1 Fungal Pathogens of Plants in the Homogocene (Newcombe et al.2001).Just as M.medusae had allowed the Mmdl gene to be detected,new pathotypes of M.x columbiana were the means by which three new genes for resistance,MxMandMre were discovered.A new gene-for-gene pathosystem had appeared with exapted resistance genes and matching e avirule in Populus otaling in to be a stin tor th svotion of inasivenesnts( 2000).and it may be so also for pathogenic fungi (Brasier 2000) Hybridization of both host and parasite that merged two separate pathosystems appears to account for the emergence of this gene-for-gene system.Reciprocal hybridization of the kind discussed here could have had an evolutionary history of repeated occurrence,as there is evidence of ancient hybridization between Populus sections Tacamahaca and Aigeiros,at least since the Miocene(Eckenwalder 1984) The nes for resistance e and virule ce that ld have be dically in the e pa: recurri Populus tricho currently hybridize naturally in part s of westem North America (Eckenwalder 1996).The ancient introgression of Pinus banksiana into Pinus contorta in western North America (Critchfield 1985).has also left a signal in terms of resistance genes,that is still evident today (Wu et al.1996). But the evolutionary basis for genes for resistance to a Eurasian poplar rust fungus,Melampsora larici-populina,that are possessed by the North American species of Populus,P.deltoides (Cervera et al.1996:Villar et al.1996).is harder to agine.Weknow was only introduced to north america ir t early 1990s N agner 1993).so the selective ford agen could not have been this fungus.The same question is raised by the above mentioned "Gasaway"gene that confers resistance to a fungus found only in North America even though the gene itself is from a European plant.Corvlus avellana. We have already mentioned that species and hybrids of Eucalyptus,introduced to South America,encountered there for the first time a novel rust fungus,Puccinia psidii,which shifted to Eucalyptus from native Myrtaceae (Grgurinovic et al. 2006).Many hybrids of E.gr ndis of widespread use in Brazilian plantation have pro he very su tible to P.psi the atter also ha wide hos range in having been reported on gener (Rayachhetry etal.2001).Nevertheless,there are individuals c are resistant;one harbors a major gene for resistance to P.psidii,the Pprl gene (Junghans et al.2003).How can we explain in terms of selection an Australian gene for resistance to a South American fungus,without going back in time to the Late Paleocene/Early Eocene thermal maximum,55 mya,when there is evidence for floristic exchange between South America and Australia that included Myrtaceae (Morley 2003)?Moreover,some evolutionarily naive species of Myrtaceae appe resistant.as species,to P.psidii(Rayachhetry et al.2001).Another exan found in the ative. Am ncan range estern gal rust,caused by Endo nartium harknessi ots pine (P)on of the most widelygrown of Eurasian pines in North America,possesses a recessive major gene for resistance
(Newcombe et al. 2001). Just as M. medusae had allowed the Mmd1 gene to be detected, new pathotypes of M. columbiana were the means by which three new genes for resistance, Mxc1, Mxc2, and Mxc3 were discovered. A new gene-for-gene pathosystem had appeared with exapted resistance genes and matching exapted avirulence genes. Resistance genes appear to be quite common in Populus, perhaps totaling in the hundreds (Tuskan et al. 2006). Hybridization has been hypothesized to be a stimulus for the evolution of invasiveness in plants (Ellstrand and Schierenbeck 2000), and it may be so also for pathogenic fungi (Brasier 2000). Hybridization of both host and parasite that merged two separate pathosystems appears to account for the emergence of this gene-for-gene system. Reciprocal hybridization of the kind discussed here could have had an evolutionary history of repeated occurrence, as there is evidence of ancient hybridization between Populus sections Tacamahaca and Aigeiros, at least since the Miocene (Eckenwalder 1984). The genes for resistance and avirulence that now appear exapted could have been selected episodically in the past in recurring, hybrid zones. Populus trichocarpa and P. deltoides do currently hybridize naturally in parts of western North America (Eckenwalder 1996). The ancient introgression of Pinus banksiana into Pinus contorta in western North America (Critchfield 1985), has also left a signal in terms of resistance genes, that is still evident today (Wu et al. 1996). But the evolutionary basis for genes for resistance to a Eurasian poplar rust fungus, Melampsora larici-populina, that are possessed by the North American species of Populus, P. deltoides (Cervera et al. 1996; Villar et al. 1996), is harder to imagine. We know that M. larici-populina was only introduced to North America in the early 1990s (Newcombe and Chastagner 1993), so the selective force or agent could not have been this fungus. The same question