Alter Spore
Alternaria alternata
Photo © George Barron
There are four common fungi causing allergies in humans: Cladosporium, Penicillium, Aspergillus and Alternaria. Alternaria alternata is an important cause of mould allergies in humans. It fruits in abundance over the surface of dying grasses and cereals. It is particularly common on corn leaves in the late fall when the straw-coloured leaves turn black with spores (conidia). Spores are dislodged during harvesting of the crop and are produced in such abundance (billions) that they form a dark cloud above the combine. Spores can affect sensitized individuals for many miles downwind. Alternaria is also found from time to time growing in damp spots on walls in homes, particularly in basements but it is not common.