Challenges in predicting invasive reservoir hosts of emerging pathogens: mapping Rhododendron ponticum as a foliar host for Phytophthora ramorum and Phytophthora kernoviae in the UK

Diversity and pathogenicity of Alternaria species associated with the invasive plant Ageratina adenophora and local plants

Pathogen accumulation after introduction is unavoidable for exotic plants over a long period of time. Therefore, it is important to understand whether plant invasion promotes novel pathogen emergence and increases the risk of pathogen movement among agricultural, horticultural, and wild native plants. In this study, we used multiple gene analysis to characterize the species composition of 104 isolates of Alternaria obtained from the invasive plant Ageratina adenophora and native plants from Yunnan, Hubei, Guizhou, Sichuan, and Guangxi in China. Phylogenetically, these strains were from A. alternata (88.5%), A. gossypina (10.6%) and A. steviae (0.9%). There was a high amount of sharing between strains associated with A. adenophora and with local plants. Pathogenicity tests indicated that most of these Alternaria strains are generalists; the isolates with a wider host range were more virulent to the plant. Woody plants were more resistant to these strains than herbaceous plants and vines. However, the invasive plant A. adenophora was highly sensitive to these strains. Our data are valuable for understanding how A. adenophora invasion impacts the Alternaria species composition of the native plant and whether A. adenophora invasion causes potential disease risks in invaded ecosystems.

Quantifying the sharing of foliar fungal pathogens by the invasive plant Ageratina adenophora and its neighbours

Local pathogens can accumulate as asymptomatic endophytes, making it difficult to detect the impacts of invasive species as propagators of disease in the invaded range. We used the invasive plant Ageratina adenophora to assess such accumulation. We intensively collected foliar fungal endophytes and leaf spot pathogens of A. adenophora and co-occurring neighbours and performed an inoculation experiment to evaluate their pathogenicity and host range. Ageratina adenophora harboured diverse necrotrophic pathogens; its communities of endophytes and leaf spot pathogens were different in composition and shared only a small number of fungal species. In the pathogen communities of local plant hosts, 21% of the operational taxonomic units (OTUs), representing 50% of strains, also occurred as leaf spot pathogens and/or endophytes of A. adenophora. The local pathogen community was more similar to the endophytes than to the pathogens of A. adenophora. The inoculation experiment showed that local pathogens could infect A. adenophora leaves asymptomatically and that local plant hosts were susceptible to both A. adenophora endophytes and pathogens. Ageratina adenophora is a highly competent host for local pathogens, and its asymptomatic latent pathogens are fungi primarily shared with local neighbours. This poses challenges for understanding the long-term ecological consequences of plant invasion.

Characterization of the fungal community in the canopy air of the invasive plant Ageratina adenophora and its potential to cause plant diseases

Airborne fungi and their ecological functions have been largely ignored in plant invasions. In this study, high-throughput sequencing technology was used to characterize the airborne fungi in the canopy air of the invasive weed Ageratina adenophora. Then, representative phytopathogenic strains were isolated from A. adenophora leaf spots and their virulence to A.adenophora as well as common native plants in the invaded range was tested. The fungal alpha diversities were not different between the sampling sites or between the high/low part of the canopy air, but fungal co-occurrences were less common in the high than in the low part of the canopy air. Interestingly, we found that the phytopathogenic Didymellaceae fungi co-occurred more frequently with themselves than with other fungi. Disease experiments indicated that all 5 Didymellaceae strains could infect A. adenophora as well as the 16 tested native plants and that there was large variation in the virulence and host range. Our data suggested that the diverse pathogens in the canopy air might be a disease infection source that weakens the competition of invasive weeds, a novel phenomenon that remains to be explored in other invasive plants.

Current and potential distribution of the ectomycorrhizal fungus Suillus lakei ((Murrill) A.H. Sm. & Thiers) in its invasion range

Suillus lakei is an ectomycorrhizal fungus native to North America and known in Europe, South America, and New Zealand. This contribution aims to illustrate the worldwide biogeography of S. lakei based on sporocarp records. Species distribution modeling was used to assess the suitable niche distribution of S. lakei, based on the climatic variables as well as distribution of its ectomycorrhizal partner, Douglas fir. In general, distribution of suitable niches of S. lakei greatly overlaps with the distribution of Douglas fir in North America. By spatial distribution modeling, we found that the precipitation of the coldest quarters, isothermality, and annual mean temperature are important factors influencing the potential distribution of S. lakei. Nevertheless, the most crucial factor limiting expansion of S. lakei in its invasion range is Douglas fir occurrence. This factor reached an 86.4% contribution for the S. lakei species distribution model. Additionally, we compare the aboveground and belowground presence of S. lakei based on surveys in the field. Our study shows that even extremely low abundance of ectomycorrhizas can open the possibility of using an ectomycorrhiza survey for their quantification as a good indicator of the presence of S. lakei in field conditions. Both sporocarps and ectomycorrhizas occurred only in gardens, where Douglas fir seedlings were outplanted at the beginning of the 1990s as an ornamental plant. Presumably, international trade of ornamental plants was one possible route of introduction of S. lakei to Poland.

Forest health in a changing world

Forest pathology, the science of forest health and tree diseases, is operating in a rapidly developing environment. Most importantly, global trade and climate change are increasing the threat to forest ecosystems posed by new diseases. Various studies relevant to forest pathology in a changing world are accumulating, thus making it necessary to provide an update of recent literature. In this contribution, we summarize research at the interface between forest pathology and landscape ecology, biogeography, global change science and research on tree endophytes. Regional outbreaks of tree diseases are requiring interdisciplinary collaboration, e.g. between forest pathologists and landscape ecologists. When tree pathogens are widely distributed, the factors determining their broad-scale distribution can be studied using a biogeographic approach. Global change, the combination of climate and land use change, increased pollution, trade and urbanization, as well as invasive species, will influence the effects of forest disturbances such as wildfires, droughts, storms, diseases and insect outbreaks, thus affecting the health and resilience of forest ecosystems worldwide. Tree endophytes can contribute to biological control of infectious diseases, enhance tolerance to environmental stress or behave as opportunistic weak pathogens potentially competing with more harmful ones. New molecular techniques are available for studying the complete tree endobiome under the influence of global change stressors from the landscape to the intercontinental level. Given that exotic tree diseases have both ecologic and economic consequences, we call for increased interdisciplinary collaboration in the coming decades between forest pathologists and researchers studying endophytes with tree geneticists, evolutionary and landscape ecologists, biogeographers, conservation biologists and global change scientists and outline interdisciplinary research gaps.