The high prevalence of hay fever in Europe has raised concerns about the implications of climate change-induced higher temperatures on pollen production. Our study focuses on downy birch pollen production across Europe by analyzing 456 catkins during 2019-2021 in 37 International Phenological Gardens (IPG) spanning a large geographic gradient. As IPGs rely on genetically identical plants, we were able to reduce the effects of genetic variability. We studied the potential association with masting behavior and three model specifications based on mean and quantile regression to assess the impact of meteorology (e.g., temperature and precipitation) and atmospheric gases (e.g., ozone (O3) and carbon-dioxide (CO2)) on pollen and catkin production, while controlling for tree age approximated by stem circumference. The results revealed a substantial geographic variability in mean pollen production, ranging from 1.9 to 2.5 million pollen grains per catkin. Regression analyses indicated that elevated average temperatures of the previous summer corresponded to increased pollen production, while higher O3 levels led to a reduction. Additionally, catkins number was positively influenced by preceding summer's temperature and precipitation but negatively by O3 levels. The investigation of quantile effects revealed that the impacts of mean temperature and O3 levels from the previous summer varied throughout the conditional response distribution. We found that temperature predominantly affected trees characterized by a high pollen production. We therefore suggest that birches modulate their physiological processes to optimize pollen production under varying temperature regimes. In turn, O3 levels negatively affected trees with pollen production levels exceeding the conditional median. We conclude that future temperature increase might exacerbate pollen production while other factors may modify (decrease in the case of O3 and amplify for precipitation) this effect. Our comprehensive study sheds light on potential impacts of climate change on downy birch pollen production, which is crucial for birch reproduction and human health.

Viruses are frequently a microbial biocontaminant of healthy plants. The occurrence of the infection can be also due to environmental stress, like urbanisation, air pollution and increased air temperature, especially under the ongoing climate change. The aim of the present study was to investigate the hypothesis that worsened air quality and fewer green areas may favour the higher frequency of common viral infections, particularly in a common tree in temperate and continental climates, Betula pendula ROTH. We examined 18 trees, during the years 2015-2017, the same always for each year, in the region of Augsburg, Germany. By specific PCR, the frequency of two viruses, Cherry leaf roll virus (CLRV, genus Nepovirus, family Secoviridae), which is frequent in birch trees, and a novel virus tentatively named birch idaeovirus (BIV), which has been only recently described, were determined in pollen samples. The occurrence of the viruses was examined against the variables of urban index, air pollution (O3 and NO2), air temperature, and tree morphometrics (trunk perimeter, tree height, crown height and diameter). Generalized Non-linear models (binomial logit with backward stepwise removal of independent variables) were employed. During the study period, both CLRV and BIV were distributed widely throughout the investigated birch individuals. CLRV seemed to be rather cosmopolitan and was present independent of any abiotic factor. BIV's occurrence was mostly determined by higher values of the urban index and of NO2. Urban birch trees, located next to high-traffic roads with higher NO2 levels, are more likely to be infected by BIV. Increased environmental stress may lead to more plant viral infections. Here we suggest that this is particularly true for urban spaces, near high-traffic roads, where plants may be more stressed, and we recommend taking mitigation measures for controlling negative human interventions.

High-altitude environments are highly susceptible to the effects of climate change. Thus, it is crucial to examine and understand the behaviour of specific plant traits along altitudinal gradients, which offer a real-life laboratory for analysing future impacts of climate change. The available information on how pollen production varies at different altitudes in mountainous areas is limited. In this study, we investigated pollen production of 17 birch (Betula pubescens Ehrh.) individuals along an altitudinal gradient in the European Alps. We sampled catkins at nine locations in the years 2020–2021 and monitored air temperatures. We investigated how birch pollen, flowers and inflorescences are produced in relation to thermal factors at various elevations. We found that mean pollen production of Betula pubescens Ehrh. varied between 0.4 and 8.3 million pollen grains per catkin. We did not observe any significant relationships between the studied reproductive metrics and altitude. However, minimum temperature of the previous summer was found to be significantly correlated to pollen (rs = 0.504, p = 0.039), flower (rs = 0.613, p = 0.009) and catkin (rs = 0.642, p = 0.005) production per volume unit of crown. Therefore, we suggest that temperature variability even at such small scales is very important for studying the response related to pollen production.