is raised by the abovementioned “Gasaway” gene that confers resistance to a fungus found only in North America even though the gene itself is from a European plant, Corylus avellana. We have already mentioned that species and hybrids of Eucalyptus, introduced to South America, encountered there for the first time a novel rust fungus, Puccinia psidii, which shifted to Eucalyptus from native Myrtaceae (Grgurinovic et al. 2006). Many hybrids of E. grandis of widespread use in Brazilian plantations have proven to be very susceptible to P. psidii; the latter also has a wide host range in the Myrtaceae having been reported on 11 genera and 31 species (Rayachhetry et al. 2001). Nevertheless, there are individuals of E. grandis that are resistant; one harbors a major gene for resistance to P. psidii, the Ppr1 gene (Junghans et al. 2003). How can we explain in terms of selection an Australian gene for resistance to a South American fungus, without going back in time to the Late Paleocene/Early Eocene thermal maximum, 55 mya, when there is evidence for floristic exchange between South America and Australia that included Myrtaceae (Morley 2003)? Moreover, some evolutionarily naive species of Myrtaceae appear resistant, as species, to P. psidii (Rayachhetry et al. 2001). Another example is found in the native, North American range of western gall rust, caused by Endocronartium harknessii. Scots pine (Pinus sylvestris), one of the most widely grown of Eurasian pines in North America, possesses a recessive major gene for resistance 1 Fungal Pathogens of Plants in the Homogocene 21
22 G.Newcombe and F.M.Dugan to western gall rust (Van der Kamp 1991).This gene may be common in Scots pine,at least in relation to the population of the western gall rust fungus in British Columbia where the study was performed.Two Asian hard pines,Pinus thunbergii and P.densiflora,are also resistant to wester gall rust (Hopkin and Blenis 1989),although genetic analyses of their resistance have not been performed. From Populus deltoides of eastemn North America were inherited QTL for resistance tPacficNorhwestem population of MyrelppiNewcombe and Bradshaw 1996).Asian sess gene to black leaf spot by the North American populati enet et al.1995). las mple in this secti hora ulmea (Be stance to riga asiatica that is found in a nonhost,Tagetes erecta,or marigold(Gowda et al.1999). 1.8 A "Tens Rule"for Novel Pathogens Exapted genes for resistance,such as the Cr.Mmd.Gasawa nes have been fou in resis nt individuals in oth rwise tible exapted r from nonhost resista As such be fixed in species outside the host range of the pathogen in question.But the evolutionary basis for this semantic distinction is unclear.In order to predict out comes of first encounters between novel pathogens and evolutionarily naive plants some estimate of frequency is needed,even though the evolutionary basis for the resistant outcome may be unknown.Examples suggest that the frequency of resistant outcomes is probably high.For example,an Asian maple species planted in north am ca is r ive with r ect to the“Ca g0门 gens (Table 1.1)of North A esp mer Nould Rhytism ct an Asiar maple grown ir the answer appears to be no in tha R.ame to North American natives,Acer rubrum and A.sace n.d.;Hudler and Jensen-Tracy 1998).Asian maples in North America are also apparently resistant to the"Category 2"R.acerinum that occurs on Norway maples A.platanoides,in North America.The"Category 1"taxa of Mycosphaerellales that are quite common on North American maples in North America also do not appear to attack Asian maples at all (farr et al.n.d.). Would this patt ern hold if we considered a north american plant that ha introduced into diffe the North that is utiliz sen noted tha est plar or nearly immune to all European rust fungi,"and"more stant than sylvestris to Phacidium infestans and Lophodermium pinastri"(Roll-Hansen 1978).Prunus serotina,or black cherry,provides another good example because it is a North American tree that has become invasive in European forests (Chabrerie et al 2008).Although eight rust taxa affect Eurasian species of Prunus in Europe,none of these "Category 1"pathogens infect p.serotin (Farr et al.n.d.).In other words none e provide biotic resistan e against this plant invader.This is not because
