Indoor "black mold" fungus has unexpected relatives.

Unitl now, scientists didn't know how to classify Stachybotrys chartarum, the black mold that can grow in humid, indoor environments and is often associated with "building sickness." But an Agricultural Research Service (ARS) scientist recently found that the toxin-producing fungus constitutes a brand-new family within the order Hypocreales.

ARS mycologist Lisa A. Castlebury discovered that the closest relatives of Stachybotrys are actually members of the genus Myrothecium, fungi that cause serious diseases in crop plants and invasive weeds. To assess this relationship, Castlebury and her colleagues at the ARS Systematic Botany and Mycology Laboratory in Beltsville, Maryland, sequenced and analyzed five genes of Stachybotrys species and similar fungi.

Stachybotrys thrives on wet, cellulose-rich materials and can become airborne. It also produces metabolic by-products known as mycotoxins. A class of these, the macrocyclic trichocenes, are especially potent and have been linked to serious illness in humans and livestock.

Up to now, when checking for fungi that produce harmful mycotoxins, environmental engineers and others have generally tested houses and buildings for the presence of Stachybotrys alone. The new finding, however, suggests that a possible source of toxic macrocyclic trichocenes also is fungi of the genus Myrothecium.

Despite concern surrounding the presence of toxin-producing fungi in homes, Stachybotrys and its relatives are relatively harmless when found in nature. It's when they occur inside artificially airtight spaces, with an abundance of moist cellulosic materials, that the accumulating toxic chemicals can reach high concentrations.

In a natural setting, moist, densely cellulosic material is scattered, and mycotoxins produced by the fungi simply diffuse into the air.