The knowledge of viruses in forest and urban trees develops rapidly. Regarding the last ten years, a remarkable number of new and previously known viruses have been discovered and described in widespread European urban and forest trees. However, so far, very little is generally known about these viruses compared to those affecting agricultural crops. This misleads to the impression that viruses in forest trees are negligible, but they are probably responsible for far greater economic losses than generally recognized. Here, we summarize the current knowledge on these tree viruses. Only the comprehensive knowledge about viruses in these trees enables the assessment and evaluation of their economic and ecological importance and their impact on other associated microbiomes. In the beginning, we outline symptomatology along samples of typical symptoms of diverse tree species and explain methods for the detection of viruses, and give information about properties they have, including current knowledge about their spread, and recommendations for current and future management in forest and urban environment.

Intraspecific genetic variation might limit the relevance of environmental factors on plant traits. For example, the interaction between genetics and (a-)biotic factors regulating pollen production are still poorly understood. In this study, we investigated pollen production of 28 birch (Betula pendula Roth) individuals in the years 2019–2021. We sampled catkins of eleven groups of genetically identical trees, which were partially topped, but of the same age and located in a seed plantation in southern Germany characterized by similar microclimatic conditions. Furthermore, we monitored environmental factors such as air temperature, characterized air quality (NO2, NOx and O3), and assessed potential solar radiation. We especially checked for differences between years as well as between and within clones and assessed the synchronicity of years with high/low pollen production. We present a robust mean for the pollen production of Betula pendula (1.66 million pollen grains per catkin). Our findings show temporal (H(2) = 46.29, p < 0.001) and clonal variations (H(4) = 21.44, p < 0.001) in pollen production. We conclude that synchronized high or low pollen production is not utterly site-specific and, in addition, not strictly dependent on genotypes. We suggest that appropriate clone selection based on application (seed plantation, urban planting) might be advantageous and encourage a long-term monitoring.

To unravel the virome in birch trees of German and Finnish origin exhibiting symptoms of birch leaf-roll disease (BRLD), high-throughput sequencing (HTS) was employed. In total five viruses, among which three were so far unknown, were detected by RNAseq. One to five virus variants were identified in the transcriptome of individual trees. The novel viruses were genetically-fully or partially-characterized, belonging to the genera Carlavirus, Idaeovirus and Capillovirus and are tentatively named birch carlavirus, birch idaeovirus, and birch capillovirus, respectively. The recently discovered birch leafroll-associated virus was systematically detected by HTS in symptomatic seedlings but not in symptomless ones. The new carlavirus was detected only in one of the three symptomatic seedlings. The novel putative Capillovirus was detected in all seedlings-irrespective of their BLRD status-while the Idaeovirus was identified in a plant without leaf symptoms at the time of sampling. Further efforts are needed to complete Koch's postulates and to clarify the possible association of the detected viruses with the BLR disease. Our study elucidates the viral population in single birch seedlings and provides a comprehensive overview for the diversities of the viral communities they harbor, to date.

Data from next generation sequencing indicate the complexity of the birch virome in the urban landscape of Berlin. It is well known that plant viruses are widespread and contribute to the decline of birch trees. A mixed infection by Cherry leaf roll virus (CLRV), Apple mosaic virus (ApMV) and two newly discovered viruses from the genus Badna- and Carlavirus were investigated in southern Berlin (Steglitz-Zehlendorf) in 2015 and 2016. To gain a more detailed view on epidemiology of this viral complex in birch, the study was enlarged in 2017 including eight districts all over Berlin. Birch trees with symptoms like defoliation and degeneration were selected for determination of viral pathogens by molecular biological methods. Within the complex occurring symptoms in birch trees, new types of symptoms have been identified. Different combinations of plant viruses in single and mixed infection were detected by Reverse Transcription- Polymerase Chain Reaction (RT-PCR.) CLRV and Badnavirus combinations have shown to be distinct and widely distributed. Heterogeneity is also known from the symptomatology of virus containing birch leaves. As the correlation of symptoms and viral infection is not shown yet for the mixed infections, it is unknown if the complexity of the virome is the cause of the variability of symptoms. Epidemiology and pathogenicity of the newly discovered viruses as well as species specificity, life cycle, mode of transmission, host plant range and phylogeny are totally unknown and have to be investigated within the next years.