to western gall rust (Van der Kamp 1991). This gene may be common in Scots pine, at least in relation to the population of the western gall rust fungus in British Columbia where the study was performed. Two Asian hard pines, Pinus thunbergii and P. densiflora, are also resistant to western gall rust (Hopkin and Blenis 1989), although genetic analyses of their resistance have not been performed. From Populus deltoides of eastern North America were inherited QTL for resistance to a Pacific Northwestern population of Mycosphaerella populicola (Newcombe and Bradshaw 1996). Asian elms possess genes for resistance to black leaf spot caused by the North American population of Stegophora ulmea (Benet et al. 1995). A last example in this section is that of the NRSA-1 gene for resistance to Striga asiatica that is found in a nonhost, Tagetes erecta, or marigold (Gowda et al. 1999). 1.8 A “Tens Rule” for Novel Pathogens Exapted genes for resistance, such as the Cr, Mmd, Gasaway, and Ppr genes, have been found in resistant individuals in otherwise susceptible species. As such, exapted resistance differs from nonhost resistance in that the latter is presumed to be fixed in species outside the host range of the pathogen in question. But the evolutionary basis for this semantic distinction is unclear. In order to predict outcomes of first encounters between novel pathogens and evolutionarily naive plants, some estimate of frequency is needed, even though the evolutionary basis for the resistant outcome may be unknown. Examples suggest that the frequency of resistant outcomes is probably high. For example, an Asian maple species planted in North America is naive with respect to the “Category 1” pathogens (Table 1.1) of North American native maples. Would Rhytisma americanum infect an Asian maple grown in the U.S.? The answer appears to be no in that R. americanum is limited to North American natives, Acer rubrum and A. saccharinum (Farr et al. n.d.; Hudler and Jensen-Tracy 1998). Asian maples in North America are also apparently resistant to the “Category 2” R. acerinum that occurs on Norway maples, A. platanoides, in North America. The “Category 1” taxa of Mycosphaerellales that are quite common on North American maples in North America also do not appear to attack Asian maples at all (Farr et al. n.d.). Would this pattern hold if we considered a North American plant that has been introduced into a different continent? Consider Pinus contorta, or the North American lodgepole pine, that is utilized quite commonly in forest plantations in northern Europe. Three decades ago, Roll-Hansen noted that P. contorta is “immune or nearly immune to all European rust fungi,” and “more resistant than P. sylvestris to Phacidium infestans and Lophodermium pinastri” (Roll-Hansen 1978). Prunus serotina, or black cherry, provides another good example because it is a North American tree that has become invasive in European forests (Chabrerie et al. 2008). Although eight rust taxa affect Eurasian species of Prunus in Europe, none of these “Category 1” pathogens infect P. serotina (Farr et al. n.d.). In other words, none provide biotic resistance against this plant invader. This is not because 22 G. Newcombe and F.M. Dugan
1 Fungal Pathogens of Plants in the Homogocene P.serotina is immune to all rust fungi;in its native range in North America,four rust taxa affect it (Farr et al.n.d.).Similarly,the"Category 2"pear trellis rust,Gymnos- porangium fuscum,has remained confined in North America to the Eurasian n genu Pyrus as indige nous rosaceous generaare not known to be aecial hosts,and indige y resistant (Ziller 197) Cat smay switch i immediatel to long-temm,alien plants,o alien.Some past switches were likely not recorded immediately,so the importance of the extended opportunities of a lag period that is used to distinguish categories 2 and 3(Table 1.1)is not yet clear.For example,Hibiscus syriacus,the popular rose-of-Sharon,was introduced to the Americas in the late sixteenth century from its native range in Asia.In the ensuing,400 years in the Americas H.syriacus acquired five rust taxa that are not t kno wn to oc cur in its native range (Farr et al nd)But th exact dates of chin not kr .Oddly, the cie that do heterogeneus. s never beer reunited with its host in its introduced range. "Category 2"pathogens are alien pathogens that have been reunited with their adaptive hosts.The latter provide these pathogens with a lag period that they may need to successfully infect naive.native plants.Podosphaera leucotricha cause powdery mildew of apple,Malus domestica,that was domesticated in Eurasia.Like Venturia inaequalis that was shown to be Eurasian in origin(Gladieux et al.2008) P.leucorricha appears to be Eurasian also.But.P.leucotricha has been reunited y part of North America in which apples are cultivated(Farr et al ch that th eve Malus tax ativ Am a hav expos noculum Six seven appea to be resistant to P.leucotr ha in that there are no records of this fungus on them.But one native species of Malus in the southeaster part of the U.S.,M.angustifolia,has proven to be susceptible (Table 1.1).Phylogenetic signal does not appear to explain this susceptible exception as M.angustifolia is no more closely related to adaptive host species of Malus than resistant species of North America (e.g.,M.coronaria) (Robinson et al.2001).So,in this case,resistance appears to be exapted rather than nonhost.P.leucotricha has also been reunited in North America with Eurasian species of Photinia. P.glabra nd p encounters America have thus s als been ssured,but out mes thus far apparently involve nothing but resistance as no records are known.This trend toward resistant outcomes of first encounters con tinues with two other genera of Rosaceae,Crataegus and Spiraea.Crataegus is especially speciose in North America (USDA nd.)but there are no records of anv of its taxa hosting P.leucotricha,even though C.cuneata in Japan does host P.leucotricha:perhaps this record is of a pathotype that has never been introduced into North America.In the case of Spiraea,P.leucotricha has been recorded on Japanese spiraea.S.bu alda,in North America,but has not been ecorded on 12 taxa of Spirded nati e to North Ame ca (Fa in the ou comes of fir e3。 n the three categories of Table 1 could collapse
