Leaf-inhabiting genera of the Gnomoniaceae, Diaporthales - CBS

So g o n o v e t a l.
species transferred from other genera. The type species of the
genus Cryptodiaporthe, C. aesculi, groups with P. euphorbiae and
its relatives, thus Cryptodiaporthe is considered a synonym of
Plagiostoma. Perithecia of Plagiostoma occur singly or in groups on
leaves and twigs of woody trees and shrubs as well as herbaceous
plants. Often the perithecia lack a stroma. Like all genera of the
Gnomoniaceae dealt with in this study except Ambarignomonia
and Gnomonia, the perithecia remain immersed in the substrate
becoming convex with the base collapsing upward when dry. The
ascospores are ellipsoidal, have one median septum that is rarely
submedian or absent, and may or may not bear appendages. The
species grow relatively fast in culture and often produce abundant
conidiomata on PDA. Species of Plagiostoma occur on a diverse
range of woody and herbaceous hosts.
Evaluation of morphological and host characteristics
The groupings of species based on the multigene phylogeny
presented here suggest that the morphological characters
previously used to define genera must be re-evaluated. The
generic classification proposed by Barr (1978) and Monod (1983)
are presented as rectangular tables, referred to as a “pigeon-hole”
system in which columns and rows show genera corresponding
to ascospore and perithecial/stromatal characteristics. The
phylogenies resulting from the analysis of multiple genes do not
agree with such a rigid definition of genera based on one or two
characteristics. Little congruence can be found between the newly
defined genera based on molecular data and the genera based on
a single morphological characteristic.
Host specificity is an important character in circumscription of
genera and species of the Gnomoniaceae. The genus Gnomonia is
almost strictly associated with plant hosts in the Betulaceae, mostly
in the subfamily Coryloideae. Likewise, most of the species of
Gnomonia are limited in their host range to a single genus and often
to a single plant species. For example, the type species, G. gnomon,
is restricted to species of Corylus except for one collection reported
from a Populus seedling that may be an accidental colonisation
of a non-specific host. The monotypic Ambarignomonia with A.
petiolorum is likewise restricted to one plant host, Liquidambar
styraciflua. The other genera of the Gnomoniaceae do not show
such consistency in host associations. The genus Apiognomonia
has the most diverse range of hosts that include hardwood trees
as well as herbaceous plants. Species of Apiognomonia exhibit a
diversity of host specificity with some species such as A. veneta
occurring on plants in at least 10 different plant families while
others such as A. acerina are restricted to one plant host. Species
of Gnomoniopsis are mostly associated with either Fagaceae or
Rosaceae also with the range of host specificity varying among
species. While the type species G. chamaemori appears to be
restricted to Rubus chamaemorus, other species are specific at the
level of host genus such as G. clavulata on Quercus spp. Species
of Ophiognomonia occur predominantly on members of the Fagales
with some exceptions such as the type species O. melanostyla that
infects overwintered leaves of Tilia spp. The genus Plagiostoma
shows a broad host range with species occurring on a variety of
hosts such as P. devexum on Persicaria, Polygonum, and Rumex
(Polygonaceae) while others are species specific such as P.
euphorbiae is known only from Euphorbia palustris. Few members
of the Gnomoniaceae are known to infect hosts outside of the
dicotyledonous plants; the asexual genus Sirococcus on conifers
provides one exception (Rossman et al. 2007).
Members of the Gnomoniaceae occur most commonly on
fallen or still attached, overwintered leaves including petioles or
herbaceous stems although some occur on woody substrates such
as species of Cryptosporella but also, for example, Apiognomonia
hystrix and Plagiostoma salicellum. When ascomata develop
on woody substrates, these are often found on relatively small
branches, one or two years old, that are dead but still attached
to the host tree. A number of species of Gnomoniaceae have
been reported as endophytes of woody plants (Viret & Petrini
1994, Cohen 1999, 2004, Danti et al. 2002, Vujanovic & Britton
2002, Green 2004, Moricca &Ragazzi 2008) but these are often
not accurately identified. In addition, some species are pathogenic
such as Apiognomonia veneta, cause of sycamore anthracnose,
and Gnomoniopsis fructicola, cause of strawberry stem rot (Maas
1998).
One morphological character that was emphasised in earlier
classification systems is the type and extent of stroma (Kobayashi
1970, Barr 1978, Monod 1980, Vasilyeva, 1998). No members
of the Gnomoniaceae have a well-developed stroma. Species of
Cryptosporella as well as Amphiporthe hranicensis, Apiognomonia
hystrix, and Plagiostoma salicellum, i.e. species that occur on
woody substrates, produce limited stromatic tissues.
These stromatic tissues may be associated with the rupture
through the surface of the substrate. In addition, three species of
Linospora, not considered in this study, produce a layer of tissue
that covers the aggregated ascomata that are superficial or slightly
immersed on leaves (Barr 1978, Monod 1983). In other families of
the Diaporthales such as the Cryphonectriaceae, Diaporthaceae,
Pseudovalsaceae, and Valsaceae, stromata are often welldeveloped
(Castlebury et al., 2002, Gryzenhout et al. 2006,
Voglmayer & Jaklitsch 2008).
Most members of the Gnomoniaceae including all species
of Ambarignomonia, Gnomonia, and Ophiognomonia, produce
ascomata singly, immersed, although some species of Gnomonia
become erumpent. Similar to the development of a rudimentary
stroma, the formation of grouped perithecia appears to be more
common in species that develop on woody substrates. This is
exemplified by Apiognomonia hystrix. Apiognomonia hystrix as
Cryptodiaporthe hystrix was traditionally placed in the Valsaceae
(Barr 1978) because of its occurrence on woody substrates with
ascomata developing a rudimentary stroma. One of its synonyms,
Gnomonia cerastis, was traditionally placed in the Gnomoniaceae
due to its occurrence on overwintered leaves with ascomata
lacking any stroma. As suggested by Monod (1983), specimens of
A. hystrix are known to occur both on woody substrates as well
as on overwintered leaves. Some species of Gnomoniopsis and
Plagiostoma, i.e. P. salicellum, produce grouped perithecia on
woody substrates. All species of Cryptosporella occur on woody
substrates and produce ascomata in groups (Mejia et al. 2008).
Ascomata of species of Gnomoniaceae are perithecial, i.e. no
cleistothecial members are known, dark brown to black, smooth,
with or without an elongated neck. In a few species, the neck is
surrounded by a distinct, powdery collar. The perithecial walls are
thin-walled, less than 30 μm diam, composed of only one or two
regions. The outer region is composed of textura angularis with cell
walls dark brown, slightly thickened, 1–2 μm. The inner region is
composed of hyaline, elongate cells. The structure of the ascomata
is relatively constant within the family. However, one characteristic of
the ascomata that has some taxonomic significance is the collapse
upon drying. In species of Ambarignomonia and Gnomonia the
ascomata collapse from the top becoming concave when dry while
in other members of the Gnomoniaceae the ascomata collapse from
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