The timber of the sweet chestnut of Europe (Castanea sativa) and the American chestnut (C. dentata) is an excellent decay-resistant hardwood, and the edible nuts produced by both species are consumed by wildlife and people. The damage caused to them by the chestnut blight fungus (also known as Asiatic blight), the ascomycete Cryphonectria parasitica, formerly known as Endothia parasitica, is thus all the more regrettable (Tattar et al., 1996). In the native range of the American chestnut along the Appalachian Mountains, 99.9% of the trees, or about 3.5 billion trees, have been killed in just a century. The chestnut formerly made up around 40-50% of the trees in the canopy over an area of some 800 000 km2 and by all accounts was a vigorous, quick-growing tree (Jacobs and Severeid, 2004). The loss is staggering and has led to very large changes in these forests: at the very least, squirrel populations crashed and seven species of moth that fed exclusively on the chestnut are now extinct.
The fungus, a canker disease, was unknowingly imported into northeastern USA on Asian chestnut trees at some time between 1882 and 1904, being noticed in Bronx Zoological Park, New York in 1904. It did not seem to cause extensive disease symptoms on Asian chestnuts, but the American chestnut C. dentata proved to be highly susceptible. Within two or three decades this tree was destroyed as a commercial crop in eastern North America. At the same time, the disease was first noted in European chestnuts (Pridnya and Cherpakov, 1996).
The fungus spreads when sticky orange spores ooze from its pycnidia and are carried long distances by insects (especially the two-lined chestnut borer Agrilus bilineatus) and birds, including migratory woodpeckers. Wind-borne ascospores, shot into the air from perithecia, also assist in dispersal. Much of the international spread has been in infected timber, resulting in a global spread, even reaching Australia (Cunnington and Pascoe, 2003).
Chestnut trees in which vigorous recovery shoots develop immediately below the lesions, or where cankers heal when the fungus fails to girdle the shoot, indicates the development of a degree of field resistance to the fungus. However, the American chestnut persists mostly as sprouts from mature trees or suppressed seedlings that become infected once big enough and so go through a repeated cycle of growth and death (Paillet, 2002). In the USA major attempts have been made to create disease-resistant hybrids between the American chestnut and Asiatic species such as Castanea crenata and especially C. mollissima. Attention is also being given to sweet chestnuts in several parts of Europe which became infected but which had subsequently recovered. This recovery has been associated with less virulent (hypovirulent) strains of the fungus that are infected with a hypovirulent agent, thought to be a fungal virus (hypovirus), a cytoplasmically transmitted double-stranded RNA, that may have entered Cryphonectria parasitica as a result of interaction with a related European fungus. This agent was successfully transferred to wild chestnut blight populations in North America (Griffin et al, 2004), but unfortunately the degree of protection offered by its hypovirulence to the American chestnut Castanea dentata has not been as great as in Europe with C. sativa.
In addition to being a source of the hypoviral equivalent of Asian 'flu, other fungi appear to have important interactions with chestnut blight, some negative, some positive. Phytophthora root rot is thought to have been an important agent contributing to American chestnut dieback prior to the arrival of chestnut blight, and it may now present a serious limitation to establishment of blight-resistant hybrid chestnut (Rhoades et al., 2003). In the Caucasus Mountains of Russia, Pridnya and Cherpakov (1996) suggest that other fungal and bacterial infections are now preventing the recovery of chestnut populations.
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