Invasions of alien plants are typically studied as invasions of individual species, yet interactions between plants and symbiotic fungi (mutualists and potential pathogens) affect plant survival, ...physiological traits, and reproduction and hence invasion success. Studies show that plant–fungal associations are frequently key drivers of plant invasion success and impact, but clear conceptual frameworks and integration across studies are needed to move beyond a series of case studies towards a more predictive understanding. Here, we consider linked plant–fungal invasions from the perspective of plant and fungal origin, simplified to the least complex representations or ‘motifs’. By characterizing these interaction motifs, parallels in invasion processes between pathogen and mutualist fungi become clear, although the outcomes are often opposite in effect. These interaction motifs provide hypotheses for fungal-driven dynamics behind observed plant invasion trajectories. In some situations, the effects of plant–fungal interactions are inconsistent or negligible. Variability in when and where different interaction motifs matter may be driven by specificity in the plant–fungal interaction, the size of the effect of the symbiosis (negative to positive) on plants and the dependence (obligate to facultative) of the plant–fungal interaction. Linked plant–fungal invasions can transform communities and ecosystem function, with potential for persistent legacies preventing ecosystem restoration.
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In the context of climate change, the effects of prolonged or more severe droughts on pest and pathogen damage are a major concern for forest ecosystems. To date, there is great uncertainty about the ...direction, magnitude and sources of variation in responses to drought by insects and fungi. We report the outcomes of a meta‐analysis of 100 pairwise comparisons of insect pest or pathogen damage to water‐stressed and control trees from 40 publications. The type of feeding substrate for insects and fungi and the water stress severity emerged as the main factors influencing the level of damage in water‐stressed trees. Overall, primary damaging agents living in wood caused significantly lower damage to the water‐stressed trees compared with the control, whereas primary pests and pathogens living on foliage caused more damage to water‐stressed trees, in all cases irrespective of stress severity. In contrast, damage by secondary agents increased with stress severity, which was best estimated by the ratio between the predawn leaf water potential in stressed trees and the xylem pressure inducing 50% loss in hydraulic conductance due to cavitation, a species‐specific index of drought tolerance. Insect and fungus feeding behaviour, affected tree part, and water stress severity are therefore proposed as three important predictors of forest damage in drought conditions.
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Little is known about the potential effect of climate warming on phyllosphere fungi, despite their important impact on the dynamics and diversity of plant communities. The structure of phyllosphere ...fungal assemblages along elevation gradients may provide information about this potential effect, because elevation gradients correspond to temperature gradients over short geographic distances.
We thus investigated variations in the composition of fungal assemblages inhabiting the phyllosphere of European beech (Fagus sylvatica) at four sites over a gradient of 1000 m of elevation in the French Pyrénées Mountains, by using tag-encoded 454 pyrosequencing.
Our results show that the composition of fungal assemblages varied significantly between elevation sites, in terms of both the relative abundance and the presence–absence of species, and that the variations in assemblage composition were well correlated with variations in the average temperatures.
Our results therefore suggest that climate warming might alter both the incidence and the abundance of phyllosphere fungal species, including potential pathogens. For example, Mycosphaerella punctiformis, a causal agent of leaf spots, showed decreasing abundance with elevation and might therefore shift to higher elevations in response to warming.
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4.
Socioeconomic legacy yields an invasion debt Essl, Franz; Dullinger, Stefan; Rabitsch, Wolfgang ...
