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Introduction and purpose: The ability of apple rootstocks to become infected by Neonectria ditissima, the cause of European canker, was studied over two years. Materials and methods: Rootstocks B9 and M9 with a size suitable for grafting (6-10 mm stem diameter, termed rootstocks), and smaller sized rootstocks (<5 mm stem diameter, termed transplants) of B9, M9, M26, MM106 and Antonovka were inoculated with N. ditissima at different times, either with contaminated map pins or with spore suspensions. In addition, the rootstocks were either defeathered (side shoots removed), topped (top shoot headed) or both, to create wounds that would normally occur during propagation, while wounds on transplants were made by removing leaves. Results and discussion: One month after inoculation, slightly sunken canker lesions had developed around the inoculation points of the map pins or wounds. No lesions developed on the non-inoculated controls. Map pin inoculation resulted in 30% to 89% infection and spore suspension sprayed on wounds from 5% to 45% infection. When the cankered areas were split open, brown lesions with necrotic tissue due to infection by N. ditissima appeared. The transplants of M9, M26 and MM106 inoculated with contaminated map pins in 2014 developed necrosis on 40% to 67% of the plants, but there were no differences in the incidence or severity among the different types. On the transplants of B9, Antonovka and M9 inoculated in 2015, there was more necrosis on B9 (42%) than on Antonovka (11%) and more sporulating lesions on B9 (29%) than on M9 (9%) or on Antonovka (4%). Conclusion: It can be concluded that rootstocks used for apple trees may become infected by N. ditissima, and wounds should thus be protected during propagation. [ABSTRACT FROM AUTHOR]

Field trials of Abies lasiocarpa were undertaken with the aim of assessing the potential for Christmas tree production in Denmark. Twenty-six provenances originating from Alaska to New Mexico were tested. Damage by the insect Adelges piceae and the fungus Neonectria neomacrospora was recorded for the first time 8 and 12 years after the initial planting. Damage from N. neomacrospora increased rapidly in the period 12-15 years after planting. Trees from the northern provenances and humid climates exhibited less damage than those from southern ones. Previous attack by A. piceae had a minor effect on N. neomacrospora infection. Greenhouse tests showed that detached shoots from healthy Abies lasiocarpa can be used to rank provenances for resistance to N. neomacrospora, but results varied according to host subspecies. [ABSTRACT FROM AUTHOR]

In 2011, Phytophthora cambivora was isolated for the first time from a European beech ( Fagus sylvatica) in the largest beech forest in Norway, in Larvik. This led to a survey of Phytophthora, both in Larvik and in a small beech stand in Ås. Both locations are in urban areas in south-eastern Norway. Trees with a circumference above 20 cm at chest height were examined for bleeding cankers. Samples from the leading edge of canker wounds were collected from selected trees in both locations and isolations were carried out on a Phytophthora-selective medium. Quantification of diseased trees in three areas of the forest in Larvik showed a variation from 1.8 to 22.7% trees with bleeding cankers. In Ås, 9.2% of the trees had bleeding cankers. Isolation from diseased beech in both Larvik and Ås yielded P. cambivora. Inoculation of healthy trees, subsequent development of disease and re-isolation of the pathogen confirmed the pathogenicity of P. cambivora. Description of morphological features of one of the obtained isolates is included. [ABSTRACT FROM AUTHOR]

Phytophthora pseudosyringae is a self-fertile pathogen of woody plants, particularly associated with tree species from the genera Fagus, Notholithocarpus, Nothofagus and Quercus, which is found across Europe and in parts of North America and Chile. It can behave as a soil pathogen infecting roots and the stem collar region, as well as an aerial pathogen infecting leaves, twigs and stem barks, causing particular damage in the United Kingdom and western North America. The population structure, migration and potential outcrossing of a worldwide collection of isolates were investigated using genotyping-by-sequencing. Coalescent-based migration analysis revealed that the North American population originated from Europe. Historical gene flow has occurred between the continents in both directions to some extent, yet contemporary migration is overwhelmingly from Europe to North America. Two broad population clusters dominate the global population of the pathogen, with a subgroup derived from one of the main clusters found only in western North America. Index of association and network analyses indicate an influential level of outcrossing has occurred in this preferentially inbreeding, homothallic oomycete. Outcrossing between the two main population clusters has created distinct subgroups of admixed individuals that are, however, less common than the main population clusters. Differences in life history traits between the two main population clusters should be further investigated together with virulence and host range tests to evaluate the risk each population poses to natural environments worldwide., (© 2024 The Authors. Molecular Plant Pathology published by British Society for Plant Pathology and John Wiley & Sons Ltd.)

