Publications
NIBIOs employees contribute to several hundred scientific articles and research reports every year. You can browse or search in our collection which contains references and links to these publications as well as other research and dissemination activities. The collection is continously updated with new and historical material.
2019
Abstract
In many areas where spring is wet, fungicides are applied in relation to rain events that trigger ejection of ascospores of Venturia inaequalis, which cause primary infections of apple scab. Past studies established the rate of ejection during rain in relation to light and temperature, and determined the wetting time required for infection. Simulation software uses this information to calculate risk and help time sprays accordingly. However, the distribution of the infection time required by a population of spores landed on leaves was never studied, and assumptions were used. To estimate this, we inoculated ascospores of V. inaequalis on potted trees at different temperatures for specific wetting times. Lesions were enumerated after incubation. Lesions increased with wetness time and leveled off once the slowest spores infected the host, closely matching the monomolecular model. Wetness hours were best adjusted for temperature using the Yin equation. The minimum infection time on the youngest leaves was about 5 h, matching results from previous studies, whereas half the lesions appeared after 7 h of infection. Infection times for leaves with ontogenic resistance were longer. Our results improve current software estimates and may improve spraying decisions.
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Authors
Katherine Ann Gredvig Nielsen Gunn Mari Strømeng Magne Nordang Skårn Kari Ørstad Arne Stensvand May Bente BrurbergAbstract
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Abstract
Invasive alien species and new plant pests are introduced into new regions at an accelerating rate, due to increasing international trade with soil, plants and plant products. Exotic, plant pathogenic oomycetes in soil from the root zone of imported plants pose a great threat to endemic ecosystems and horticultural production. Detecting them via baiting and isolation, with subsequent identification of the isolated cultures by Sanger sequencing, is labour intensive and may introduce bias due to the selective baiting process. We used metabarcoding to detect and identify oomycetes present in soil samples from imported plants from six different countries. We compared metabarcoding directly from soil both before and after baiting to a traditional approach using Sanger-based barcoding of cultures after baiting. For this, we developed a standardized analysis workflow for Illumina paired-end oomycete ITS metabarcodes that is applicable to future surveillance efforts. In total, 73 soil samples from the rhizosphere of woody plants from 33 genera, in addition to three samples from transport debris, were analysed by metabarcoding the ITS1 region with primers optimized for oomycetes. We detected various Phytophthora and Pythium species, with Pythium spp. being highly abundant in all samples. We also found that the baiting procedure, which included submerging the soil samples in water, resulted in the enrichment of organisms other than oomycetes, compared to non-baited soil samples.
2018
Abstract
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Abstract
Dieback of European ash, caused by the ascomycete Hymenoscyphus fraxineus originating from Asia, has rapidly spread across Europe, and is threatening this keystone tree at a continental scale. High propagule pressure is characteristic to invasive species. Consistently, the enormous production of windborne ascospores by H. fraxineus in an ash forest with epidemic level of disease obviously facilitates its invasiveness and long distance spread. To understand the rate of build-up of propagule pressure by this pathogen following its local introduction, during 2011–2017 we monitored its sporulation at a newly infested ash stand in south-western Norway characterized with mild winters and cool summers. We also monitored the propagule pressure by Hymenoscyphus albidus, a non-pathogenic native species that competes for the same sporulation niche with H. fraxineus. During the monitoring period, crown condition of ash trees had impaired, and 20% of the dominant trees were severely damaged in 2017. H. fraxineus showed an exponential increase in spore production between 2012 and 2015, followed by drastic decline in 2016 and 2017. During 2011–2013, the two Hymenoscyphus species showed similar sporulation level, but thereafter spores of H. albidus were no longer detected. The data suggest that following local introduction, the population of H. fraxineus reaches rapidly an exponential growth stage if the local weather conditions are favorable for ascomata maturation across years. In the North Atlantic climate, summer temperatures critically influence the pathogen infection pressure, warm summers allowing the population to grow according to its biotic potential, whereas cold summers can cause a drastic decline in propagule pressure.
Abstract
No abstract has been registered
Authors
Ari Hietala Isabella Børja Hugh B. Cross Nina Elisabeth Nagy Jørn Henrik Sønstebø Volkmar Timmermann Adam Vivian-Smith Halvor SolheimAbstract
No abstract has been registered
Authors
Ari Hietala Isabella Børja Hugh Cross Nina Elisabeth Nagy Halvor Solheim Volkmar Timmermann Adam Vivian-SmithAbstract
European ash (Fraxinus excelsior), a keystone species with wide distribution and habitat range in Europe, is threatened at a continental scale by an invasive alien ascomycete, Hymenoscyphus fraxineus. In its native range of Asia, this fungus is a leaf endophyte with weak parasitic capacity and robust saprobic competence in local ash species that are closely related to European ash. In European ash, H. fraxineus has a similar functional role as in Asia, but the fungus also aggressively kills shoots, resulting in crown dieback and tree death. H. fraxineus is a typical invasive species, as its spread relies on high propagule pressure. While crown dieback of European ash is the most obvious symptom of ash dieback, the annual colonization of ash leaves is a crucial key dependency for the invasiveness of H. fraxineus, since its fruiting bodies are formed on overwintered leaf vein tissues in soil debris. Leaves of European ash host a wide range of indigenous epiphytes, endophytes, facultative parasites and biotrophic fungi, including Hymenoscyphus albidus, a relative of H. fraxineus that competes for the same sporulation niche as the invader. At face value, leaves of European ash are colonized by a large and diverse indigenous mycobiome. In order to understand why this invader became successful in Europe, we discuss and summarize the current knowledge of diversity, seasonal dynamics and traits of H. fraxineus and indigenous fungi associated with leaves of European ash.