Proceedings of the National Academy of Sciences - PNAS,
01/2011, Volume:
108, Issue:
1
Journal Article
Peer reviewed
Open access
Globalization and economic growth are widely recognized as important drivers of biological invasions. Consequently, there is an increasing need for governments to address the role of international ...trade in their strategies to prevent species introductions. However, many of the most problematic alien species are not recent arrivals but were introduced several decades ago. Hence, current patterns of alien-species richness may better reflect historical rather than contemporary human activities, a phenomenon which might be called "invasion debt." Here, we show that across 10 taxonomic groups (vascular plants, bryophytes, fungi, birds, mammals, reptiles, amphibians, fish, terrestrial insects, and aquatic invertebrates) in 28 European countries, current numbers of alien species established in the wild are indeed more closely related to indicators of socioeconomic activity from the year 1900 than to those from 2000, although the majority of species introductions occurred during the second half of the 20th century. The strength of the historical signal varies among taxonomic groups, with those possessing good capabilities for dispersal (birds, insects) more strongly associated with recent socioeconomic drivers. Nevertheless, our results suggest a considerable historical legacy for the majority of the taxa analyzed. The consequences of the current high levels of socioeconomic activity on the extent of biological invasions will thus probably not be completely realized until several decades into the future.
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1. The timing of tree flushing follows strong phenotypic and genetic clines across environmental gradients. It may be seen as an adaptive response to abiotic (escape of spring frost and maximizing ...growing season length) and biotic (escape of pest and disease) hazards. However, few studies have investigated jointly both types of hazards. 2. We assessed exposure to both abiotic (spring frost) and biotic (powdery mildew) hazards within and between sessile oak populations along elevation gradients, during the flushing period in several years. For each population and phenological phenotype (early- vs. late-flushing trees), we estimated safety margins, defined as the time period separating budburst from the hazard occurrence (spore emission, spring frost). 3. We observed that powdery mildew phenology (initiation of spore release in spring) was less responsive to the elevation gradient than oak phenology (budburst) and that it was not correlated with tree phenology within populations. The spring frost and disease safety margins varied considerably between oak populations as a function of elevation and within populations in relation to tree phenological phenotype. For both hazards, safety margins decreased significantly with increasing elevation. The safety margin for spring frost was mostly positive (i.e. escape), whereas the safety margin for powdery mildew was mostly negative (i.e. exposure), leading to infection. The abiotic and biotic hazards interact in opposite directions with phenology, especially at low elevations (< 500 m) where early flushing enabled trees to escape disease while late flushing provided a higher safety margin against late frost. 4. Synthesis. The observed patterns suggest that oak populations are better adapted to escape spring frost than pathogen exposure all along the elevation gradient. The combination of the biotic and abiotic selective pressures may have contributed to the maintenance of phenological diversity within low-elevation tree populations. As tree and pathogen respond differently to environmental cues, climate change is likely to affect the phenological (a)synchrony between host and parasite, both within and between populations.
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• Exotic pathogens cause severe damage in natural populations in the absence of coevolutionary dynamics with their hosts. However, some resistance to such pathogens may occur in naive populations. ...The objective of this study was to investigate the genetics of this so-called ‘exapted’ resistance to two pathogens of Asian origin (Erysiphe alphitoides and Phytophthora cinnamomi) in European oak.
• Host–pathogen compatibility was assessed by recording infection success and pathogen growth in a full-sib family of Quercus robur under controlled and natural conditions. Two high-resolution genetic maps anchored on the reference genome were used to study the genetic architecture of resistance and to identify positional candidate genes.
• Two genomic regions, each containing six strong and stable quantitative trait loci (QTLs) accounting for 12–19% of the phenotypic variation, were mainly associated with E. alphitoides infection. Candidate genes, especially genes encoding receptor-like-kinases and galactinol synthases, were identified in these regions. The three QTLs associated with P. cinnamomi infection did not colocate with QTLs found for E. alphitoides.
• These findings provide evidence that exapted resistance to E. alphitoides and P. cinnamomi is present in Q. robur and suggest that the underlying molecular mechanisms involve genes encoding proteins with extracellular signaling functions.
