6 Wood Discoloration

8.3 Tree Rots by Macrofungi 189
and A. ostoyae penetrated Picea sitchensis root bark without prior wounding,
but neither species formed rhizomorphs. The rhizomorphs grow in the soil
from tree to tree and serve for nutrient translocation and infection. If the tree
does not succeed in defending the fungus by histological or chemical barriers
(Woodward 1992a; Wahlström and Johansson 1992), the fungus spreads between
bark and xylem in the cambial region. The sap stream is interrupted, and
toxic metabolites are excreted by the fungus. If the whole cambium is colonized
around the stem, the tree dies rapidly (“cabium killer”). Beside the parasitic
way of life, the fungus can spread via the wood rays in the heartwood of the
root and stem basis (butt rot). Armillaria species and Heterobasidion annosum
showed an increased occurrence in forest dieback sites (Kehr and Wulf 1993).
AdirectcontrolofArmillaria spp. (e.g., Fox 1990) is practically impossible,
particularly since the fungus occurs almost everywhere in the soil. In Oregon,
the upper ground layer was colonized over an area of about 9 km 2 by only
one mycelial clone of A. ostoyae, whose age was supposed to be 2,400 years.
In England, a clone of A. gallica of about 500 years of age covered an area of
9 ha. In France and Germany, clone diameter reached about 200 m in diameter
(Marxmüller and Holdenrieder 2000).
Armillaria is more frequent on soils with balanced microclimate and high
air humidity at ground level as well as on nutrient-rich soils of about pH 5.
Since young conifers are particularly susceptible on former hardwood soils,
oldstumpsandrootsshouldberootedoutbeforeplantingconiferstolimit
the vitality of the fungus, which, during its saprobic phase, depends on easily
degradable nutrients (Butin 1995). Isolation of infected tree groups by 30 to 50cm-deep
ditches is usually unsuccessful. Armillaria-infected plants in gardens
and parks should be promptly removed. The resistance of the plant hosts can
be increased by suitable soil preparation, good planting, and tree care. Douglas
fir, Sitka spruce, fir and larch are lesser susceptible species. The application of
chemicals within the root range is strenuous and therefore only suitable for
valuable garden and park trees (Schönhar 1989).
Pinosylvin from Pinus strobus inhibited mycelial growth of A. ostoyae
(Mwangi et al. 1990). Growth rate, spread and survival of rhizomorphs decreased
by several bacteria, particularly Pseudomonas fluorescens Migula (Dumas
1992), Trichoderma species (Dumas and Boyonoski 1992), wood-inhabiting
Basidiomycetes (Pearce 1990) and mycorrhizal fungi (Kutscheidt 1992).
8.3.2
Heterobasidion annosum s.l. Root Rot Fungus, Fomes Butt Rot
From the Root rot fungus, several intersterility groups have been distinguished,
which differ in relation to distribution, fruit body morphology and host tree
(Korhonen 1978a). In Europe, three groups have been referred to as P-group
www.taq.ir