 5 patients had serum precipitins against S. lacrymans, and three experienced a late forced expiratory volume (FEV) response when exposed to nebulised extracts of the fungus {3}. However, the authors did not associate allergic alveolitis specifically to S. lacrymans exposure, emphasizing the concomitant presence of moulds in patients’ homes and possible synergistic reactions between these sources of allergenic reactions.

Possible health effects

The findings of this literature review are coherent with those of Pottier et al. {20} publication; we could not locate any other scientific or medical publications at the time of updating this Compendium. These reviews also highlight the very low number of scientific publications involving S. lacrymans for other possible health effects; moreover, the most recent communication in this respect was published in 1991.

Pottier et al. {20} and Garon et al {21} specified that the various clinical symptoms reported by the occupants of the investigated buildings, in a study on buildings degraded by S. lacrymans, were not necessarily correlated with the presence of spores of this dry rot fungus. They highlighted that buildings contaminated by wood decay fungi (like S. lacrymans) are also contaminated by several moulds, because the same environmental conditions support the growth of these two groups of fungi (high relative humidity and dampness). There is sufficient epidemiological evidence to show that occupants of moldy buildings are at increased risk of health problems {22, 23}. Garon et al {21} also reported the importance of molds involvement in health effects reported by some occupants of investigated buildings.

According to the scientific and medical information collected to date, S. lacrymans is clearly not a pathogenic or infectious fungus, and does not produce toxins. With regard to the alleged allergic effects, the scientific and medical literature published is very scarce and does not provide any clear evidence. The absence of medical publication on this subject for at least two decades reinforces this conclusion. Consequently, the possibility of a link between exposure to spores or fragments of S. lacrymans and specific effects on human health is extremely weak.

Specific settings

Nosocomial infections

No  Serpula infections have been reported: no particular outbreaks due to exposure to  S. lacrymans have been reported in the hospital setting.

Occupational diseases

No  Serpula infections have been reported: no particular outbreaks due to exposure to  S. lacrymans have been reported in the workplace.

Diagnostic tools


No infections or colonisations have been reported: cultures are thus not warranted. 


No infections or colonisations have been reported: histological examinations are thus not warranted. 


Neither  Serpula  lacrymans allergens, nor antigens are part of routine immunology panels. 

No allergen is registered with the American «Biological Products Deviation Reporting»  surveillance program of the Federal Drug Administration (FDA) {13}.

Allergen extracts are available for skin tests in Europe.

Test IgE IgG Antigens Other
Skin Tests X      
Complement fixation        


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