6 G.Newcombe and F.M.Dugan hypothesis"of a comparatively pest-and disease-free agriculture in Neolithic Europe (Dark and Gent 2001),or the introduction of Ascochyta blight of chickpea. resulting in summer cropping systems in the Levant (Abbo et al.2003).Literature on movement of plant pathogenic fungi from the Neolithic through classical antiquity has been summarized recently (Dugan 2008).Archaeobotanical or text- based analyses are particularly numerous for tracing the complex introduction of crops into medieval Europe (Behre 1992:Campbell 1988:Harvey 1984.1992: Kroll 2005:P ton et al.2004:Taavits etal.1998) roduced re not all ecologically.Intr nts car be ome (Richardson et al.2000b).Of course.only a small fraction of introduced plants become naturalized.For example,in Florida,of approximately 25,000 non-native or alien plant species,only 900 have become naturalized(Pimentel et al.2005).A further winnowing occurs as only a small fraction of naturalized species become invasive,with invaders defined as species that have successfully spread away from sites of introduction (Richardson et al.2000b).These successive winno owing e what is called the“t nd fitter 1996).a rule of fact s be come invaders(Kola ed in understand ing this phenomenon. In this chapter,we shall see how well concepts and definitions borrowed from invasion biology might apply to fungi,especially fungal pathogens of plants.Is there a"tens rule"for fungal pathogens,or are all alien fungal pathogens equally likely to attack evolutionarily naive plants or a host from which they had been separated?In describing the plants and pathogens that take part in"first encounters' aive and n el, re followin f Parke ng the t(2004).f does apply. we op.hypoth gal pat gen and vill be ecom invaders?The threat of fungi as novel path gens is a tradit onal topi for plant pathologists and mycologists(Rossman 2001).But,apart from the notori ous example of chestnut blight,do novel pathogens generally act as"transformers' that"change the character.condition.form or nature of ecosystems over a substan tial area"(Pysek et al.2004)?What are the roles of fungi as potential facilitators of plant invasions in the Homogocene?Recognizing and predicting invasions are the central obiectives of invasion biology (kolar and lodge 2001)But both obiective to h dicated on kn of the e p ng ative,geographic ranges organisms question.a problematic area for mycology. 1.2 Native Ranges of Fungi In the eighteenth century.the French naturalist.Georges Buffon.had observed that different continents had different assemblages of macrobes(i.e.plants and animals)(Cox and Moore 2005).In the nine teenth century, Humboldt had
hypothesis” of a comparatively pest- and disease-free agriculture in Neolithic Europe (Dark and Gent 2001), or the introduction of Ascochyta blight of chickpea, resulting in summer cropping systems in the Levant (Abbo et al. 2003). Literature on movement of plant pathogenic fungi from the Neolithic through classical antiquity has been summarized recently (Dugan 2008). Archaeobotanical or textbased analyses are particularly numerous for tracing the complex introduction of crops into medieval Europe (Behre 1992; Campbell 1988; Harvey 1984, 1992; Kroll 2005 ; Preston et al. 2004; Taavitsainen et al. 1998). Introduced plants are not all equal ecologically. Introduced or alien plants can become naturalized if they survive and regularly reproduce outside of cultivation (Richardson et al. 2000b). Of course, only a small fraction of introduced plants become naturalized. For example, in Florida, of approximately 25,000 non-native or alien plant species, only 900 have become naturalized (Pimentel et al. 2005). A further winnowing occurs as only a small fraction of naturalized species become invasive, with invaders defined as species that have successfully spread away from sites of introduction (Richardson et al. 2000b). These successive winnowings characterize what is called the “tens rule” (Williamson and Fitter 1996), a rule of thumb that reflects the fact that relatively few aliens become invaders (Kolar and Lodge 2001). Plant and community ecologists are keenly interested in understanding this phenomenon. In this chapter, we shall see how well concepts and definitions borrowed from invasion biology might apply to fungi, especially fungal pathogens of plants. Is there a “tens rule” for fungal pathogens, or are all alien fungal pathogens equally likely to attack evolutionarily naive plants or a host from which they had been separated? In describing the plants and pathogens that take part in “first encounters” as evolutionarily “naive” and “novel,” respectively, we are following the example of Parker and Gilbert (2004). If the “tens rule” does apply, do we have, or can we develop, hypotheses to predict which fungal pathogens will naturalize and which will become invaders? The threat of fungi as novel pathogens is a traditional topic for plant pathologists and mycologists (Rossman 2001). But, apart from the notorious example of chestnut blight, do novel pathogens generally act as “transformers” that “change the character, condition, form or nature of ecosystems over a substantial area” (Pysˇek et al. 2004)? What are the roles of fungi as potential facilitators of plant invasions in the Homogocene? Recognizing and predicting invasions are the central objectives of invasion biology (Kolar and Lodge 2001). But both objectives seem to be predicated on knowing the native, geographic ranges of the organisms in question, a problematic area for mycology. 1.2 Native Ranges of Fungi In the eighteenth century, the French naturalist, Georges Buffon, had observed that different continents had different assemblages of macrobes (i.e., plants and animals) (Cox and Moore 2005). In the nineteenth century, Humboldt had 6 G. Newcombe and F.M. Dugan