P. serotina is immune to all rust fungi; in its native range in North America, four rust taxa affect it (Farr et al. n.d.). Similarly, the “Category 2” pear trellis rust, Gymnosporangium fuscum, has remained confined in North America to the Eurasian genus Pyrus as indigenous rosaceous genera are not known to be aecial hosts, and indigenous Juniperus populations are apparently resistant (Ziller 1974). “Category 1” pathogens may switch immediately to long-term, alien plants, or they may eventually produce some virulent propagules that successfully infect the alien. Some past switches were likely not recorded immediately, so the importance of the extended opportunities of a lag period that is used to distinguish categories 2 and 3 (Table 1.1) is not yet clear. For example, Hibiscus syriacus, the popular rose-of-Sharon, was introduced to the Americas in the late sixteenth century from its native range in Asia. In the ensuing, 400 years in the Americas H. syriacus acquired five rust taxa that are not known to occur in its native range (Farr et al. n.d.). But the exact dates of switching are not known. Oddly, the one rust species that does occur on H. syriacus in India, Uromyces heterogeneus, has never been reunited with its host in its introduced range. “Category 2” pathogens are alien pathogens that have been reunited with their adaptive hosts. The latter provide these pathogens with a lag period that they may need to successfully infect naive, native plants. Podosphaera leucotricha causes powdery mildew of apple, Malus domestica, that was domesticated in Eurasia. Like Venturia inaequalis that was shown to be Eurasian in origin (Gladieux et al. 2008) P. leucotricha appears to be Eurasian also. But, P. leucotricha has been reunited with apple in every part of North America in which apples are cultivated (Farr et al. n.d.) such that the seven Malus taxa native to North America have undoubtedly been exposed to its inoculum. Six of the seven appear to be resistant to P. leucotricha in that there are no records of this fungus on them. But one native species of Malus in the southeastern part of the U.S., M. angustifolia, has proven to be susceptible (Table 1.1). Phylogenetic signal does not appear to explain this susceptible exception as M. angustifolia is no more closely related to adaptive host species of Malus than resistant species of North America (e.g., M. coronaria) (Robinson et al. 2001). So, in this case, resistance appears to be exapted rather than nonhost. P. leucotricha has also been reunited in North America with Eurasian species of Photinia, P. glabra and P. serratifolia. Extended opportunities for first encounters with three species of Photinia native to North America have thus also been assured, but outcomes thus far apparently involve nothing but resistance as no records are known. This trend toward resistant outcomes of first encounters continues with two other genera of Rosaceae, Crataegus and Spiraea. Crataegus is especially speciose in North America (USDA n.d.) but there are no records of any of its taxa hosting P. leucotricha, even though C. cuneata in Japan does host P. leucotricha; perhaps this record is of a pathotype that has never been introduced into North America. In the case of Spiraea, P. leucotricha has been recorded on Japanese spiraea, S. bumalda, in North America, but has not been recorded on 12 taxa of Spiraea native to North America (Farr et al. n.d.). If lag periods do not figure in the outcomes of first encounters, then the three categories of Table 1.1 could be collapsed. 1 Fungal Pathogens of Plants in the Homogocene 23
24 G.Newcombe and F.M.Dugan Examples such as these have not been subjected to genetic analysis,but they do suggest a relatively high frequency of resistant outcomes when novel pathogens and naive plants meet.Furthermore,first encounters must be common as naturalized plants in the U.S.belong to 549 genera(USDA n.d.).of which 305,or 56%,are represented in the U.S.by both the naturalized species and native congeners.Other parts of the homogenized world are likely similar in affording many opportunities for plants to encounter"Category 1"or"Category 2"pathog ens in pa icular. alie ngi in that only a fracti t in su sceptibl outco This analogous to the fa t that only a fractio invasions.This analogy,of course.does not imply that the same mechanism explains both