Introduction to Fungi, Third Edition

15
Hymenoascomycetes: Helotiales (inoperculate
discomycetes)
15.1 Introduction
In contrast to the Pezizales (see preceding
chapter) which produce apothecia with asci discharging
their spores through a detachable lid at
their apex, the asci of inoperculate discomycetes
liberate their spores either through a valve or
a slit. In the inoperculate as well as operculate
discomycetes, the asci may contain two or more
layers, i.e. they are often described as bitunicate.
However, these layers do not separate during
ascus discharge, i.e. they are non-fissitunicate.
Fine structural details of the asci of Helotiales
have been described by Verkley (1993, 1994,
1996). Two large ecological groups of inoperculate
discomycetes can be distinguished: the
lichenized and non-lichenized species. This feature
correlates approximately with the taxonomy
at the level of orders, and here we shall
discuss the Helotiales (sometimes alternatively
called Leotiales) which contain mostly
non-lichenized fungi. The Lecanorales and other
orders with mainly or exclusively lichen-forming
fungi are described in Chapter 16.
Those relatively few phylogenetic studies that
have so far been performed on the Helotiales
lack the necessary power of resolution to delimit
natural groups. Thus, it is not clear at present
whether this order is monophyletic or not,
and several different classification schemes are
in use (Gernandt et al., 2001; Kirk et al., 2001;
Pfister & Kimbrough, 2001). The families
currently associated with the Helotiales are
listed in Table 15.1. Thus circumscribed, the
order Helotiales contains some 2300 species.
The Orbiliaceae, formerly included here (Pfister,
1997), are now considered to be more closely
related to the Pezizales. Since the conidial forms
of some of them are of interest as nematodetrapping
and aquatic fungi, they will be considered
in Chapter 25.
Fungi belonging to the Helotiales have
adapted to several different ecological situations.
Many species are necrotrophic, hemibiotrophic or
biotrophic plant pathogens, and some can cause
considerable damage in economically important
crops. Other species are saprotrophic, colonizing
dead leaves and shoots of herbaceous and woody
plants. Endophytic species are also known, and
it would not be too surprising if many of the
inoperculate discomycetes known as saprotrophs
were found to be already present in the living
plant as endophytes. Some species fruit on plant
debris submerged in freshwater streams and have
adapted to this habitat by producing conidia of
unusual shapes (Chapter 25). Other species form
ericoid mycorrhizal associations with the roots of
Ericaceae (p. 442). The Thelebolaceae are a group
of coprophilous species.
15.2 Sclerotiniaceae
The Sclerotiniaceae and Rutstroemiaceae are
closely related but can be separated by