1 Fungal Pathogens of Plants in the Homogocene 7 discovered the predictability of species-area relationships in that larger areas held more species(Rosenzweig 1995).but again this was known to apply only to macrobes.Spatial scaling and diversification of fungi were little st died until recently wher species. area relationshins of fungi were demonstrated to be ilar to the crobes (Gre al 2004)This s not a trivial find cause even toda some microb maintai ia eukaryotes have global ranges (Fenche of Beijer inck,that species of bacteria were cosmopolitan,or of Baas-Becking,that "everything is everywhere"(Fenchel and Finlay 2004),have also been challenged recently by application of the sequence-based phylogenetic species concept of fungi (Taylor et al.2006). Mvcologists are now learning that most fungi do conform to Buffon's Law and to spatial scaling rules for macrobes.However.it does not follow that it will be easy to dete ine the native s of those fungi i that do onfor,for reasons that will he ussed.And the are undor ly fungi tha not conform Fo example,some saprophytic hyphomycetes,such as common Cladosporium species are associated with a very broad range of substrata.Such species do indeed seem to have cosmopolitan distributions as evidenced by their incorporation over long time periods into Arctic ice,alpine glaciers,and permafrost throughout the Northern Hemisphere (summarized in Dugan 2008). Macrobiologists may be surprised to learn that the native ranges of fungi are largely unknown.Yet,how could it be otherwise?Today,83%and 90%of vascular plants and vertebrates. are kno wn to obiologists.wher as.at best 7%to described (Cox and Mo 205::Ro sman2009). ow when a new species of fungus described,its current,geographic pattem of occurrence might suggest an original native range.Unfortunately,that pattern could also be the product of homogeniza tion since 1500.In contrast,the native ranges of macrobes are largely known.no only because the species are largely known,but because their ranges were docu mented early in the Homogocene before homogenization hadhad large effects Disputes do exist.but they appear minor in scope to a mycologist.For instance Gayther Plumer proposed that the most mysterious of native trees of North or the Franklin e as a ally om As ades before the Bartrams discover a sma grove in 1 (Rowland 2006).M st botanists,however,disagree with Plummer(USD .n.d. The native ranges of annual brome-grasses have more recently presented more serious challenges to botanists (Smith 1986),and other examples exist of course but botanists have a set of criteria for dealing with problematic taxa:paleobotanical evidence of native status,records of their presence in their current range by early botanists,and current presence in natural habitats(Pysek et al.2004). Mycologists face the unknown species problem,and the problem of the near total lack of knowledge of pre-Ho distributions.My board the as iti "in its in ive even argu that the fungi lack"any significant fossil record"(Cain 1972).Even today fungi
discovered the predictability of speciesarea relationships in that larger areas held more species (Rosenzweig 1995), but again this was known to apply only to macrobes. Spatial scaling and diversification of fungi were little studied until recently when species-area relationships of fungi were demonstrated to be similar to those of macrobes (Green et al. 2004). This was not a trivial finding because even today some microbiologists maintain the view that microbial eukaryotes have global ranges (Fenchel and Finlay 2004). The views of Beijerinck, that species of bacteria were cosmopolitan, or of Baas-Becking, that “everything is everywhere” (Fenchel and Finlay 2004), have also been challenged recently by application of the sequence-based phylogenetic species concept of fungi (Taylor et al. 2006). Mycologists are now learning that most fungi do conform to Buffon’s Law and to spatial scaling rules for macrobes. However, it does not follow that it will be easy to determine the native ranges of those fungi that do conform, for reasons that will be discussed. And then there are undoubtedly fungi that do not conform. For example, some saprophytic hyphomycetes, such as common Cladosporium species, are associated with a very broad range of substrata. Such species do indeed seem to have cosmopolitan distributions