4.3. Endophyte diversity and ecology
The twelve fungal endophyte strains isolated in this study represent seven genera in six families and five orders in the phylum Ascomycota. Six of the 12 strains are classified as
Diaporthe, a speciose genus consisting of endophytes, saprotrophs, and pathogens that colonize a wide range of host plants [
41]. This genus has more than 800 described species, and more than 950 species in its asexual state (formerly
Phomopsis) [
42]. Two of the isolated strains (En20-4 and En01-1) appear to represent putative novel species, based on their phylogenetic distinction from described species with sequences available in GenBank. Four strains are closely related to
D. vacuae, a recently described species in the
D. eres complex that has been associated with dieback and twig blight of
Vaccinium corymbosum [
43]. These strains also had similar extrolite profiles (
Figure S2). Recent work resulted in
D. vacuae and other related species in the
D. eres complex being synonymized with
D. eres [
44]. En20-4. species are commonly associated with diseases of grapevine worldwide, notably Phomopsis cane and leaf spot, which is attributed to
D. ampelina (=
D. viticola), and symptoms such as stem and branch dieback, perennial cankers, vascular discoloration, and rachis necrosis [
45,
46,
47,
48]. Phomopsis cane and leaf spot can affect most parts of the grapevine, including the flowers and berries, and its potential to cause large crop losses often leads to the application of fungicides and other control meaures [
49]. Although
D. eres causes minor disease in a broad range of hosts, it can also result in serious canker disease in grapevine, apple, and blueberry [
43]. For example, strains of
D. eres isolated from symptomatic apple rootstocks in the same region as this study (Annapolis Valley, NS) caused necrosis, cankers, and eventually death in young apple plants within four weeks [
19]. Other
Diaporthe species show varying degrees of virulence in grapevine: for example, the closely related
D. ampelina,
D. hispaniae, and
D. hungariae are highly virulent, while other species, such as
D. bohemiae, appear to be avirulent endophytes ). Overall, most
Diaporthe species, including
D. eres, tested in grapevine pathogenicity experiments show some degree of virulence [
45,
50,
51]. Reveglia
, et al. [
52] demonstrated that a
D. eres strain isolated from symptomatic grapevine wood produced phytotoxic secondary metabolites; one phytotoxin, nectriapyrone, was identified in several of our
Diaporthe strains. Guarnaccia
, et al. [
53] reported that
D. eres was the
Diaporthe species most commonly isolated in grapevines sampled in eight countries. Taken together, the fact that
Diaporthe represents the majority of endophytes isolated in this study is not unexpected. Its presence does not necessarily reflect the overall health of the sampled grapevines, given its frequency of occurrence on asymptomatic grapevines and potential host resistance [
54]. However, it is notable that Kernaghan, Mayerhofer and Griffin [
16] did not report any
Diaporthe endophytes in wild and hybrid
Vitis leaves sampled from wild grapes and vineyards in eastern Canada. Although pathogenicity experiments were not conducted in this study, the virulence of En20-4 and En01-1 should be assessed in the future, especially considering the identification of the phytotoxin nectriapyrone in
Although
Gnomoniopsis paraclavulata (En61-1) was isolated from grapevine leaves, this species is best known from
Quercus spp. in the US and Europe, where it is found in the stems, wood, asymptomatic leaves, overwintering leaves, leaf litter, and acorns [
55,
56,
57,
58]. The most dominant species isolated from asymptomatic and diseased stems of
Quercus robur in Poland included
G. paraclavulata (and, incidentally,
D. eres and
C. fioriniae) [
59,
60]. Subsequent pathogenicity tests on young
Q. robur seedlings demonstrated that
G. paraclavulata was the most pathogenic species tested, causing dieback symptoms and small lesions on stems. Costa
, et al. [
61] also identified
G. paraclavulata in symptomatic and declining
Q. suber in Portugal, while Tosi
, et al. [
62] implicated
G. paraclavulata in chestnut bud and shoot blight (
Castanea sativa). Given
G. paraclavulata’s apparently narrow association with
Quercus, or Fagaceae in general, its isolation from grape leaves is somewhat unexpected and its interaction with this host is unknown.