S3-O3. Application of High-throughput-sequencing for detection of known and identification of novel plant viruses in broad-leaved forest trees and woody species of urban green space. Susanne von Bargen1, Marius Rehanek1, Thomas Gaskin1, Max Tischendorf1, Maria Landgraf1, Martina Bandte1, Thierry Candresse2, Armelle Marais2, Chantal Faure2, Thierry Wetzel3, Dag-Ragnar Blystad4, Markus Rott1,5, Juliane Langer1, Jenny Roßbach1, Artemis Rumbou1, Michael Kube6, Marleen Botermans7, Risto Jalkanen8, Jean-Sebastien Reynard9 and Carmen Büttner1

The application of bioinformatic tools on NGS data of birch enables to identify new plant viruses continuously in birch samples from different origin. The virome of birch seems to be more complex and heterogenic as compared to other investigated deciduous trees and this is in some way special. The virome includes DNA as well as RNA viruses with their specific characteristics and they consequently influence the host plants diverse. In 2015 and 2016 the incidence of 4 viruses (CLRV, ApMV, Carlavirus and Badnavirus (Rumbou et al. 2017) of the heterogenic virome of birch was investigated in symptomatic leaves in Berlin (Landgraf et al. 2017). Different combinations of viruses in single and mixed infection were detected by RT-PCR. Some viral combinations are more distinct distributed in Berlin birch than others, so that the question of their relevance in birch and the observed “leaf roll disease” poses. The picture of heterogeneity is also known from the leaf symptomatology in virus containing birch leaves. As the correlation of symptoms and viral infection is not shown yet for the mixed infections, it is unknown if the complexity of the virome is causative for the complicated symptomatology. Epidemiology and pathogenicity of the new discovered viruses as well as species specificity, life cycle, mode of transmission, host plant range and phylogeny are totally unknown and have to be investigated within the next years. References: Rumbou A, Candresse T, Marais A, Theil S, Langer J, Jalkanen R, Büttner C 2017: A novel Badnavirus discovered from betula sp. Affected by birch leaf roll disease, PLOS ONE PONED-17-44140 Landgraf M, Langer J, Gröhner J, Zinnert L, Bandte M, von Bargen S, Schreiner M, Jäckel B, Büttner C, 2017: Viruserkrankungen im urbanen Grün – eine Studie an Birken im Berliner Bezirk Steglitz-Zehlendorf Viral diseases in urban areas – a study on birch in Berlin SteglitzZehlendorf Jahrbuch der Baumpflege, 21. Jg., S. 327–332

Until today virus diseases of trees are not well investigated and research focusing on occurrence and impact is underrepresented compared to other diseases of broadleaved perennials (Büttner et al. 2013). High-throughput sequencing technologies enable the characterization of virus communities affecting important diseased broadleaved tree species. The identification of previously unknown viruses as well as the determination of mixed infections with several different plant viruses is a prerequisite to determine the impact of viruses on long-living forest trees as well as woody species dominating the urban habitats. The virome of several deciduous tree species exhibiting virus-suspicious symptoms such as chlorotic ringspots, veinbanding and necroses of leaves associated with dieback of twigs and parts of the canopy were determined by a Highthroughput sequencing (NGS) approach (RNA-Seq, Illumina). Analyses of NGSgenerated sequence contigs identified plant viruses known to affect investigated tree species such as Cherry leaf roll virus (CLRV), Elm mottle virus (EMoV) and European mountain ash ringspot-associated virus (EMARaV). Further, nearly complete genomes of new, previously unidentified plant viruses could be retrieved from the generated data. NGS contributed to the discovery of six previously unknown viruses belonging to the genera Badna-, Carla-, and Emaravirus, respectively, infecting important tree species of European forests and urban stands. Virus-incidence in birch, elm, maple, poplar, and oak could be confirmed by virus-specific RT-PCR in diseased trees providing first insights into geographical distribution,impact of known and newly identified viruses as well as the occurrence of mixed infections. References Büttner C, von Bargen S, Bandte M, Mühlbach H-P 2013. Forest diseases caused by viruses. In: Infectious forest diseases. Gonthier P., Nicolotti G. (eds), CABI, pp. 50-75.