as evidenced by their incorporation over long time periods into Arctic ice, alpine glaciers, and permafrost throughout the Northern Hemisphere (summarized in Dugan 2008). Macrobiologists may be surprised to learn that the native ranges of fungi are largely unknown. Yet, how could it be otherwise? Today, 83% and 90% of vascular plants and vertebrates, respectively, are known to macrobiologists, whereas, at best, from 7% to perhaps 20% of the fungi are presently described (Cox and Moore 2005; Hawksworth 2001; Rossman 2009). Now when a new species of fungus is described, its current, geographic pattern of occurrence might suggest an original native range. Unfortunately, that pattern could also be the product of homogenization since 1500. In contrast, the native ranges of macrobes are largely known, not only because the species are largely known, but because their ranges were documented early in the Homogocene before homogenization had had large effects. Disputes do exist, but they appear minor in scope to a mycologist. For instance, Gayther Plummer proposed that the most mysterious of native trees of North America, Franklinia alatamaha, or the Franklin tree, was actually introduced from Asia a few decades before the Bartrams discovered a small grove in 1765 (Rowland 2006). Most botanists, however, disagree with Plummer (USDA n.d.). The native ranges of annual brome-grasses have more recently presented more serious challenges to botanists (Smith 1986), and other examples exist of course, but botanists have a set of criteria for dealing with problematic taxa: paleobotanical evidence of native status, records of their presence in their current range by early botanists, and current presence in natural habitats (Pysˇek et al. 2004). Mycologists face the unknown species problem, and the problem of the near total lack of knowledge of pre-Homogocene distributions. Mycologists were not on board the ships of the explorers and palaeomycology can hardly arbitrate disputes, as it is “in its infancy” (Stubblefield and Taylor 1988); others have even argued that the fungi lack “any significant fossil record” (Cain 1972). Even today fungi 1 Fungal Pathogens of Plants in the Homogocene 7
G.Newcombe and F.M.Dugan are more intensively studied in managed habitats,as pathogens of agicultural crops than in natural habitats where fungi provide ecosystem services on a massive scale.Last.but not least for a book on the molecular identification of fungi "molecular diagnostic tools are only as good as the systematic underpinnings upon which these tools are based"(Rossman 2009).and upheavals in fungal systematics are common today. What can we make of a new species such as Cladosporium subtilissimum that ntly mat rial in Slovenia and the rthwestern United ld h ve been c smopolitan prior the Homo gocene,bu can we rule e out the role e of homogenization in produ cing its curren distribution?Climate change can of course also cause range shi fts (Parmesan 2006) but homogenization is more likely to be the source of the error that we are concemed with here:calling an invaded or naturalized range a native range or part of a native range.How can this error be avoided?And to what extent have previously isolated mycobiotas already been cryptically homogenized? 1.3 Pathogen Release A roundabout but fruitful way to approach the latter question is through the pathogen release hypothesis,according to which alien plants are less regulated by pathogens than native plants(Keane and Crawley 2002).But first,if all fungi were everywhere,as Beijerinck.Baas-Becking.Fenchel,and Finlay have asserted is the case for other microbes (Fenchel and finlay 2004).there would be no pathoger release for plants fro plant pathoge uld have the et of ns in h their native and invaded ranges. The e environment (i.e.the wo Is pathogen release a real phenomenon?Using the USDA Fungus-Host Dis tributions database of the Systematic Mycology and Microbiology Laboratory Mitchell and Power showed that for"473 plant species naturalized to the United States from Europe there were 84%fewer rust.smut.and powdery mildew species infecting plants in their naturalized ranges than in their native ranges (Mitchell and Power 2003).The SMML database is by far the most extensive of its kind with r94.000 cies"(Ro n2009.0 uld als introduced for classical bio cite s ngi tha onrol of weedy plants th directly confirm the pathogen release hypothesis for phytopathogenic fungi(Cullen et al.1973),but Mitchell and Power's paper was the first study to show the generality of this phenomenon.Another way of phrasing this is that if phytopatho genic fungi were everywhere,then pathogen introductions would not be a threat.To prove that this belief represents a completely false sense of security,one has to look no further than the chestnut blight fungus that transformed an ecosystem (Cox 1999:Liebhold et al.1995:Rizzo and Garbelotto 2003)
