Occurrence and pathogenicity of fungi in necrotic and non-symptomatic shoots of declining common ash (Fraxinus excelsior) in Sweden

Currently, massive dieback of Fraxinus excelsior is observed in countries of eastern, northern and central Europe, and the reasons for it are unclear. The aims of the present work were (a) to study fungal communities in declining F. excelsior crowns; (b) to clarify role of fungi in the decline. Shoots from symptomatic crowns were collected in four localities in central Sweden, and distributed into the following categories: (a) visually healthy; (b) initial necroses; (c) advanced necroses; (c) dead tops. The most frequently isolated fungi were Gibberella avenacea, Alternaria alternata, Epicoccum nigrum, Botryosphaeria stevensii, Valsa sp., Lewia sp., Aureobasidium pullulans and Phomopsis sp., and these taxa were consistently found in shoots of all four symptomatic categories. Forty-eight taxa of other fungi were isolated, and fungal diversity was not exhausted by the sampling effort. The same taxa of fungi were dominant in F. excelsior shoots of different symptomatic categories, and moderate to high similarity of fungal communities was observed in shoots despite the symptoms. Forty-four isolates from 24 fungal taxa were used for artificial inoculations of 277 1-year-old F. excelsior seedlings in bare root nursery. After 2 years, only four fungi caused symptomatic necroses of bark and cambium: A. alternata, E. nigrum, Chalara fraxinea and Phomopsis sp. The most pathogenic was C. fraxinea, inducing symptoms on 50% of inoculated trees, while three other fungi caused necroses on 3-17% of inoculated trees. Infection biology of C. fraxinea and environmental factors determining susceptibility of F. excelsior to decline deserve future investigations.