Abstract
Studies on fungal communities in the previous year’s leaf petioles of Fraxinus excelsior found in litter in five ash stands in southern Poland were made in 2017. Fungi were identified on the basis of isolation from 300 surface sterilized leaf petioles and by in situ inventory of fruit bodies (on 600 petioles, in spring and autumn). Identification was based on morphology of colonies and fruit bodies, and sequencing of ITS region of the rRNA gene cluster. In total, 2832 isolates from 117 taxa (Ascomycota—100; Basidiomycota—15; Mucoromycota—2 taxa) were obtained with the isolation method. The most frequent taxa (with frequency >10%) were: Nemania serpens, Hymenoscyphus fraxineus, Alternaria sp. 1, Boeremia sp., Helotiales sp. 1, Epicoccum nigrum, Venturia fraxini, Fusarium sp., Fusarium lateritium, Nemania diffusa, Typhula sp. 2 (in descending order). In total, 45 taxa were detected with the in situ inventory method. Eleven taxa were classified as dominant: Hymenoscyphus fraxineus, Venturia fraxini, Leptosphaeria sp. 2, Cyathicula fraxinophila, Typhula sp. 2, Hypoderma rubi, Pyrenopeziza petiolaris, Cyathicula coronata, Hymenoscyphus scutula, Leptosphaeria sclerotioides and Hymenoscyphus caudatus. Among 202 leaf petioles colonized by H. fraxineus, 177 petioles also showed fructification of 26 other fungi. All the isolated saprotrophs were tested in dual-culture assay for antagonism to two strains of H. fraxineus. Three interaction types were observed: type A, mutual direct contact, when the two fungi meet along the contact line (occurred with 43.3% of test fungi); type B, with inhibition zone between colonies (with 46.9% of test fungi); type C, when the test fungus overgrows the colony of H. fraxineus (with 9.8% of test fungi). The possible contribution of the fungal saprotrophs in limiting of the expansion of H. fraxineus in ash leaf petioles, which may result in reduction in the inoculum of ash dieback causal agent, is discussed.
Highlights
European ash (Fraxinus excelsior L.) started to show symptoms of serious disease in the early 1990s and, currently, dieback can be seen in most of its range in Europe [1,2,3,4,5,6,7,8]
Since a single taxon could be represented by 1–5 visually different morphotypes, sequencing of ITS region of the rRNA gene cluster of representative isolates resulted in detection of 117 fungal taxa (Supplementary Material Tables S1 and S2)
Growth of H. fraxineus was apparently limited by the same fungi that produced inhibition zones or made physical contact by overgrowth and replacement of H. fraxineus. These results show that the spectrum of fungi able to limit growth of H. fraxineus in situ is wide and includes more than 50% of species fruiting on leaf petioles
Summary
European ash (Fraxinus excelsior L.) started to show symptoms of serious disease in the early 1990s and, currently, dieback can be seen in most of its range in Europe [1,2,3,4,5,6,7,8]. The disease is caused by an alien ascomycete, Hymenoscyphus fraxineus The role of leaves, mainly petioles, as sites of infection in the H. fraxineus life cycle and ash dieback has been emphasized. Each year H. fraxineus forms apothecia, predominantly on the previous year’s leaf petioles and leaflet veins, in the litter on the forest floor [5,7,8]. In Poland, apothecia of H. fraxineus are formed mainly in July and August. In other European regions, apothecia can be found much earlier, from the middle of May [13]
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