6 Wood Discoloration

2.2 Growth and Spreading 15
Fig.2.7. Rhizomorph of Armillaria mellea. Left: Apex with hair-like microhyphae (A), cortex
(B) andpith(C). (from Hartig 1882); right: cross section (LM; from Schmid and Liese
1970)
fiber hyphae (Hartig 1882). By means of rhizomorphs, Armillaria species grow
in the soil and infect the roots of living trees (Chap. 8.3.1).
Under unfavorable conditions, resistance stages are formed. Spores are more
resistant to heat, dryness, and wood preservatives than their mycelium. The hyphal
cell water content is reduced, nutrients are concentrated, parts of the protoplasts
or storage substances of neighboring cells are translocated in resting
cells, and enzyme activity decreases (“latent life”). Chlamydospores (Fig. 2.8)
are thick-walled spores with a brown cell wall, which occur in many blue-stain
fungi.
Formerly, it was believed that the vegetative mycelium of some wood-decay
fungi is also resistant to dryness (Chap. 3.3) and heat (Chap. 3.4). Recent results
show that this must not be true: When cultured on agar at about 28 ◦ C,
the dikaryotic hyphae of Serpula lacrymans tend to revert to the monokaryotic
condition, which regularly shows abundant arthrospores (Schmidt and
Moreth-Kebernik 1990). In wood samples that were slowly dried or warmed,
the substrate mycelium of S. lacrymans, C. puteana, Donkioporia expansa,
and Gloeophyllum trabeum also formed arthrospores (Huckfeldt 2003). It was
therefore assumed that these arthrospores are the agents for resistance against
drying and heat.
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