6 Wood Discoloration

2.2 Growth and Spreading 25
and Ishikawa 1973). Yeast extract, vitamin B1, traumatic influences through
physical distortions to the mycelium, and the presence of another fungus or its
mycelial extract or culture filtrate may be favorable (Stahl and Esser 1976; Leslie
and Leonard 1979; Matsuo et al. 1992; Kawchuk et al. 1993). In S. commune,the
development of a fruit-body-inducing substance (FIS) is genetically controlled
(Leslie and Leonard 1979). In a Polyporus species, there are fi+ genes (fruiting
initiation) (Stahl and Esser 1976; Esser 1989). The force of gravity determines
that the yearly hymenial layers in the bracket-like, perennial fruit bodies of
Fomes fomentarius also point to the earth center if the host tree is lying on the
ground (Chap. 3.6).
2.2.5
Production, Dispersal and Germination of Spores
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 (Rypáček 1966)
and Piptoporus betulinus 31,000,000 (Kramer 1982) spores per hour and cm 2 of
hymenium. Many forest mycorrhizal fungi fruit at higher air moisture content
and lower temperature in the autumn. Among the tree parasites, Heterobasidion
annosum disperse spores almost over the whole year, Laetiporus sulphureus
in the autumn.
Many Basidiomycetes disperse their spores actively for 0.1–0.2 mm (ballistospores)
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 (Müller and Loeffler 1992). Möykkynen (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 × 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 (Chap. 8.1.2.1).
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 numerous
in the air in spring and in autumn. Conidia have a maximum from June
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