Spores represent in the life cycle of a fungus a state of rest (low water content, high nutrient content; "latent life") between the active phase of spore dispersal and start of new growth. Serpula lacrymans produces 300,000 (Falck 1912) to 360,000 (Rypaek 1966) and Piptoporus betulinus 31,000,000 (Kramer 1982) spores per hour and cm2 of hymenium. Many forest mycorrhizal fungi fruit at higher air moisture content and lower temperature in the autumn. Among the tree parasites, Heterobasi-dion annosum disperse spores almost over the whole year, Laetiporus sul-phureus in the autumn.
Many Basidiomycetes disperse their spores actively for 0.1-0.2 mm (ballis-tospores) so that the spores more easily reach the open air (Schwantes 1996). In Schizophyllum commune, a liquid drop at the sterigma becomes larger and hurls the spore into the airflow (Muller and Loeffler 1992). Moykkynen (1997), using a wind tunnel, measured for the conidia of Heterobasidion annosum that a threshold speed of an airflow of 1.8 m/s liberates the spores.
Falck (1912) calculated the mass of a spore of S. lacrymans as 171 x 10-12 g. Fungal spores exhibit a density of 1.1 dp. In standing air, spores sink with sedimentation speeds of 0.03 - 0.55 cm/s (Reiss 1997).
A continuously colonized area can expand 50 km over the year. In an appropriate air stream, spores can be transported up to 1,000 km (Burnett 1976).
Furthermore, spores are spread by rain and snow. Animals distribute spores that are attached by the spore surface sculpturing (see Fig. 2.9) or remain indigested. Assumably by international trade, the causal agent of the Dutch Elm disease, Ophiostoma ulmi, was imported from Asia to Europe in 1918 .
The spore content in air is subject to characteristic rhythms. In Central Europe, it is higher in the summer at warm temperatures and low relative air humidity than in the winter. Basidiospores and ascospores are numer-ous in the air in spring and in autumn. Conidia have a maximum from June to September. In cities in temperate regions, the spore concentration of Cla-dosporium, mainly C. herbarum, often rises up to 10,000 - 15,000 spores/m3 air with peaks of more than 50,000 spores/m3 (Nolard 2004).
Air turbulence during stable areas of high pressure may result in daily rhythms, the concen-tration rising during the midday (Reig 1997). Interiors with high dust content (e.g., the wood-processing industry) may exhibit increased spore contents.
The lifespan of spores in free air is affected by temperature, air humidity, and sun exposure. As unpigmented spores are sensitive to UV light, pigmented spores predominate in the air.
Exogenously dormant spores only germinate when the environmental conditions (nutrients, temperature, pH value) become favor-able. Endogenously dormant spores fail to germinate even under favorable conditions, which is due to factors within the spore such as nutrient imper-meability or the presence of endogenous inhibitors. Dormancy within these spores is broken by ageing when nutrients begin to enter or the inhibitors leach out (Robson 1999).
Prior to the emergence of one or more germ tubes, spores undergo a process of swelling during which they increase in diameter due to the uptake of water. The metabolic activity, production of protein, DNA and RNA all increase. The percentage of germinating spores depends on fungal species, spore age, temperature, available moisture, and substrate. In Serpula lacrymans, only 30% of sampled spores germinated in vitro (Hegarty et al. 1987). For the conidia of Heterobasidion annosum, the thermal cardinal points were 0 °C minimum, be-tween 12 and 28 'C optimum and 34 °C maximum (Courtois 1972).
Depending on the species, the duration of the germination ability reaches from a few days or weeks, like in Stereum species, to several years in Chaetomium globosum, and can reach up to about 20 years in S. lacrymans (Grosser et al. 2003). Germination of spores of wood fungi is favored by high air humidity, warmth, and pH values of 4-6. In Serpula lacrymans, citric acid (Hegarty et al. 1987) and vitamin B1 (Czaja and Pommer 1959) stimulated germination. Heartwood compounds may inhibit.
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