6 Wood Discoloration

172 8 Habitat of Wood Fungi
through the cambium under the bark. In the cambial layer particularly in red
oak, 5 to 8-cm-large sporulation mats (usually conidia) develop from May
to October, which cause bark detachment and fissure by means of pressure
cushions.
There are two different ways for long-distance transmission by insects (about
100 m/year): First, oak bark beetles (Pseudopityophthorus spp.) breed in dying
or dead oaks, and the young beetles transfer the pathogen during the maturation
feeding on shoots and twigs of healthy oaks (Fig. 8.5b). Since asexual
spores do not develop in the larval galleries, this transmission way has only
less significance. Second, sap beetles, particularly Nitidulidae, are attracted by
the smell of the sporulating mats and transmit infectious material to healthy
trees into fresh wounds, attracted by their smell (Fig. 8.5c) (Appel et al. 1990).
The nitidulids effect that the bipolar heterothallic fungus is dikaryotized and
develops ascospores, if conidia with contrary mating factor were introduced
from other sporulation mats. Since wounds are infectious only a few days in
healthy oaks, this infection way has also less significance. Furthermore, the
subcortical mats of C. fagacearum were observed to be rapidly overgrown by
Graphium pyrinum Goid. (anamorph of Ophiostoma piceae). This colonization
reduces the chance of contamination of the insect vectors with spores of the
pathogen and is likely to contribute to the low efficacy of insect transmission
(Rütze and Parameswaran 1984).
Since 1951, the import of unpeeled oak logs from North America to Germany
was allowed according to a plant protection order, if the wood derives
from healthy areas (“white counties”), in accordance with the plant protection
departmentoftheUSDepartmentofAgriculture.Ithadhowevertobeconsidered
that also the European oaks, although usually white oaks (Quercus petraea
and Q. robur), are more susceptible from nature and that the European oak
bark beetle Scolytus intricatus is more aggressive in its transmission behavior
than the North American species. In order to prevent the import of the fungus
(Gibbs et al. 1984), oak wood became subject to specific treatment requirements
under Council Directive 77/93/EC including bark removal, kiln drying,
etc.Sincesuchwoodcannotbeconvertedtoveneers,thosemeasureswould
have equaled practically an import stop for oak logs and the endangerment of
the European veneer industry. Thus, experiments were performed in a cooperative
venture between the Federal Research Center for Forestry and Forest
Products Hamburg and the Universities of Minnesota and West Virginia on log
fumigation with bromomethane (methyl bromide) as a means of ensuring that
thelogswerefreefromC. fagacearum (Liese et al. 1981; Schmidt 1988). The
European community permitted by EEC Plant protection guidelines of 1978
the import of unpeeled oak logs if they were disinfected before export with
240 g bromomethane per m 3 of wood for 3 days at a minimum temperature
of 3 ◦ C in plastic tents (Rütze and Liese 1983). The use of bromomethane has
fallen off considerably since the Montreal Conference of 1997 because of its
www.taq.ir