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Pathogens are embedded in multi‐trophic food webs, which often include co‐occurring cryptic species within the same pathogen complex. Nonetheless, we still lack an understanding of what dimensions of ...the ecological niche might allow these cryptic species to coexist. We explored the role of climate, host characteristics (tree autumn phenology) and attack by the fungal hyperparasite Ampelomyces (a group of fungi attacking plant pathogens) in defining the niches of three powdery mildew species (Erysiphe alphitoides, E. hypophylla and E. quercicola) within a cryptic pathogen complex on the pedunculate oak Quercus robur at the continental (Europe), national (Sweden and France) and landscape scales (a 5 km2 island in southwestern Finland). Previous studies have shown that climate separated the niches of three powdery mildew species (E. alphitoides, E. hypophylla and E. quercicola) in Europe and two species (E. alphitoides and E. quercicola) in France. In our study, we did not detect a significant relationship between temperature or precipitation and the distribution of E. alphitoides and E. hypophylla present in Sweden, while at the landscape scale, temperature, but not relative humidity, negatively affected disease incidence of E. alphitoides in an exceptionally warm year. Tree variation in autumn phenology did not influence disease incidence of powdery mildew species, and hyperparasite presence did not differ among powdery mildew species at the continental, national and landscape scale. Climate did not affect the distribution of the hyperparasite at the continental scale and at the national scale in Sweden. However, climate affected the hyperparasite distribution in France, with a negative relationship between non‐growing season temperature and presence of the hyperparasite. Overall, our findings, in combination with earlier evidence, suggest that climatic factors are more important than species interactions in defining the niches of cryptic species within a pathogen complex on oak.
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This review synthesizes the available knowledge on drought-disease interactions in forest trees with a focus on (1) evidence and patterns of drought-disease interactions, (2) current understanding of ...processes and mechanisms, and (3) three well documented cases studies. The first part is based on the analysis of a database of slightly more than one hundred studies, obtained by keyword searches combining drought, diseases or pathogens, and forest trees. A large majority of published studies referred to a positive association between drought and disease, i.e. disease favoured by drought or drought and disease acting synergistically on tree health status, with a predominance of canker/dieback diseases, caused by pathogens like Botryosphaeria, Sphaeropsis, Cytospora and Biscognauxia (Hypoxylon). The type of disease-related variables (incidence vs. severity) and the intensity and timing of water stress were shown to be significant factors affecting the drought- infection interaction. Interactions with other abiotic stresses and species-specific and genetic effects, related to host or pathogen, have also been reported. Direct effects of drought on pathogens are generally negative, although most fungal pathogens exhibit an important plasticity and can grow at water potentials well below the minimum for growth of their host plants. Studies on indirect effects of drought on pathogens through other community interactions are still relatively scarce. Positive drought-infection effects can mostly be explained by indirect effects of drought on host physiology. The predisposition and the multiple stress hypotheses are presented, as well as recent developments in the study of the molecular basis of abiotic and biotic stress, and their interactions. Sphaeropsis sapinea on pines, Biscognauxia mediterranea on oaks and root pathogens in declines associated with drought provide illustrative examples, treated as case studies, of pathogens of current significance associated with drought. The conclusion highlights some knowledge gaps, e.g. the role of latent parasites and the shift to a pathogenic stage, or t
