The Spores (conidia) of Alternaria Click here for more Alternaria in chains
Alternaria belongs to a Division of the fungi called the Deuteromycota. The word means "neuter fungi" and they are called this because they have no sexual spore stage (meiospores). As can be seen in the photo, however, Alternaria still produces spores. The nuclei result from mitosis and the spores are genetically identical (mitospores). The spores in Alternaria are multi-celled and pigmented and they are produced in chains or branching chains. The spores have a distinctive appearance that makes them easy to recognize. They are broadest near the base and taper gradually to an elongate beak, Alternaria species are cellulolytic (break down cellulose to glucose for energy) and commonly grow on dead plant materials, particularly cereals and grasses. Some species are also parasitic on living plants and cause early blight of tomato and potato in our region. When Alternaria attacks the host leaf, it produces a series of concentric rings around the initial site of attack. This gives a "target spot" effect that is associated with early blight. In fall saprobic (=saprophytic) species of Alternaria, and another Deuteromycota species called Cladosporium, grow on senescent corn leaves and dead grasses and turn them black with spores. When the corn is harvested, the spores are released in black clouds above the combine to drift downwind for many miles. Alternaria species can grow on other sources of cellulose if free water is available. I have found it growing on wallpaper in the bedroom of a home in Ontario. Because it can sometimes be produced in the air in large numbers, susceptible individuals can become sensitized to the proteins on the spore surface and develop allergies. Since Alternaria is a seasonal fungus then this type of allergy would be more prevalent in the fall. Unless, of course, it's growing on your bedroom wall! Alternaria, fortunately, is not a common component of the home environment. Alternaria alternata is perhaps the commonly encountered saprophytic species. In general the beak (extended tip) of Alternaria is much longer in species parasitic on plants.
The common fungi responsible for allergies in humans are Alternaria, Aspergillus, Penicillium and Cladosporium. Aspergillus and Penicillium grow quite happily in your basement in summer and Cladosporium is the dark brown mould that grows around the edge of the bath or shower stall or even on the ceiling in bathrooms. All of these fungi belong in the Deuteromycota and produce large numbers of spores that become airborne (bioaerosols) and are inhaled to cause pulmonary problems in humans and sometimes animals.