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

Abstract

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 eVort. The same taxa of fungi were dominant in F. excelsior shoots of diVerent 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 artiWcial 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.

Fungi on stems and twigs in initial and advanced stages of dieback of European ash (Fraxinus excelsior) in Poland

“…Similar results were reported from Sweden (Bakys et al 2009a, b). Most species that occurred commonly in this study have been found on stems and twigs of F. excelsior in various habitats (Butin and Kowalski 1986;Griffith and Boddy 1988, 1991Lygis et al 2005;Bakys et al 2009a;Chen 2012;Davydenko et al 2013).…”

“…A similarly high diversity of fungi on ash with dieback has been recorded by others. In Sweden, Bakys et al (2009a) identified 56 fungal taxa on shoots of 20-to 30-year-old F. excelsior trees with various dieback symptoms, and in New Zealand, Chen (2012) identified 90 fungal taxa in bark and wood or buds of symptomless twigs of 60-to 70-year-old F. excelsior. In contrast, the base of dead twigs was colonized by only 22 species in healthy, naturally pruned F. excelsior trees (Butin and Kowalski 1986).…”

“…Alternaria alternata was detected in 22.9 %, Phomopsis in 51.4 % and Fusarium, which occurred exclusively in F. excelsior, in 5.7 % of branches (Kowalski and Kehr 1992). In Sweden, A. alternata was very frequent in both symptomless, healthy-looking shoots and in shoots with initial and advanced necroses (Bakys et al 2009a). In the Ukraine, A. alternata occurred in symptomatic shoots at a frequency more than twice that in the present study, but was absent from symptomless, healthy-looking shoots (Davydenko et al 2013).…”

“…and Phomopsis spp. are generally reported as the most common endophytes in ash bark and wood (Griffith and Boddy 1991;Kowalski and Kehr 1992;Sieber 2007;Bakys et al 2009a;Chen 2012). In symptomless branch bases, these three taxa were more frequent in F. excelsior than in ten other tree species examined (Kowalski and Kehr 1992).…”

Genetic analysis of European common ash (Fraxinus excelsior L.) populations affected by ash dieback

“…European ash (Fraxinus excelsior L.) is threatened by ash dieback, a disease caused by the fungus Hymenoscyphus fraxineus (BARAL et al, 2014), occurring first in Poland and meanwhile in most parts of Europe (PUKACKI and PRZYBYL, 2005;BAKYS et al, 2009a;WOOD-WARD and BOA, 2012;VASAITIS et al, 2013). The origin of the fungus was unknown until recently when it was proved to originate from East Asia, where it is a saprophytic fungus and plays a similar ecological role on F. manshurica as H. albidus on F. excelsior (VASAITIS et al, 2013;ZHAO et al, 2012).…”