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6 Wood Discoloration

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166 8 Habitat of <strong>Wood</strong> Fungi<br />

Fig.8.2. Pathogenesis of Chestnut blight by Cryphonectria parasitica (translated from<br />

Heiniger 1999, with permission of Swiss Federal Institute for Forest, Snow and Landscape<br />

Research)<br />

time the most important hardwood species. The disease appeared in Europe<br />

first in 1938 in Genoa in the European chestnut sites (Castanea sativa)ofItaly,<br />

then in southern France, Spain, Switzerland (1948), Germany (1992), and Eastern<br />

Europe. The fungus penetrates as a spore by means of wind, rain, insects,<br />

or birds through wounds into the bark until the cambium. Then reddish-brown<br />

bark spots that break to longitudinal fissures, branch-surrounding necroses,<br />

wilt, and death of the affected branch or crown region occur. One- to 2mm-large,<br />

orange-yellow-ochre pustules (conidiomata, ascomata) develop on<br />

the bark.<br />

The disease in Europe does not run however as intensively as in the USA<br />

probably due to lesser aggressive fungal strains. The reduced pathogenicity is<br />

caused by Cryphonectria-hypovirus 1 that infests the fungus, that is, it becomes<br />

lesser virulent and only produces superficial cankers, which soon heal up. The<br />

virusisalsofoundinthenaturalC. parasitica populations in Japan and China,<br />

but not in the North American populations. To limit the distribution of the<br />

fungus in non-infested countries, there are official regulations (European and<br />

Mediterranean Plant Protection Organization) (Heiniger 2003).<br />

Breeding experiments are performed between C. dentata and resistant Asian<br />

species. There are also attempts on a biological control based on vegetative<br />

pairing of hypo-virulent fungal isolates with virulent strains. Infested sites<br />

are inoculated with hypo-virulent isolates that can transfer the virus in the<br />

www.taq.ir

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