Gray mold, caused by Botrytis spp., is a serious problem in Norway spruce seedling production in forest nurseries. From 2013 to 2019, 125 isolates of Botrytis were obtained from eight forest nurseries in Norway: 53 from Norway spruce seedlings, 16 from indoor air, 52 from indoor surfaces, and four from weeds growing close to seedlings. The majority of isolates were identified as B. cinerea , and over 60% of these were characterized as Botrytis group S. B. pseudocinerea isolates were obtained along with isolates with DNA sequence similarities to B. prunorum . Fungicide resistance was assessed with a mycelial growth assay, and resistance was found for the following: boscalid (8.8%), fenhexamid (33.6%), fludioxonil (17.6%), pyraclostrobin (36.0%), pyrimethanil (13.6%), and thiophanate-methyl (50.4%). Many isolates (38.4%) were resistant to two to six different fungicides. A selection of isolates was analyzed for the presence of known resistance-conferring mutations in the cytb , erg27 , mrr1 , sdhB , and tubA genes, and mutations leading to G143A, F412S, ΔL497, H272R, and E198A/F200Y were detected, respectively. Detection of fungicide resistance in Botrytis from Norway spruce and forest nursery facilities reinforces the necessity of employing resistance management strategies to improve control and delay development of fungicide resistance in the gray mold pathogens.[Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license., Competing Interests: The author(s) declare no conflict of interest.

This study aimed to determine the differences and drivers of oomycete diversity and community composition in alder- and birch-dominated park and natural forest soils of the Fennoscandian and Baltic countries of Estonia, Finland, Lithuania, Norway, and Sweden. For this, we sequenced libraries of PCR products generated from the DNA of 111 soil samples collected across a climate gradient using oomycete-specific primers on a PacBio high-throughput sequencing platform. We found that oomycete communities are most affected by temperature seasonality, annual mean temperature, and mean temperature of the warmest quarter. Differences in composition were partly explained by the higher diversity of Saprolegniales in Sweden and Norway, as both total oomycete and Saprolegniales richness decreased significantly at higher longitudes, potentially indicating the preference of this group of oomycetes for a more temperate maritime climate. None of the evaluated climatic variables significantly affected the richness of Pythiales or Peronosporales. Interestingly, the relative abundance and richness of Pythiales was higher at urban sites compared to forest sites, whereas the opposite was true for Saprolegniales. Additionally, this is the first report of Phytophthora gallica and P . plurivora in Estonia. Our results indicate that the composition of oomycetes in soils is strongly influenced by climatic factors, and, therefore, changes in climate conditions associated with global warming may have the potential to significantly alter the distribution range of these microbes, which comprise many important pathogens of plants.

Non-native pests, climate change, and their interactions are likely to alter relationships between trees and tree-associated organisms with consequences for forest health. To understand and predict such changes, factors structuring tree-associated communities need to be determined. Here, we analysed the data consisting of records of insects and fungi collected from dormant twigs from 155 tree species at 51 botanical gardens or arboreta in 32 countries. Generalized dissimilarity models revealed similar relative importance of studied climatic, host-related and geographic factors on differences in tree-associated communities. Mean annual temperature, phylogenetic distance between hosts and geographic distance between locations were the major drivers of dissimilarities. The increasing importance of high temperatures on differences in studied communities indicate that climate change could affect tree-associated organisms directly and indirectly through host range shifts. Insect and fungal communities were more similar between closely related vs. distant hosts suggesting that host range shifts may facilitate the emergence of new pests. Moreover, dissimilarities among tree-associated communities increased with geographic distance indicating that human-mediated transport may serve as a pathway of the introductions of new pests. The results of this study highlight the need to limit the establishment of tree pests and increase the resilience of forest ecosystems to changes in climate., (© 2023. The Author(s).)

International trade in plants and climate change are two of the main factors causing damaging tree pests (i.e. fungi and insects) to spread into new areas. To mitigate these risks, a large-scale assessment of tree-associated fungi and insects is needed. We present records of endophytic fungi and insects in twigs of 17 angiosperm and gymnosperm genera, from 51 locations in 32 countries worldwide. Endophytic fungi were characterized by high-throughput sequencing of 352 samples from 145 tree species in 28 countries. Insects were reared from 227 samples of 109 tree species in 18 countries and sorted into taxonomic orders and feeding guilds. Herbivorous insects were grouped into morphospecies and were identified using molecular and morphological approaches. This dataset reveals the diversity of tree-associated taxa, as it contains 12,721 fungal Amplicon Sequence Variants and 208 herbivorous insect morphospecies, sampled across broad geographic and climatic gradients and for many tree species. This dataset will facilitate applied and fundamental studies on the distribution of fungal endophytes and insects in trees., (© 2022. The Author(s).)