are more intensively studied in managed habitats, as pathogens of agicultural crops, than in natural habitats where fungi provide ecosystem services on a massive scale. Last, but not least for a book on the molecular identification of fungi, “molecular diagnostic tools are only as good as the systematic underpinnings upon which these tools are based” (Rossman 2009), and upheavals in fungal systematics are common today. What can we make of a new species such as Cladosporium subtilissimum that was described recently from material in Slovenia and the northwestern United States (Schubert et al. 2007)? It could have been cosmopolitan prior to the Homogocene, but can we rule out the role of homogenization in producing its current distribution? Climate change can of course also cause range shifts (Parmesan 2006), but homogenization is more likely to be the source of the error that we are concerned with here: calling an invaded or naturalized range a native range or part of a native range. How can this error be avoided? And to what extent have previously isolated mycobiotas already been cryptically homogenized? 1.3 Pathogen Release A roundabout but fruitful way to approach the latter question is through the pathogen release hypothesis, according to which alien plants are less regulated by pathogens than native plants (Keane and Crawley 2002). But first, if all fungi were everywhere, as Beijerinck, Baas-Becking, Fenchel, and Finlay have asserted is the case for other microbes (Fenchel and Finlay 2004), there would be no pathogen release for plants from plant pathogenic fungi. Plants would have the same set of fungal pathogens in both their native and invaded ranges. The environment (i.e., the host plant) would select. Is pathogen release a real phenomenon? Using the USDA Fungus–Host Distributions database of the Systematic Mycology and Microbiology Laboratory, Mitchell and Power showed that for “473 plant species naturalized to the United States from Europe” there were 84% fewer rust, smut, and powdery mildew species infecting plants in their naturalized ranges than in their native ranges (Mitchell and Power 2003). The SMML database is by far the most extensive of its kind with “reports of fungi on plant hosts throughout the world that includes over 94,000 fungal species” (Rossman 2009). One could also cite specific examples of fungi that have been deliberately introduced for classical biocontrol of weedy plants that more directly confirm the pathogen release hypothesis for phytopathogenic fungi (Cullen et al. 1973), but Mitchell and Power’s paper was the first study to show the generality of this phenomenon. Another way of phrasing this is that if phytopathogenic fungi were everywhere, then pathogen introductions would not be a threat. To prove that this belief represents a completely false sense of security, one has to look no further than the chestnut blight fungus that transformed an ecosystem (Cox 1999; Liebhold et al. 1995; Rizzo and Garbelotto 2003). 8 G. Newcombe and F.M. Dugan
1 Fungal Pathogens of Plants in the Homogocene 9 Although plants may at first leave their fungal enemies behind when introduced outside their native ranges,in keeping with pathogen release,one can imagine that inadvertent introductions of those same enemies would slowly counter the pathogen the best that we hav of the of introduction of fungi Gie hon ogenization)around the world.For ex when rus occ an invasive plant of Eurasian origin. in Nort 1989 (Mortensen et al.1989)or for the first time in the United States in 1992 (Dugan and Carris 1992:Palm et al.1992).these pathogen reunions ended more than 80 years of release from rust dating from 1907,the year that C.diffusa itself was introduced into North America(Maddox 1982).This rust,Puccinia jaceae var. diffusae of Eurasian origins (Savile 1970b),is easily distinguished from the only rust fungus,Puccinia irrequiseta,that occurs on the only North American species (Savile 1970a) the of ta vulgare,plants were introduced and colonists in North America for culinary and medicina purposes (Haughton 1978).Rust,common in its native range,was absent from this introduced range.Some 400 vears after the introduction of tansy.Puccinia tanaceti was finally reunited with its host for the first time in the North American range of tansy (Newcombe 2003b). cochliobolus carbonum provides an example of serial pathogen reunions in that it must have followed the introduction of its host,Zeamays,around the world to the oint where the fun s itself isr opolitan.So e of the eunions wer nly re ched G e Britain in 1972 (oe and Baker 207.which is presumably mays wa s introduce there,as the plant was introduced into Europe in 1493 by Columbus(Rebourg et al 2003).C.carbonum had reached Australia 6 years before it arrived in the U.K.(Farr et al.n.d.).Soybean rust,caused by Phakopsora pachyrhizi and P.meibomiae,also took considerable time to be reunited with its agriculturally important host around the world(Rossman 2009). It is not clear how lengthy periods of pathogen release might potentially be as yet to oc ur.Morus alba,the white mulberry tely in oduced from Chi esta in the U.S ore t an400 years ago (Duncan and 988). The tree uralized.and even become ally invasive in the U.S but its rus fungi (i.e.species of Cerotelium,Peridiopsora,Phakopsora,and Kuehneola) have remained in the native range of their host (Farr et al.n.d.).Powdery mildews,on the other hand,have reunited with introduced populations of white mulberry in western Europe and Central America,although not yet in North America (farr et al n d) Similarly.St-John's-wort.Hypericum perforatumn.was introduced into North America by Rosicrucian pilgrims in 1696(Haughton 1978).and it has since bec edy and invasive acros the entire ntinent (USDA n.d.).But t more t a 400 years later Melampsora hypericorum has yet to be reunited with H.perforatm
