Eutypella parasitica R.W. Davidson and R.C. Lorenz is the causative agent of Eutypella canker of maple, a destructive disease of maples in Europe and North America. The fungus E. parasitica infects the trunk through a branch stub or bark wound. Because the fungal community may have an impact on infection and colonization by E. parasitica, the composition of fungi colonizing wood of dead branches of sycamore maple (Acer pseudoplatanus L.) was investigated in five sampling sites in Slovenia. Forty samples from each sampling site were collected between the November 2017 and March 2018 period. Isolations were made from the wood in the outer part of dead branches and from discoloured wood in the trunk that originated from a dead branch. Pure cultures were divided into morphotypes, and one representative culture per morphotype was selected for further molecular identification. From a total of 2700 cultured subsamples, 1744 fungal cultures were obtained, which were grouped into 212 morphotypes. The investigated samples were colonized by a broad spectrum of fungi. The most frequently isolated species were Eutypa maura (Fr.) Sacc., Eutypa sp. Tul. and C. Tul., Fusarium avenaceum (Fr.) Sacc., Neocucurbitaria acerina Wanas., Camporesi, E.B.G. Jones and K.D. Hyde and E. parasitica. In this study, we distinguished species diversity and the fungal community. There were no significant differences in the diversity of fungal species between the five sampling sites, and branch thickness did not prove to be a statistically significant factor in fungal species diversity. Nevertheless, relatively low Jaccard similarity index values suggested possible differences in the fungal communities from different sampling sites. This was confirmed by an analysis of similarities, which showed that the isolated fungal community distinctly differed between the five sampling sites and between the different isolation sources. Eutypella parasitica was isolated from all five investigated sampling sites, although Eutypella cankers were observed in only three sampling sites, indicating the possibility of asymptomatic infection.

Reports of sooty bark disease of maples caused by the fungus Cryptostroma corticale have recently been emerging from across Europe. The aims of our study were to describe the first report of sooty bark disease in Slovenia, to determine the pathogenicity of C. corticale, to confirm the optimum temperature for the growth of the fungus, and to determine the mass loss of Acer pseudoplatanus wood inoculated by C. corticale. We confirmed the presence of C. corticale on A. pseudoplatanus via morphological and molecular analysis. The optimal growth of C. corticale was measured in vitro on potato dextrose agar and was determined to occur at 25 °C. Pathogenicity tests were performed on 30 saplings of A. pseudoplatanus under two treatments, humid and drought stress, and the fungus was pathogenic in both treatments. The mean length of bark lesions and wood discoloration of the drought-stressed saplings was significantly greater than that in the humid treatment. Re-isolations of C. corticale were successful from all inoculated saplings, and thus Koch’s postulates were confirmed. The mass loss of A. pseudoplatanus wood was determined by mini-block test in a period of 10 weeks and was observed as minimal. Based on the results, we conclude that C. corticale is a weak and opportunistic pathogen that most likely expresses itself intensively under hot and dry conditions.

New and emerging Phytophthora-related diseases in small trees, shrubs and herbaceous plants typical of subalpine vegetation have recently been observed in Italy and Slovenia. Diseased plants showed a complex symptomatology including foliar necrosis, fruit rot, shoot blight and branch bleeding cankers. Since little information is available about the aetiology of these aerial Phytophthora diseases, from 2019 to 2022, field surveys were conducted in 54 sites to define the occurrence, distribution and impact of the Phytophthora species on mountain vegetation. A total of 360 Phytophthora isolates were obtained from 397 samples collected from 33 herbaceous and woody host species. Based on phylogenetic analysis and morphometric data, 17 Phytophthora species were identified: P. pseudosyringae (201 isolates), P. plurivora (54), P. gonapodyides (21), P. ilicis (20), P. alpina (17), P. acerina (11), P. cactorum (7), P. pseudocryptogea (6), P. cambivora (5), P. idaei (4), P. psychrophila (3), P. bilorbang (2), P. chlamydospora (2), P. hedraiandra (1), P. kelmanii (1), P. rosacearum (1) and P. syringae (1). In addition, three isolates of a new putative Phytophthora species obtained from Alnus viridis, Juniperus communis and Rhododendron ferrugineum are described here as Phytophthora pseudogregata sp. nov. Overall, the results highlighted an unexpectedly high diversity of Phytophthora species in mountain areas, with many species able to cause aerial infections due to the production of caducous sporangia.

The ten most frequently isolated fungi from the wood of the dead branches of Acer pseudoplatanus L. were tested in dual cultures to evaluate their in vitro antagonistic activity against Eutypella parasitica R.W. Davidson and R.C. Lorenz, the causative agent of a destructive disease of maples in Europe and North America. The tested fungi, treated also as challenge isolates, were Diaporthe sp., Eutypa sp., Eu. maura, E. parasitica, Fusarium avenaceum, Neocucurbitaria acerina, Neonectria sp., Peniophora incarnata, Petrakia irregularis, and Phomopsis pustulata. The antagonistic ability of each challenge isolate was evaluated by calculating an index of antagonism (AI) based on the interaction type in the dual cultures. The results of competition between the fungal isolates were quantified after re-isolations from the interaction zone (s). The dual cultures revealed two main types of competitive interactions: Deadlock, consisting of mutual inhibition after mycelial contact or at a distance, and replacement, reflecting in the inhibition of E. parasitica, followed by partial overgrowth by the replacing fungus. Statistical analysis showed significant differences in average AI and s of challenge isolates between different dual culture assays. Based on the results of the antagonism index, Eutypa sp., Eu. maura, Neonectria sp., and P. incarnata had the highest inhibitory effect on E. parasitica growth and were recognized as the most promising candidates for further biocontrol studies of E. parasitica. The mycelium of E. parasitica at the interaction zones remained mostly viable, except in dual cultures with Eutypa sp., F. avenaceum, and Neonectria sp., where re-isolations did not yield any colony of the E. parasitica isolate. Based on the results, we assume that E. parasitica is a weak competitor, which invests less energy in direct mycelial competition. We discuss the potential of the observed antagonists as a possible biocontrol of Eutypella canker of maple. Nevertheless, additional experiments should be performed for a solid conclusion about competitive ability of E. parasitica and usefulness of antagonists as biocontrol.

Verticillium dahliae Kleb., the causal agent of Verticillium wilts, is a devastating plant disease affecting many plant species. Fungus V. dahliae was detected in a partially artificially established Acer pseudoplatanus L. forest stand in central Slovenia. This finding incited further study about the risk of different sources of V. dahliae isolates for maples in forests and the pathogenicity of three V. dahliae isolates of different origins was tested on saplings of A. pseudoplatanus, A. platanoides L., and A. campestre L. The inoculated saplings exhibited disease symptoms, i.e., leaf necrosis and wilting. At the end of the pathogenicity test, typical xylem browning was visible on the cross-sections, and the pathogen was successfully re-isolated. The isolates showed significant differences in their pathogenicity to specific maple hosts, with the agricultural isolate (originated from bell pepper) being the most aggressive on all three maple species. The disease severity index (DSI) and relative area under the disease progress curve (rAUDPC), as well as the success of re-isolation, indicate that A. platanoides is the most susceptible to inoculation with V. dahliae. In addition, significant differences in sapling biomass were observed between treated and control plants. These results suggest that maples in forest stands are threatened by V. dahliae, and biosecurity measures should be considered and implemented in forest management to reduce the transmission and potential spread of the pathogen.