Résumé:
Fungi were isolated in pure cultures from decaying giant sequoias in Geneva (Switzerland). Isolates were genetically identified by ITS rDNA sequencing. Young giant sequoia trees were artificially infected with a pure culture of Botryosphaeria parva. Henle-Koch’s Postulates demonstrated that Botryosphaeria parva was pathogenic to Sequoiadendron giganteum. When analysing the microorganisms associated to canker and dieback symptoms in a giant sequoias (Sequoiadendron giganteum) in Geneva, the fungus Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips, teleomorph Botryosphaeria parva (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips, was isolated, whereas such symptoms are commonly associated to Fusicoccum aesculi (teleomorph Botryosphaeria dothidea). These two fungal species belong to the same genus Botryosphaeria of the Botryosphaeriaceae family. Because Neofusicoccum parvum was causing cankers and diebacks in other woody species around the world, we extended the analysis to other trees displaying sequoia dieback symptoms in order to evaluate the involvement of Neofusicoccum parvum in such increasing symptoms in sequoias in Geneva. Dried twigs, trunk, and branch cankers from diseased trees were sampled on several distinct sites. From all samples, isolated fungi in pure cultures showed a phenotype typical of Botryosphaeriaceae species. Isolates were then genetically identified at the species level. Subsequently Neofusicoccum parvum was inoculated to young giant sequoia trees, re-isolated in pure culture from provoked symptoms, and re-identified to fulfil Henle-Koch’s postulates. The identification confirmed that Neofusicoccum parvum was present on all sites, while Fusicoccum aesculi was retrieved only once alone. The inoculation of Neofusicoccum parvum isolates on young sequoias demonstrated for the first time that this fungus was able to develop cankers in Sequoiadendron giganteum. Neofusicoccum parvum could, therefore, be the major cause for dying of giant sequoias in the Geneva Lake area.
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