The spatial distribution and niche differentiation of three closely related species (Erysiphe alphitoides, E. quercicola and E. hypophylla) causing oak powdery mildew was studied at scales ranging from the European continent, where they are invasive, to a single leaf. While E. alphitoides was dominant at all scales, E. quercicola and E. hypophylla had restricted geographic, stand and leaf distributions. The large-scale distributions were likely explained by climatic factors and species environmental tolerances, with E. quercicola being more frequent in warmer climates and E. hypophylla in colder climates. The extensive sampling and molecular analyses revealed the cryptic invasion of E. quercicola in nine countries from which it was not yet recorded. The presence of the three species was also strongly affected by host factors, such as oak species and developmental stage. Segregation patterns between Erysiphe species were finally observed at the leaf scale, between and within leaf surfaces, suggesting competitive effects.

Summary The occurrence and distribution of Dothistroma needle blight (DNB) were studied in 2014–2017 around Slovakia. A total of 84 localities, both native and planted, were investigated, and the presence of DNB was confirmed in 73 of them. In all positive locations, symptoms typical of DNB were observed and the Dothistroma species was confirmed using species‐specific primers either from fungal cultures or directly from needles. Both Dothistroma species—D. septosporum and D. pini—were identified. Both species occurred together in 29 locations, only D. septosporum in 42 and only D. pini in two locations. The host range of D. septosporum included 10 pine species and two spruce species. The host range of D. pini comprised the same number of pine hosts but only one spruce species. Five pine hosts, P. aristata, P. coulteri, P. densiflora, P. jeffreyi, P. × schwerinii, and one spruce host P. abies are new hosts species of D. pini. P. densiflora and Picea pungens have earlier been reported to be susceptible for DNB. In this study, D. septosporum was found from both tree species.

The diversity of fungi on branches and leaves of ashes (Fraxinus angustifolia, F. excelsior, F. ornus) in Slovakia was studied. Symptomatic material collected in Slovakia during the period of 2013 to 2017 and herbarium specimens previously collected were examined. In total, 30 fungal taxa (15 Deuteromycetes, 14 Ascomycetes and one Basidiomycetes) were recorded. Twenty-three of them have never been recorded on ashes in the country. The most frequently occurring fungi were Hymenoscyphus fraxineus (anamorph Chalara fraxinea) that causes necrosis of shoots and branches, and Phyllactinia fraxini, a foliar pathogen that causes powdery mildew disease. Fungal diversity on ashes growing in different types of stands was compared. Species richness was the greatest in seed orchards (20 fungal taxa) compared to private gardens, which contained the lowest (two fungal taxa). Species diversity in forest stands comprised 18 fungal taxa and the urban greenery was represented by 10 fungal taxa. Nine fungal taxa were recorded in tree alley along the road. The widest fungal species spectrum was recorded on F. excelsior.

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Interactions between 3 pathogenic fungi damaging horse-chestnut (Aesculus hippocastanum) leaves and fruits -Phyllosticta sphaeropsoidea, Phomopsis carposchiza, and Diaporthe padi -and the antagonistic fungus Trichoderma harzianum were studied to determine their mutual influence in vitro. Antibiosis of colonies developing on 5 nutrient media was tested. The 3 studied T. harzianum isolates differed in their antagonistic potential. although T. harzianum isolates significantly inhibited the growth of Phomopsis carposchiza, the mycelium growth of some of the re-isolates on fresh medium indicates an inadequate antagonistic effect of T. harzianum on this species. The tested Trichoderma isolates showed stronger antagonism towards the other pathogens, reflected in overgrowing of Phyllosticta sphaeropsoidea and Diaporthe padi and reducing their growth. Granulation of the cytoplasm and lysis of hyphae of the fungal pathogens were the most frequently observed effects of the interaction.

The health of 34 different Fraxinus taxa in association with the pathogenic fungus Hymenoscyphus fraxineus was assessed in four Slovak arboreta. Averaged across all arboreta, nearly one-quarter (24.9%) of all evaluated trees showed ash dieback symptoms. The damage was most serious on the common ash F. excelsior, a native species. The percentage of dead trees did not exceed 2% for all evaluated trees. Generally, ash trees of all ages were affected, though the intensity of the damage varied among the sites. The identity of H. fraxineus was confirmed by conventional PCR targeting the internal transcribed spacer (ITS) sequences of the nuclear ribosomal DNA, as well as the 18S gene/ITS-2 region of the rDNA operon. In Slovakia, the pathogen has expanded its host range from native species not only to their ornamental cultivars, but also to introduced North American (F. cinerea, F. latifolia, F. pennsylvanica, F. quadrangulata) and Asian (F. bungeana, F. chinensis ssp. rhynchophylla, F. mandshurica) ash species. H. fraxineus was also observed on the previous year’s leaf petioles of the native European species F. ornus, considered a weakly susceptible host. In Slovak arboreta, H. fraxineus was found on 23 Fraxinus taxa; 21 of them represent first records for the country. F. bungeana is recorded as a new host species of H. fraxineus.

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