Plants with roots and soil clumps transported over long distances in plant trading can harbor plant pathogenic oomycetes, facilitating disease outbreaks that threaten ecosystems, biodiversity, and food security. Tools to detect the presence of such oomycetes with a sufficiently high throughput and broad scope are currently not part of international phytosanitary testing regimes. In this work, DNA metabarcoding targeting the internal transcribed spacer (ITS) region was employed to broadly detect and identify oomycetes present in soil from internationally shipped plants. This method was compared to traditional isolation-based detection and identification after an enrichment step. DNA metabarcoding showed widespread presence of potentially plant pathogenic Phytophthora and Pythium species in internationally transported rhizospheric soil with Pythium being the overall most abundant genus observed. Baiting, a commonly employed enrichment method for Phytophthora species, led to an increase of golden-brown algae in the soil samples, but did not increase the relative or absolute abundance of potentially plant pathogenic oomycetes. Metabarcoding of rhizospheric soil yielded DNA sequences corresponding to oomycete isolates obtained after enrichment and identified them correctly but did not always detect the isolated oomycetes in the same samples. This work provides a proof of concept and outlines necessary improvements for the use of environmental DNA (eDNA) and metabarcoding as a standalone phytosanitary assessment tool for broad detection and identification of plant pathogenic oomycetes., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Rossmann, Lysøe, Skogen, Talgø and Brurberg.)

Production of inoculum of Colletotrichum acutatum from both previously infected and overwintered tissue, as well as newly developed plant tissue of sour cherry (Prunus cerasus), was studied in southern Norway. Plant parts were sampled from commercial, private, or research orchards, and incubated for 2 to 14 days (time depended on tissue type) in saturated air at 20°C. In early spring, abundant sporulation was found on scales of overwintered buds and shoots. A mean of 35% infected buds in four cultivars was observed, with a maximum of 72% of the buds infected in one of the samples. Over 3 years, the seasonal production of overwintered fruit and peduncles of cv. Fanal infected the previous year was investigated. In all three years, the infected plant material was placed in the trees throughout the winter and the following growing season; in two of the years, fruit and peduncles were also placed on the ground in the autumn or the following spring. Old fruit and peduncles formed conidia throughout the season, with a peak in May and June. Spore numbers declined over the season, but the decline was more rapid for plant material on the ground than in the trees. On average over 2 years, 68.7, 24.0, or 7.3% of the inoculum came from fruit placed in the trees, placed on the ground in spring, or placed on the ground the preceding autumn, respectively. The number of fruit and peduncles attached to the trees in a planting of cv. Hardangerkirsebær was followed from February to July one year, and although there was a decline over time, fruit and/or their peduncles were still attached in substantial numbers in July, thus illustrating their potential as sources of inoculum. In observations over 2 years in a heavily infected orchard of cv. Stevnsbær, 75 and 47% of flowers and newly emerged fruit, respectively, were infected. Artificially inoculated flowers and fruit produced conidia until harvest, with a peak in mid-July. It may be concluded that previously infected and overwintered, as well as newly emerged tissue of sour cherry, may serve as sources of inoculum of C. acutatum throughout the growing season.

Current season needle necrosis (CSNN) has been a serious foliage disorder on true fir Christmas trees and bough material in Europe and North America for more than 25y. Approximately 2-4 weeks after bud break, needles develop chlorotic spots or bands that later turn necrotic. The symptoms have been observed on noble fir (Abies procera), Nordmann fir (A. nordmanniana) and grand fir (A. grandis) on both continents. CSNN was reported as a physiological disorder with unknown aetiology from USA, Denmark, and Ireland, but was associated with the fungus Kabatina abietis in Germany, Austria and Norway. In 2007, a fungus that morphologically resembled K. abietis was isolated from symptomatic needle samples from Nordmann fir from Austria, Denmark, Germany, Norway, and USA. Sequencing of the internal transcribed spacer (ITS) region of ribosomal DNA of these cultures, plus a K. abietis reference culture from Germany (CBS 248.93), resulted in Hormonema dematioides, the imperfect stage of Sydowia polyspora, and thus the taxonomy is further discussed. Inoculation tests on Nordmann fir seedlings and transplants with isolates of S. polyspora from all five countries resulted in the development of CSNN symptoms. In 2009, S. polyspora was also isolated from symptomatic needles from Nordmann fir collected in Slovakia., (Copyright © 2010 The British Mycological Society. Published by Elsevier Ltd. All rights reserved.)