Although plants may at first leave their fungal enemies behind when introduced outside their native ranges, in keeping with pathogen release, one can imagine that inadvertent introductions of those same enemies would slowly counter the pathogen release effect over time. These “pathogen reunions” do occur, and they are perhaps the best measure that we have of the rate of introduction of fungi (i.e., fungal homogenization) around the world. For example, when rust occurred on Centaurea diffusa, an invasive plant of Eurasian origin, for the first time in North America in 1989 (Mortensen et al. 1989) or for the first time in the United States in 1992 (Dugan and Carris 1992; Palm et al. 1992), these pathogen reunions ended more than 80 years of release from rust dating from 1907, the year that C. diffusa itself was introduced into North America (Maddox 1982). This rust, Puccinia jaceae var. diffusae of Eurasian origins (Savile 1970b), is easily distinguished from the only rust fungus, Puccinia irrequiseta, that occurs on the only North American species of Centaurea, C. americana (Savile 1970a). In the case of tansy, or Tanacetum vulgare, plants were introduced and cultivated by English colonists in North America for culinary and medicinal purposes (Haughton 1978). Rust, common in its native range, was absent from this introduced range. Some 400 years after the introduction of tansy, Puccinia tanaceti was finally reunited with its host for the first time in the North American range of tansy (Newcombe 2003b). Cochliobolus carbonum provides an example of serial pathogen reunions in that it must have followed the introduction of its host, Zea mays, around the world to the point where the fungus itself is now cosmopolitan. Some of the reunions were relatively recent. For instance, C. carbonum only reached Great Britain in 1972 (Jones and Baker 2007), which is presumably long after Z. mays was introduced there, as the plant was introduced into Europe in 1493 by Columbus (Rebourg et al. 2003). C. carbonum had reached Australia 6 years before it arrived in the U.K. (Farr et al. n.d.). Soybean rust, caused by Phakopsora pachyrhizi and P. meibomiae, also took considerable time to be reunited with its agriculturally important host around the world (Rossman 2009). It is not clear how lengthy periods of pathogen release might potentially be as many pathogen reunions have yet to occur. Morus alba, the white mulberry, was deliberately introduced from China in an attempt to establish a silk industry in the U.S. more than 400 years ago (Duncan and Duncan 1988). The tree has naturalized, and even become locally invasive in the U.S., but its rust fungi (i.e., species of Cerotelium, Peridiopsora, Phakopsora, and Kuehneola) have remained in the native range of their host (Farr et al. n.d.). Powdery mildews, on the other hand, have reunited with introduced populations of white mulberry in western Europe and Central America, although not yet in North America (Farr et al. n.d.). Similarly, St.-John’s-wort, Hypericum perforatum, was introduced into North America by Rosicrucian pilgrims in 1696 (Haughton 1978), and it has since become weedy and invasive across the entire continent (USDA n.d.). But more than 400 years later Melampsora hypericorum has yet to be reunited with H. perforatum, 1 Fungal Pathogens of Plants in the Homogocene 9
0 G.Newcombe and F.M.Dugan as this rust fungus has only been recorded in St.-John's-wort's native range in Europe (Farr et al.n.d.).On the other hand,it was close to 2,000 years ago that the Romans introduced H.perforatum to the U.K.(Haughton 1978),where reunion with M.hypericorum eventually did occur sometime before 1913(Grove 1913 The"honeymoon hypothesis"of Dark and Gent (2001).mentioned above,posited that some reunions of grave consequence to European agriculture were postponed for centuries,but these reunions eventually took place as long distance movement of se eds b e in the late Iron Age and Ro n time Plant America were also int where some remain in a at least partial pat ogen release.Helia inappropriately named"Jerusalem artichoke," was brought to Europe from North America in the early 1600s (Hedrick 1950).Although Puccinia helianthi was ther reunited with its host in westem Europe nearly 400 years later,other rust fungi remain restricted to the native range (e.g.,Coleosporium helianthi)(Farr et al.n.d.) A similar patter of pathogen release is known for Helianthus annuus,the cultivated sundower.also a native of north america that became