Fungal Spore Types and Their Release

In conidial fungi or molds, dry conidia are often released by passive mechanisms or by disturbance such as gravitation force, convection currents, deflation force, or mechanical disturbance. Some spores are actively released by osmotic force built up with water. Movement and feeding of arthropods, such as insects and mites, are also a common release-dispersal mechanism.97-99 Drifting mist or tiny water droplets in air current can serve as a secondary process to carry and disperse spores of Cladosporium91 and C. herbarum.100 Dry-spored fungi include such common indoor fungi as species of Alternatia, Aspergillus, Cladosporium, and Penicillium. Wet-spored or slimy-spored fungi include species of Acremonium, Aureoba-sidium, Fusarium, Phoma, Stachybotrys, and Trichoderma, to name only a few. Active discharge mechanisms by osmotic force are not uncommon in fungi, but rare in molds or hyphomycetes. Epicoccum nigrum and Arthrinium cuspidatum are discharged when sudden rounding of the basal cell under the spore occurs.97 Pressure buildup from water absorption is involved in spore discharge in many species of ascomycetes and basidiomycetes.100 The well-known "cannonball" fungus, Sphaerobolus stellatus, absorbs moisture to build up pressure in the inner wall of the sporophore. The pressure and strain cause the inner wall to turn inside out and catapult the spore mass covered in a sporing tissue (the entire structure is called gleba) for a distance of several meters. The cannonball fungus is a basidiomy-cete and common on very rotten wood, dung of herbaceous animals, or mulch.100 The black dots on exterior walls of houses and buildings are occasionally reported when rainfall occurs after new mulch is applied. The gleba is covered with a sticky liquid, which becomes tightly bound to the wall after drying.

Ascomycetes may actively discharge their ascospores by internal pressure of asci. Some ascomycetes, however, may dissolve their asci and release ascospores in wet masses for insect pickup and dispersal. Chaetomium species are wet-spore producers. Their ascospores are released in wet mass after their asci are dissolved.86 Wind or air movement plays an important role in spore dispersal of most spore types and basidiospores. Air movement carries and disperses basidiospores after they are ejected from basidia and fall out from between the gills of the basidioma.47 Basidiospores of Coprinus species can be carried away and dispersed by insects, which are attracted by odors produced by the mushroom and pick up slimy spore masses by contact. Although wet, slimy spore masses are likely to be picked up and dispersed by insects, they may become dry and airborne.

In hyphomycetes, dry-spore release is often influenced by air movement and wind speed.48 The minimum wind speed needed to release dry conidia varies with each species, between 0.4 and 2.0 m/s.97 Sutton et al.46 reported that Botrytis squamosa spores were apparently released at very low wind speeds. Maximum wind speed was found to negatively correlate with spore concentrations of Cladosporium and

Alternaria and unidentified ascospores and basidiospores. It was the only factor among 10 weather parameters found to significantly correlate with all groups. Wind and air movement is known to play an important role in basidiospore dispersal. Minimum wind was directly correlated with spore counts, while maximum wind was inversely correlated.48 High wind speed is also likely to disperse spore clouds and dilute spore concentrations. Li63 found that wind speed was negatively correlated with basidiospore release of Amanita muscaria var. alba. A study of mushroom wood model and fresh Agaricus bisporus var. bisporus suggested that decreasing wind speed may reduce the number of the basidiospores being blown back into the cap.101 Basidiospores are dispersed primarily by air movement after being released from their basidiomata.47

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