A global genetic analysis of herbarium specimens reveals the invasion dynamics of an introduced plant pathogen
Introduction
Invasive plant pathogens can cause substantial damage to ecosystems throughout the world (Mack, 2000; Ellison et al., 2005; Loo, 2008; Stajich et al., 2009). Due to increases in global trade, and in particular, the importation of plants, many detrimental plant pathogens have been introduced relatively recently (Brasier, 1990; Gómez-Alpizar et al., 2007; Rellou, 2018). For example, during the 1900s, the causative agents of Dutch elm disease (Ophiostoma ulmi) (Brasier, 1990), chestnut blight (Cryphonectria parasitica) (Rellou, 2018), and white pine blister rust (Cronartium ribicola) (Maloy, 1997) were introduced to North America where they have caused major declines in their host trees and in the case of American chestnut, functional extinction (Anagnostakis, 1987). The introduction of non-native fungal pathogens can be particularly difficult to manage due to their small size and ability to arrive without detection on asymptomatic host plants (Migliorini et al., 2015; Burgess et al., 2016).
Fungi, as model organisms to study biological invasions, have often been overlooked due to their inconspicuous nature and the difficulty in identifying them to the species level. The widespread nature of fungi and their fast rate of evolution also makes them valuable study organisms for elucidating ecological and evolutionary processes involved in pathogen success in new environments (Gladieux et al., 2014; Burgess et al., 2016). Herbarium specimens of fungal plant pathogens can provide unique insights into the evolutionary history of pathogen-host interactions (Ristaino et al., 2001; Yoshida et al., 2014; Miller et al., 2016; Ristaino, 2020). For example, the evaluation of herbarium specimens of the Oomycete pathogen Phytophthora infestans lead to the discovery that the genotype present now is distinct from the genotype that caused the Irish potato famine (Martin et al., 2013, Martin et al., 2013; Yoshida et al., 2013). Among fungal pathogens, powdery mildew is an ideal model system to study invasions due to its cosmopolitan distribution (Braun and Cook, 2012), high rate of evolution (Glawe, 2008), and rapid adaptation to plant hosts (Brown and Rant, 2013).
Powdery mildews are known to infect >10,000 flowering plant species worldwide (Amano, 1986), with an estimated 873 described species (Braun and Cook, 2012). Symptoms of powdery mildew first appear on its host plants as white powdery spots that can spread over large areas of the plant, decreasing growth, and reducing flower and fruit quantity (Daughtrey and Benson, 2005). Powdery mildew conidia, an asexual spore stage, can aerially disperse and greatly facilitate its spread, resulting in severe epidemics over a relatively short time period (Ale-Agha et al., 2000, 2004; Brown et al., 1991; Kiss, 2005).
In recent years, tree mortality in western North America has increased at a higher rate than what is thought to be expected under historical conditions (van Mantgem et al., 2009; Cohen et al., 2016). In some tree species, fungal plant pathogens have been shown to contribute to mortality (Farr et al., 2005; Stone et al., 2008). In 2018, severe powdery mildew infections were observed on bigleaf maple, Acer macrophyllum, in and around the University of Washington campus in Seattle, Washington, USA (Fig. 1A); A. macrophyllum has been reported to be in decline in western North America (Washington Department of Natural Resources, 2016; Betzen, 2018, Oregon State University Extension, 2019). We were motivated by this observation, and the possibility that this fungal pathogen might be detrimentally affecting A. macrophyllum. We thus sought to (1) identify the powdery mildew species infecting A. macrophyllum from samples collected from the University of Washington campus, (2) evaluate the susceptibility of A. macrophyllum and other Acer species to this powdery mildew species, and (3) conduct a global genetic analysis on old (i.e., >150 years) herbarium and newly collected specimens of powdery mildew on Acer species. The genetic analysis allowed us to identify multiple powdery mildew haplotypes, provided evidence on the likely native range of this powdery mildew species, and provided a means to estimate the timing of its introduction to western Washington.
Section snippets
Species identification
We used morphological and genetic analyses to identify the powdery mildew species infecting A. macrophyllum from the University of Washington campus. In late summer 2019, 519 A. macrophyllum trees were inspected for signs and symptoms of powdery mildew (Fig. 1). Powdery mildew was also noted on the congeneric species A. circinatum, A. campestre, and A. platanoides on campus. For identification, we collected 30 powdery mildew specimens from 30 different A. macrophyllum trees throughout campus
Species identification
Morphological and genetic analyses conducted on powdery mildew on A. macrophyllum, A. campestre, and A. circinatum at the University of Washington revealed the species in question to be S. bicornis. Signs of powdery mildew occured as a white ‘powdery substance’ on both the abaxial and adaxial leaf surface. Powdery mildew was also found to form on young Acer spp. stems. The morphology (Fig. 2) matched the description of S. bicornis from Braun and Cook (2012). Morphological species identification
Discussion
Powdery mildew caused by S. bicornis was collected at several locations in western North America (Fig. 4), and the epidemic recently observed from the University of Washington campus is likely widespread throughout this region. Acer macrophyllum trees are also particularly susceptible to S. bicornis (Fig. 3). Although the current study did not evaluate if there was an association between A. macrophyllum decline and powdery mildew severity, the percentage of leaf area on A. macrophyllum infected
Statement of authorship
The main research was done by MB in partial fulfillment of the requirements for the Ph.D degree from the University of Washington. Study design and manuscript writing was done by MB and PT. ME conducted some of the statistics and helped with the study design. UB, JK, SYL and GG collected data. All authors contributed substantially to revisions.
Data accessibility statement
Data from this study will be deposited in the appropriate public repository (GenBank).
Author contributions
The main research was done by MB. Study design and manuscript writing was done by MB and PT. ME conducted some of the statistics and helped with the study design. UB, JK, SYL and GG collected data. All authors contributed substantially to revisions.
Acknowledgments
We would like to thank Dr. Jay Pscheidt, Dr. Jerry Cooper and Dr. Gregory Gilbert for helping with fresh collections, the curators of WTU, HMJA, NY, DAOM, PDD, and TNS for helping supply the various herbarium collections and Ellie Reese at the SEFS Genomics Lab for her help supporting the genomics aspect of this research. In addition, we would like to thank the following organizations for funding this research: The Daniel E. Stuntz Memorial Foundation (Grant nos. A132796 and A143262), the Puget
References (96)
- et al.
A new powdery mildew disease on Aesculus spp. introduced in Europe
Cryptog. Mycolog.
(2000) - et al.
Forest disturbance across the conterminous United States from 1985–2012: the emerging dominance of forest decline
For. Ecol. Manag.
(2016) - et al.
Testing significance of incongruence
Cladistics
(1994) The conditioned reconstructed process
J. Theor. Biol.
(2008)- et al.
Nucleotide sequence diversity of rDNA internal transcribed spacers extracted from conidia and cleistothecia of several powdery mildew fungi
Mycoscience
(1996) - et al.
Molecular phylogeny and evolution of the maple powdery mildew (Sawadaea, Erysiphaceae) inferred from nuclear rDNA sequences
Mycol. Res.
(2005) Powdery mildew as invasive plant pathogens: new epidemics caused by two North American species in Europe
Mycol. Res.
(2005)The measurement of diversity in different types of biological collections
J. Theor. Biol.
(1966)- et al.
Geographic and temporal distributions of four genotypes found in Erysiphe gracilis var. gracilis, a powdery mildew of evergreen oaks (Erysiphales)
Mycoscience
(2018) - et al.
Sawadaea nankinensis comb. nov.: a powdery mildew fungus of Acer buergerianum
Mycoscience
(2008)