N. niveniae (En61-2) is related
to Phaeomoniella chlamydospora, believed to be one of the primary causal agents of Petri disease and esca, two serious grapevine trunk diseases. Other Phaeomoniellales taxa have recently been described as a result of spore trapping efforts in vineyards, but do not appear to be pathogenic [
63]. Similarly,
Neophaeomoniella spp. have not been implicated in grapevine disease, although
N. niveniae has been identified from symptomatic plants, albeit in low relative abundance [
64]. Interestingly,
N.
niveniae was isolated from wild olive trees (
Olea europaea subsp.
cuspidata) in South Africa, and was found to be present in 7.1% of the trees sampled; two strains exhibited low-to-zero virulence and intermediate virulence, respectively, when inoculated on shoots of ‘Frantoio’ olive trees (
O. europaea L. subsp.
europaea) in South Africa [
65,
66].
N. niveniae was first described from leaves of
Nivenia stokoei in South Africa, where it was isolated from leaf spots, but the causality of this was not tested [
67]. Our isolation of
N. niveniae is likely a first record in Canada, and its interactions with grapevines are unknown. Interestingly, among the tested strains, En61-2 produced a number of dipeptides, all of which contain Ile or Leu. Although fungi are known to produce a wide range of cyclic dipeptides [
68], all the ones reported here are linear and have unknown bioactivity.
Species of
Colletotrichum have been implicated in various diseases of agricultural crops and other plants, such as bitter rot of pear and apple [
69,
70],anthracnose of apple [
71],fruit rot of cranberry and blueberry [
72], anthracnose fruit rot of raspberry [
73],leaf spot disease of walnut [
74],anthracnose of eggplant [
75], seedling blight of poison ivy (
Toxicodendron radicans; [
76], and leaf blight of
Mahonia aquifolium [
77]. Bitter rot of apple, for example, is considered a major disease of apples in warm and humid regions [
58]. On the other hand, several
species in this genus are capable of forming symbiotic and mutualistic relationships with plant species [
78,
79]
. C. fioriniae was isolated as a leaf endophyte from apple trees in orchards and from various plant species growing nearby, including angiosperm trees, wild grapes and others, leading the authors to hypothesize that the species’ main ecological role was indeed that of a leaf endophyte [
58]. In the same study, the
C. fioriniae strains isolated were also found to be pathogenic on apple fruit. Recently,
C. fioriniae was reported as the causal agent of grapevine anthracnose in New York [
80], and is also associated with grapevine ripe rot [
81].
C. fioriniae not only has been widely reported as an endophyte and pathogen in a broad range of hosts, but is also cited as an entomopathogen of the elongate hemlock scale (
Fiorinia externa), the origin of its specific epithet [
82,
83,
84]. Therefore,
C. fioriniae may act as a benign or potentially beneficial foliar endophyte in some plant hosts, while also reducing the quality of and accelerating decay in, fruit.
C. gloeosponoides has also been reported to cause epizootics of the scale insect Orthezia praelonga, a major pest of citrus in Brazil [
83]
.
The family Xylariaceae contains a large number of commonly reported endophytes in a very broad range of host plants, including bryophytes, liverworts, angiosperms and conifers, even occurring in the form of endolichenic fungi [
85]. These fungi are not only ubiquitous endophytes, but also remarkably prolific producers of bioactive natural products [
86,
87]. The genus
Nemania consists of approximately 80 named species (
www.indexfungorum.org), including species characterized as endophytes producing described natural products [
88,
89,
90,
91,
92].
N. serpens and
Nemania spp. have occasionally been isolated as endophytes of cultivated and wild grapes [
88,
93]. We isolated
N. aureolutea (En25-1), a rarely observed species that is closely related to the common
N. aenea. Our strain produced the characteristic slow-growing orange colonies on PDA as described by Fournier
, et al. [
94] (
Figure 3a). Little is known about
N. aureolutea because it is rarely collected, with a small number of reports from Europe, the continental US, and Hawaii, where it grows in the wood, and sometimes on the outer bark, of
Acer,
Corylus,
Populus,
Quercus, and
Salix; most collections appear to be from
Quercus {Vasilyeva, 2006 #95;Fournier, 2020 #39}. Our strain may be the first report from Canada; however,
N. aureolutea is likely more common than perceived and some specimens identified as
N. aenea are actually
N. aureolutea [
95].
