Mold forms a necessary part of our environment. It plays a major role in breaking down materials in the outdoor environment, but the growth of mold in indoor environments is of concern and should be avoided. The molds reproduce by spores and are disseminated by various agents. For spores to germinate there are a variety of requirements such as water, an acceptable climate, a suitable substrate, and appropriate nutrients for that particular fungus. In this article, we will be discussing those fungi for which harvested wood provides an acceptable substrate for germination, as well as sufficient nutrients to sustain growth.
In living trees, the bark forms a very good barrier against fungal and insect attacks. This excellent barrier is lost after the harvest of trees. Inside the bark, the wood can be divided into the heartwood and the sapwood. Heartwood is located in the center of the tree and is so named due to its central position. It is physiologically dead. Sapwood forms the outermost wood, just inside, or next to the bark. The innermost layer of sapwood is living and conducts water and nutrients.
When the wood is processed for lumber, sugars and other nutrients are exposed and readily available for use by fungal spores that arrive. Spores can be transferred to lumber through a variety of agents, but primarily by air transfer. Note that it is generally felt that to avoid mold growth; the moisture content of the wood should be reduced to 20% or less. Fungi that use wood as a substrate for growth can be broadly grouped into three types: surface fungi, sap-stain fungi and decay fungi.
Surface moisture supports growth for a variety of molds and can easily be removed. This constitutes the surface fungi group and causes discoloration of wood surfaces due to presence of spores, hyphae and mycelia (thread-like structures). It is mostly confined to the wood surface and generally does not penetrate or damage the wood structure.
Decay fungi grow primarily in the heartwood and sapwood, degrading the structural components of the wood. They utilize the major components of wood which include cellulose, hemicelluloses and lignin.
Sap-stain fungi are only found in the sapwood and utilize the stored nutrients such as sugars, starches, proteins, and fats that are found there. These fungi cause staining in the sapwood of living trees as well as processed lumber. Since they are common in lumberyards, they are often called “lumberyard mold.” The sap stain itself has also been called blue stain due to bluish or grayish black discolorations of the sapwood which develops due to fungal growth. Sap-stain fungi often thrive in wood processing mills where a variety of tree species and a variety of trees at different ages can be mixed together. This provides many opportunities for contact and cross infection. Insects and arthropods often aid this process by acting as causal vectors.
Fungi that are associated with sap-stain are placed into three groups:
Ophiostomatoid fungi, which includes species of Ceratocystis, Ophiostoma and other related fungi
Black yeasts such as Aureobasidium and Rhinocladiella
Dematiaceous molds such as Alternaria and Cladosporium
The Ceratocystis and Ophiostoma (C/O) group of fungi are a very common example of sap-stain fungi and are often the first to colonize freshly cut wood. Some species of Ceratocystis and Ophiostoma are pathogenic to trees, while a few others are saprophytes that grow on decaying organic matter. Commonly recorded species include C. coerulescens, C. fagacearum, C. pluriannulata, C. pilifera, O. piceae, O. ulmi and O. novo-ulmi.
The “C/O group” belongs to the division/phylum ascomycete and are comprised of a few genera. Morphologically they are very similar which one reason they are grouped together is. The fruiting bodies (ascomata) of Ceratocystis and Ophiostoma are perithecia. These structures are dark, globose, with elongated necks that produce spores in a sticky mass at their apices. The small sticky spore mass make it possible to easily adhere to insect and other arthropod bodies, particularly bark beetles, resulting in easy and efficient dispersal.
It is nearly impossible to identify C/O spores on spore trap samples. Ophiostoma grows in culture on media that contains cycloheximides, an agricultural fungicide that inhibits protein synthesis. Ceratocystis is sensitive to cycloheximides and consequently will not grow in media that contain it. Really, the best method of sampling for Ceratocystis or Ophiostoma is by tapes, swabs or bulks intended for analysis by direct examination by the laboratory.
Presence of these fungi in indoor environments is common but, for the most part, is not associated with any moisture problems and may be present on wood surfaces prior to the time construction begins. The sap stain penetrates into the sapwood and cannot be removed by surface or chemical cleaning. The stain is allowed in varying degrees in most lumber grades. Although it reduces the aesthetic value, the strength of the lumber is unaffected. Occasionally the brown hyphae can become airborne through sanding or other remediation work and can be seen on spore trap samples.
Mycotoxins have not been reported for the C/O group. At least one species of Ophiostoma is an important human pathogen.
It is interesting to note that the fungus, Gonatobotryum, lives as a mycoparastitism on the C/O group and has therefore been associated with lumberyard mold. The spores are very distinct and can be identified in spore trap samples. Gonatobotryum has broad hyphae, swollen nodes conidiophores, conidia (asexual spores) that are smooth, brown, ellipsoidal and balloon-like. The presence of Gonatobotryum spores is generally indicative of the C/O group in the vicinity. Production of mycotoxins has not been reported and little information is available regarding toxicity, health effects or Allergenicity for Gonatobotryum.