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The atmosphere is filled with abundant airborne spores. These are produced by fungi in the natural environment. These may be fungi growing in the soil, on leaf litter on the soil, on leaf surfaces of living plants, or on other parts of living plants. Although the majority of these fungi are saprobes, existing on dead or non-living organic material, other fungi may be plant or animal pathogens. Although the news media often refer to fungal spores as "mold spores", many of the airborne spores are not from molds. They are spores of mushrooms, bracket fungi, lichens, morels and other fleshy fungi. Of course, molds are also fungi, specifically microfungi, so "fungal spores" is the terminology that includes the reproductive structures of all fungi. In many parts of the world, fungal spores occur in the atmosphere year-round. In most temperate climates, spores will be present in the air unless the ground is covered with snow and ice. Concentrations of airborne spores outdoors can be high, especially in the late summer or fall. We have occasionally recorded hourly concentrations in Tulsa above 100,000 spores per cubic meter of air.
Dry Air Spora
Highest spore levels typically occur in the summer and fall on warm windy days. Spores called the dry air spora, are released into the air passively by simple air movement. These spores are easily detached from the fungal mycelium by the wind and can be deposited locally or carried long distances depending on meteorological variables. Common members of the dry air spora include Cladosporium, Alternaria, Epicoccum, Pithomyces, Curvularia, Drechslera-type conidia as well as smut spores and others. The image on the left, is the typical appearance of the atmosphere in Tulsa during the fall. Most of the spores pictured here are members of the dry air spora.
Wet Weather Air Spora
Although many spore types are washed from the atmosphere during rainfall, other airborne spore types require moisture and are actively released during or soon after rainy weather or during times of high humidity. Ascospores and basidiospores are typical of the what can be referred to as the wet weather air spora. Ascospores may be released during light rain or shortly after rain has ended. The image on the right shows abundant ascospores after rainfall. Some ascospores only require high humidity for spore release, and these are found in the atmosphere whenever the humidity is high, usually late night or early morning. Rain from spring to fall will often stimulate the development of mushrooms and other fleshy fungi in an area. Therefore, a few days after rainfall, there may be a peak in airborne basidiospores that are produced by these fleshy fungi. The active release of basidiospores from mushrooms and bracket fungi also requires moisture and their atmospheric levels are usually high during late night and early morning hours when the humidity is high.
In addition to the active dispersal described here, some spores are passively dispersed by rain. Rain splash is one of these passive methods. As rain drops hit a leaf surface or other substrate with hyphae and conidia, the rain drops may cause some spores to be splashed off the substrate into the air. Basidiospores produced by puffballs are also dispersed by rain drops. As rain drops hit the wall of the fruiting body, a puff of spores are released into the atmosphere.
Airborne spores also occur in indoor environments. Some are introduced from outside through open doors and windows, while other spores are produced by fungi that are growing on indoor substrates.
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