Introduction to Fungi, Third Edition

CERATOBASIDIALES
595
affected (Sneh et al., 1991), the question of the
boundaries of biological species remains unanswerable
at present.
The basidiocarps of Ceratobasidiales are thin,
gelatinous and often resupinate. Their formation
can sometimes be induced by covering an agar
culture with soil (Warcup & Talbot, 1966). Hyphal
tips due to develop into a probasidium are
generally binucleate, and karyogamy is followed
swiftly by meiosis. During the later stages of
meiosis, four prominent sterigmata referred to
as epibasidia are formed (Wells & Bandoni, 2001),
but septa are not laid down, even in the mature
basidium (Fig. 21.1a). The basidiospores are uninucleate
and frequently germinate by repetition
to produce uninucleate ballistoconidia. Eventually,
hyphal germination occurs. Mating systems
in the Ceratobasidiales are not well understood,
and several species may be homothallic.
The basidium of the Tulasnellales differs in
that the four epibasidia become separated from
the metabasidium by septation after meiosis.
Maturation of the four epibasidia may occur at
different times (Fig. 21.1b).
21.2.1 Rhizoctonia in agriculture
The most important taxon is R. (Moniliopsis) solani
(teleomorph Thanatephorus cucumeris), which causes
a wide array of soil-borne necrotrophic diseases
especially of herbaceous plants including all
kinds of vegetables, rice, turfgrasses, and less
frequently also woody tree hosts (Adam, 1988;
Agrios, 2005). The most common disease is
damping off of seedlings in which the infected
hypocotyl region becomes water-soaked and no
longer provides structural integrity (see p. 95),
leading to pre- or post-emergence death of
the seedling. In older plants, infection may not
immediately cover the entire circumference of
the plant stem, so that cankers and girdling of the
stem may result. Roots are also frequently
infected, as are aerial plant organs in contact
with the soil or exposed to watersplash from the
soil surface. The fungus can survive in the soil
for several years as small sclerotia about 1 3mm
in diameter. These are often associated with
the debris of host plants killed by the fungus.
Sclerotia are occasionally seen as black scurf on
the surface of potato tubers because they are not
easily removed, even by assiduous washing.
Rhizoctonia diseases are typically most severe
at cool temperatures, presumably because the
seedling stage of host plants is prolonged due
to their slow growth. The infection process
is similar to that of Gaeumannomyces graminis,
i.e. hyphae branch and build up a cushion-like
compound appressorium on the host plant
surface, from which penetration is achieved.
Fig 21.1 (a) Basidium of Thanatephorus cucumeris
(Ceratobasidiales). Note the four prominent
epibasidia and the lack of septation within the
basidium. (b) Basidium of Gloeotulasnella cystidiophora
(Tulasnellales) showing various stages of maturity.
The epibasidia are separated from the metabasidium
by septation.Three epibasidia have already
discharged their basidiospore, with one about to
produce it. (a) redrawn from Warcup and Talbot
(1962) with permission from Elsevier, (b) redrawn
from Wells and Bandoni (2001) with kind permission
of Springer Science and Business Media.