phenomena.Improvements in our knowledge of the "tens rule"for novel fungal pathogens of plants will be built upon advances in the systematics and diagnostics of fungi that allow us to distinguish between pathogen reunions and first encounters.Improvements in our ability to predict which first encounters will result in relatively r re,but devastating.susceptible outcomes will come with a deeper understanding of the evolution and retention of genes for resistar 1.9 Transformers counters can however be"ransformative"if the nge dition orm or of ecosystems over are y The tnut b was clearly a "transformer in the range of Castanea dentata,the American chestnut.The latter was an abundant species in eastern North America at the time of the introduction of the blight fungus (Paillet 2002).C.dentata is no longer a dominant.overstory tree species in those deciduous forests that are starting to be dominated by oak and hickory (McGormick and Platt 1980).Unfortunately,"we know very little concering ecosystem response to the loss of chestnut"(Orwig 2002).Effects of blight on the food ofound,but they y.The ica iven to extin ch ely were (Opler 1978) A particular class of tran ormer among novel fungal pathogens would be one which does cause the extinction of an evolutionarily naive plant species.However examples of this are not known.Possibly,novel pathogens came closest with the above-mentioned Franklinia alatamaha.This tree species is not now extinct,but its only natural population was extirpated shortly after the bartrams discovered it Were it not for ex sit cultivation,F.alatamaha would now be extinct.Speculation about the causes of the loss of the single,naturally occurring population abound nd that speculation clude the i popu 100 s山 extin pathogens (Rosenzweig 200 1b).as we have also briefly
Examples such as these have not been subjected to genetic analysis, but they do suggest a relatively high frequency of resistant outcomes when novel pathogens and naive plants meet. Furthermore, first encounters must be common as naturalized plants in the U.S. belong to 549 genera (USDA n.d.), of which 305, or 56%, are represented in the U.S. by both the naturalized species and native congeners. Other parts of the homogenized world are likely similar in affording many opportunities for plants to encounter “Category 1” or “Category 2” pathogens in particular. A variant of the “tens rule” may thus apply to alien, plant pathogenic fungi in that only a fraction of all first encounters result in susceptible outcomes. This is analogous to the fact that only a fraction of all plant introductions result in plant invasions. This analogy, of course, does not imply that the same mechanism explains both phenomena. Improvements in our knowledge of the “tens rule” for novel fungal pathogens of plants will be built upon advances in the systematics and diagnostics of fungi that allow us to distinguish between pathogen reunions and first encounters. Improvements in our ability to predict which first encounters will result in relatively rare, but devastating, susceptible outcomes will come with a deeper understanding of the evolution and retention of genes for resistance. 1.9 Transformers Susceptible outcomes of novel encounters can however be “transformative” if they change the “character, condition, form or nature of ecosystems over a substantial area” (Pysˇek et al. 2004). The chestnut blight fungus, Cryphonectria parasitica, was clearly a “transformer” in the range of Castanea dentata, the American chestnut. The latter was an abundant species in eastern North America at the time of the introduction of the blight fungus (Paillet 2002). C. dentata is no longer a dominant, overstory tree species in those deciduous forests that are starting to be dominated by oak and hickory (McGormick and Platt 1980). Unfortunately, “we know very little concerning ecosystem response to the loss of chestnut” (Orwig 2002). Effects of blight on the food web were probably profound, but they were not studied except anecdotally. The American chestnut itself was not driven to extinction by blight, but chestnut-specific insects likely were (Opler 1978). A particular class of transformer among novel fungal pathogens would be one which does cause the extinction of an evolutionarily naive plant species. However, examples of this are not known. Possibly, novel pathogens came closest with the above-mentioned Franklinia alatamaha. This tree species is not now extinct, but its only natural population was extirpated shortly after the Bartrams discovered it. Were it not for ex situ cultivation, F. alatamaha would now be extinct. Speculation about the causes of the loss of the single, naturally occurring population abounds (Rowland 2006), and that speculation includes the introduction of novel pathogens. Small populations are notoriously susceptible to stochastic forces of extinction that might include pathogens (Rosenzweig 2001b), as we have also briefly 24 G. Newcombe and F.M. Dugan
