Parasitic fungi such a smuts and rusts are numerous during harvest time, as their spores become airborne. Several types of fungi comprise the daily mold count. Alternaria, Cladosporium, and Epicoccum are dry weather spores. During dry periods, fungi increase in number until noon and decrease throughout the evening. Ascospores and Basidiospores are considered wet weather spores, and are plentiful during light rainfall or in predawn hours when condensation is heavy.
Many fungi have internal mechanisms which actively discharge their spores into the air.
Other fungi rely on air movement for their dispersal. Pollen grains and mold spores are reported as the number of particles per cubic meter of air sampled. Efficient equipment collects tiny particles. The device is mounted on the roof of a centrally located, Saint Louis County Department of Health building, where it collects airborne particles of pollen and mold. Pollen grains and mold spores, ranging in size between one and 100 microns, flow through the sampling apparatus and stick to a greased flat surface.