Cette revue synthétise les connaissances actuelles sur les interactions entre sécheresse et maladies chez les arbres forestiers, avec trois grandes parties : (1) description des types d’interaction ; (2) connaissances acquises sur les mécanismes impliqués ; (3) trois études de cas bien étudiées. La première partie est basée sur l’analyse d’une base de données d’une centaine d’études, sélectionnées par recherche sur mots clés. La plupart de ces études se rapportent à des maladies favorisées par la sécheresse ou à un effet synergique entre sécheresse et maladie sur l’état sanitaire des arbres, avec une prédominance de maladies à chancres ou de rameaux, causées par des espèces des genres Botryosphaeria, Sphaeropsis, Cytospora et Biscognauxia (Hypoxylon). Un effet significatif de l’intensité et de la durée du stress est montré dans certains cas sur le développement des maladies. Les effets de la sécheresse peuvent être différents selon qu’il s’agit de variables liées à l’incidence ou à la sévérité de la maladie. Des interactions avec d’autres stress abiotiques ou en relation avec des effets génétiques ou spécifiques ont également été décrits. Les effets directs de la sécheresse sur les pathogènes sont généralement négatifs, mais les champignons montrent une grande plasticité et peuvent généralement se développer à des potentiels hydriques inférieurs à ceux inhibant la croissance de leurs hôtes. Les effets positifs entre sécheresse et infection s’expliquent principalement par des effets indirects sur la physiologie des hôtes. Les hypothèses sur la prédisposition et les stress multiples sont présentées, ainsi que les développements récents sur les bases moléculaires des stress et de leurs interactions. Les trois études de cas concernent Sphaeropsis sapinea sur pins, Biscognauxia mediterranea sur chênes et l’interaction entre pathogènes racinaires et sécheresse dans les dépérissements. Dans la conclusion, quelques perspectives de recherche à privilégier sont mentionnées, comme le rôle et la biologie des parasites latents, ou la génétique de quelques groupes fongiques. Sur un plan pratique, la nécessité de limiter la dissémination des parasites, particulièrement cruciale dans le cas des parasites latents, est soulignée
Forest disease management relies principally on a preventive approach in which epidemiological surveillance plays a crucial role. However, efficient and cost‐effective surveillance methods are not ...currently available for large spatial scales. Nevertheless, aerobiological networks have been set up for several decades in many countries to monitor pollen dispersal and provide real‐time assessments of allergenic risk. Here, we suggest that the same approach could be used for the surveillance of forest pathogens. Using molecular methods, we analysed samples from 12 sites of the French aerobiological network, at different dates. Both metabarcoding by high‐throughput sequencing (using two markers and two different bioinformatics approaches) and real‐time PCR targeting eight important forest pathogens were conducted. To validate the approach, temporal and spatial trends of spore detection were compared with field disease data. The metabarcoding approach demonstrated that many fungal plant pathogens could be found in aerobiological samples. Moreover, five of the eight targeted forest pathogens were detected by real‐time PCR, with temporal and spatial trends of spore capture consistent with field data. In particular, Hymenoscyphus fraxineus was detected at high frequency in aerobiological samples in the areas where ash dieback has been present for the longest period of time, and at lower frequency in areas with more recent invasion. Spore detection of seasonal pathogens showed a temporal pattern similar to that of disease reports. Overall, our study provides a proof of concept that permanent aerobiological networks combined with molecular methods may provide a useful tool for large‐scale surveillance of forest pathogens.
The network of Hirst‐type volumetric traps installed on city rooftops to monitor pollen concentrations in the air could provide useful information for the monitoring of several forest pathogens. (Photo credit: RNSA)
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• CONTEXT : Powdery mildew is one of the most common diseases of oaks in Europe. After alarming reports in the beginning of the twentieth century following the presumed introduction of the invasive ...fungus, the disease has become familiar to foresters. However, its impact may vary greatly according to intrinsic and extrinsic factors. • AIMS : We aimed at providing updated and synthesised information on the impact of powdery mildew on oak and on the effects of environment on disease. • METHODS : A comprehensive literature review was performed, including old reports of the early epidemics to more recent data. • RESULTS : Tree growth patterns are of critical importance to explain the severity of the disease and the differences between juvenile and mature trees. A critical element, especially for infection of mature trees, is the availability of spores during the production of the first leaf flush. High disease impact is often related to modified growth patterns, either by environmental factors (insects or frost) or silvicultural practices (e.g., coppicing). • CONCLUSION : Powdery mildew can have important impacts in natural oak regenerations and a significant role in decline of mature trees. Climate change might influence the disease severity mainly by altering the host pathogen phenological synchrony. Process-based models are required for reliable predictions.
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