widely cultivated rou nd the world. note that pat oge unio 010 lly simila r fungi that are n natur d range of an intro duced plant.For instance.Popuus nigra is Eurasian,but it has been widely plante in North America as cv.'Italica,'the columnar Lombardy poplar.Venturia infec- tion of P.nigra in North America could potentially represent pathogen reunion by a Eurasian Venturia,or host switching by a Venturia that is native to North American Populus.As it turns out,Venturia populina,a Eurasian fungus,was determined to be causing leaf and shoot blight of P.nigra (Newcombe 2003a).so this was a case ion.Venturia in a North An cies of P to but it has gra.It was th specicitieso r the e two s heir respective expressed in a common environment,that actually led t discerment of subtle,but consistent,differences in morphology and in ITS sequences.In retrospect,Venturia blight of P.nigra in North America could easily have been misinterpreted as host switching.or simply as the product of a fungus with a broader host range and larger geographic range than either of these species of Venturia actually has In general,most of the introductions of alien or so-called "invasive"plant appe to be oge rece ,0f1970-2004 among the i 0 the U.K.85%wer reunions on plar roduced.Or were first reports o pathogens on native,wild plants of the U.K.and not all of these e were necessarily reports of alien pathogens(Jones and Baker 2007):some could have been native pathogens that had been overlooked because their hosts lacked economic impor tance.Pathogen reunions may be the best measure that we have of the rate of introduction of fungi.but what do they tell us about the native ranges of fungi.the primary question of this section?
as this rust fungus has only been recorded in St.-John’s-wort’s native range in Europe (Farr et al. n.d.). On the other hand, it was close to 2,000 years ago that the Romans introduced H. perforatum to the U.K. (Haughton 1978), where reunion with M. hypericorum eventually did occur sometime before 1913 (Grove 1913 ). The “honeymoon hypothesis” of Dark and Gent (2001), mentioned above, posited that some reunions of grave consequence to European agriculture were postponed for centuries, but these reunions eventually took place as long distance movement of seeds became more routine in the late Iron Age and Roman times. Plants native to North America were also introduced to Europe where some remain in a state of at least partial pathogen release. Helianthus tuberosus, the inappropriately named “Jerusalem artichoke,” was brought to Europe from North America in the early 1600s (Hedrick 1950). Although Puccinia helianthi was then reunited with its host in western Europe nearly 400 years later, other rust fungi remain restricted to the native range (e.g., Coleosporium helianthi) (Farr et al. n.d.). A similar pattern of pathogen release is known for Helianthus annuus, the cultivated sunflower, also a native of North America that became widely cultivated around the world. It is important to note that pathogen reunions may be confused with infection by morphologically similar fungi that are native to the naturalized range of an introduced plant. For instance, Populus nigra is Eurasian, but it has been widely planted in North America as cv. ‘Italica,’ the columnar Lombardy poplar. Venturia infection of P. nigra in North America could potentially represent pathogen reunion by a Eurasian Venturia, or host switching by a Venturia that is native to North American Populus. As it turns out, Venturia populina, a Eurasian fungus, was determined to be causing leaf and shoot blight of P. nigra (Newcombe 2003a), so this was a case of pathogen reunion. Venturia inopina, occurring on a North American species of Populus, P. trichocarpa, is morphologically similar to V. populina, but it has not switched to P. nigra. It was the specificities of these two species of Venturia for their respective hosts, expressed in a common environment, that actually led to discernment of subtle, but consistent, differences in morphology and in ITS sequences. In retrospect, Venturia blight of P. nigra in North America could easily have been misinterpreted as host switching, or simply as the product of a fungus with a broader host range and larger geographic range than either of these species of Venturia actually has. In general, most of the introductions of alien or so-called “invasive” plant pathogens appear to be pathogen reunions. In a recent study of 1970–2004, among the introductions of non-native plant pathogens into the U.K., 85% were reunions on plants that were themselves introduced. Only 15% were first reports of pathogens on native, wild plants of the U.K., and not all of these were necessarily reports of alien pathogens (Jones and Baker 2007); some could have been native pathogens that had been overlooked because their hosts lacked economic importance. Pathogen reunions may be the best measure that we have of the rate of introduction of fungi, but what do they tell us about the native ranges of fungi, the primary question of this section? 10 G. Newcombe and F.M. Dugan