The genus
Sphaerulina comprises more than 200 named species (
www.indexfungorum.com), with some causing leaf spot on various plant species; notably,
S. musiva is responsible for one of the most damaging diseases of poplar in northeastern and north-central North America [
96].
S. amelanchier (En26-1) was originally described from
Amelanchier sp. leaf litter in the Netherlands, and has also been isolated from leaves of unidentified species of
Betula,
Castanea, and
Quercus ([
97,
98]. Given the infrequent reports, little is known about the ecology of
S. amelanchier. A related species,
S. vaccinii, causes a common leaf spot and stem canker disease in lowbush blueberry (
Vaccinium angustifolium) in eastern Canada and the US [
99,
100,
101]. We also isolated another Mycosphaerellaceae species, a strain (En60) of an unknown
Ramularia species that is a sister species to
R. heraclei and
R. lamii var.
lamii.
Ramularia are also well-known and common associates of leaf spot diseases of varying significance in a broad range of plant hosts. For example, Ramularia leaf spot of sugar beet and barley, caused by
R. beticola and
R. collo-cygni, respectively, are serious emerging disease [
102,
103].
Species of Ramularia have been reported as endophytes of grapevine [
16]
, while a Ramularia sp. isolated from Rumex gmelini Turcz has been noted to produce the bioactive secondary metabolite chrysophanol [
104]
. According to the results of the phylogenetic analysis and the lack of similarity to available sequences,
Ramularia sp. (En60) is possibly a novel species.
In general, the host preferences of the endophyte species that we isolated from grapevine are either demonstrably broad (e.g.,
Colletotrichum fioriniae,
D. eres), suggestive of a broader host range than currently known (e.g.,
G. paraclavulata,
Nemania aureolata, Neophaeomoniella niveniae), or are unknown due to possible novelty (e.g.,
Diaporthe spp. En20-4 and En01-1,
Ramularia sp.). Many of these endophytes produce slimy, bright yellow-orange masses of conidia that are presumably spread via rain splash and water runoff, and potentially vectored by insects, including pollinators [
63,
105,
106]. Endophytes in the Xylariaceae family (e.g.,
Nemania aureolata) may be transmitted horizontally to new leaves via dry, airborne conidia and ascospores. While grapevine is probably a secondary host for
N. aureolata, endophytes occurring on leaves as foliar endophytes may colonize primary hosts (i.e., host substrate that supports sexual reproduction, such as hardwood trees) through direct contact (viaphytism) [
107]. Therefore, the senescent and overwintered leaves and wood of both grapevine and other distantly related plant hosts may serve as inoculum for new infections. Vertical transmission of endophytes is also possible in some cases, for example, as demonstrated or suggested in
Diaporthe,
Ramularia, and
G. paraclavulata [
57,
108,
109].
Some grapevine endophytes are potentially opportunistic pathogens, such as
Diaporthe spp. and
C. fioriniae, which may affect vines, leaves, and/or fruit. Other species may be commensal or mutualistic, such as
N. aureolata or even
C. fioriniae, an endophyte that may protect against insect herbivory and, as demonstrated in other
Colletotrichum species, provide protection from plant pathogens [
72,
110,
111,
112]. Interactions between endophytes and their hosts span the mutualist-pathogen continuum and depend on the host’s status at the given moment [
113]. Consequently, the overall effect of endophyte colonization on host fitness may be difficult to generalize; for example,
C. fioriniae could benefit its host by protecting leaves against insect herbivory, but its colonization of fruit could reduce seed dispersal by altering signals that attract animal seed dispersers, i.e., by reducing palatability and altering nutrient content [
114]. Conversely, the fungal infection of fruit may actually increase dispersal, for example, by increasing levels of attractive volatiles, in which case an anthropocentric view of fruit diseases may lead to the misinterpretation of a fungus’ effects on its host’s fitness in natural systems [
115].