1 Fungal Pathogens of Plants in the Homogocene 5 discussed.This is a serious concern for Wollemia nobilis that lacks genetic variation (Peakall et al.2003):that finding might indicate that W.nobilis has lost through e ohe h et n shown to be sus eptible to Phytophtho cin and to a haeria (Bullock etal.200) Ge uniform rtality ca Dutch elm fu Enslish em that turned out to bea.00 e magn ee mor year-old Roman clone (Gil et al.2004). 1.10 Deliberate Introductions of Fungi Deliberate introductions of fungi have likely been unc mmon.So me introductior have been made to control plant invaders (i.e.class ical biological control)with pathogens with narrow host ranges such as rust fungi(Bruckart and Dowler 1986). Edible.cultivable mushrooms are certainly cultivated outside their native ranges (Arora 1986).Australian ectomycorrhizal fungi"were likely introduced with euca- lypt seedlings brought into peninsular Spain before plant quarantine restrictions were observed"(Diez 2005).and this introduction may have been deliberate if the ing the seedlin ngs knew of the d epe of eucal Fo on thes purp se. ectomy iate P eratel eld et al gens of some concern. For example.Rhizina undulata.native to the northern hemisphere.now causes root disease in plantations of northern hemisphere conifers grown in plantations in southern Africa(Wingfield et al.2001).Armillaria mellea,the root rot fungus,may also have entered South africa in this way(Coetzee et al.2001).Another nontarge effect of these omycomhizal introductions,deliberate or inadvertent,has involved 2005 otic tree plantations Diez are no doub examples, e ummary ,deliberately introduced fungi represent just a tiny fraction of global,fungal diversity. 1.11 Inadvertent Co-Introductions of Fungi in Plants Brasier highlights the da plant ade(Bras ersof inadverction of fung bche er 2008).Even trees"upto 10m tall with large root b are being moved from one country to another.Homogenization of previously isolated fungal communities above and belowground is thought to be inevitable if this trade persists.Not only can such shipments not be made safe,but the exotic plant itself contributes to changes in microbial community structure and function in
discussed. This is a serious concern for Wollemia nobilis that lacks genetic variation (Peakall et al. 2003); that finding might indicate that W. nobilis has lost through genetic drift genes for exapted resistance. Little is yet known however of the susceptibilities of W. nobilis other than that it has been shown to be susceptible to Phytophthora cinnamomi and to a species of Botryosphaeria (Bullock et al. 2000). Genetic uniformity certainly affected the magnitude of tree mortality caused by the Dutch elm fungus to Ulmus procera, the English elm, that turned out to be a 2,000- year-old Roman clone (Gil et al. 2004). 1.10 Deliberate Introductions of Fungi Deliberate introductions of fungi have likely been uncommon. Some introductions have been made to control plant invaders (i.e., classical biological control) with pathogens with narrow host ranges such as rust fungi (Bruckart and Dowler 1986). Edible, cultivable mushrooms are certainly cultivated outside their native ranges (Arora 1986). Australian ectomycorrhizal fungi “were likely introduced with eucalypt seedlings brought into peninsular Spain before plant quarantine restrictions were observed” (Dı´ez 2005), and this introduction may have been deliberate if the people transporting the seedlings knew of the dependence of eucalypts on these fungi. For the same purpose, ectomycorrhizal associates of pine seedlings were deliberately introduced into the southern hemisphere (Wingfield et al. 2001). Unfortunately, these introductions also inadvertently brought with them soil pathogens of some concern. For example, Rhizina undulata, native to the northern hemisphere, now causes root disease in plantations of northern hemisphere conifers grown in plantations in southern Africa (Wingfield et al. 2001). Armillaria mellea, the root rot fungus, may also have entered South Africa in this way (Coetzee et al. 2001). Another nontarget effect of these ectomycorrhizal introductions, deliberate or inadvertent, has involved competition with native ectomycorrhizal fungi in the exotic tree plantations (Dı´ez 2005). There are no doubt other examples, but in brief summary, deliberately introduced fungi represent just a tiny fraction of global, fungal diversity. 1.11 Inadvertent Co-Introductions of Fungi in Plants Brasier highlights the dangers of inadvertent introductions of fungi by the modern plant trade (Brasier 2008). Even trees “up to 10 m tall with large root balls attached” are being moved from one country to another. Homogenization of previously isolated fungal communities above and belowground is thought to be inevitable if this trade persists. Not only can such shipments not be made safe, but the exotic plant itself contributes to changes in microbial community structure and function in 1 Fungal Pathogens of Plants in the Homogocene 25