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Widespread but locally severe disease of Nothofagus cunninghamii occurs in Tasmanian rainforests. Signs and symptoms of disease in individual trees include attack by the mountain pinhole borer Platypus subgranosus, chlorosis-necrosis and abscission of foliage and sudden wilting of shoots and foliage of affected trees, discoloration of sapwood from the roots to the upper stem and the formation of dark grey-black fungal felts on the bark or cross-cut surfaces of infected stems. Naturally infected trees may take from less than 1 year to more than 2.5 years to die from the time of initial infection. The hyphomycete Chalara australis J. Walker & G. A. Kile, sp. nov. is isolated consistently from the discoloured wood in dying trees. The species is described and its taxonomic affinities with C. quercina, C. ungeri, C. neocaledoniae, C. nothofagi and the Chalara anamorphs of Ceratocystis virescens discussed. No teleomorph has been found. Artificial inoculation of C. australis into healthy seedlings, saplings and large trees of N. cunninghamii reproduced the disease and confirmed that the species is an aggressive primary pathogen. None of the other fungi isolated directly from P. subgranosus, its tunnels or adjacent discoloured wood or the upper stems and branches of dying N. cunninghamii were pathogenic in seedlings. It is uncertain whether C. australis is vectored by P. subgranosus or if fungal infection precedes P. subgranosus infestation. The general significance of Chalara spp. in relation to the Fagaceae is discussed.

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