Introduction to Fungi, Third Edition

484 LOCULOASCOMYCETES
incompatible interactions results if the product
of the Avr gene is recognized by the host plant.
Several Avr genes have been characterized, and
their products are usually small proteins
secreted by C. fulvum into the apoplast. Many of
the corresponding Cf genes of tomato are also
known; they encode proteins anchored in the
plasma membrane of tomato cells, with large
extracellular domains. The examination of the
products of matching avirulence and resistance
genes should provide an opportunity to examine
their interactions, and thus the molecular basis
of recognition events involved in specific resistance
(Rivas & Thomas, 2005). This work is still
ongoing.
Fig17.21 Cercospora beticola.Conidiophores and conidia from
sugar beet seed.
(chlorotic) leaf areas are produced as a result of
such systemic infections (Fig. 17.24), and eventually
conidiophores are emitted through
stomata especially on the lower leaf surface,
forming a lawn of spores resembling powdery
mildews but being light brown in colour.
The interaction between C. fulvum and its
tomato host is governed by a classical genefor-gene
relationship based on dominant host
resistance genes (Cf genes, for C. fulvum) and
dominant avirulence (Avr) genes in C. fulvum,
i.e. virulence is a recessive trait (Joosten et al.,
1997). The hypersensitive response of
17.3.5 Aureobasidium and black yeasts
Aureobasidium pullulans is a ubiquitous saprotroph
whose main habitats are the phylloplane
and other surfaces of living and senescent plants,
but it can also occur as a symptomless endophyte.
It grows in soil, but is often not recorded
because it is temperature-sensitive and is not
seen on soil plates prepared with warm agar. It
has been isolated from fresh water, estuarine
and marine sediments, sea water, sewage and
other liquid waste (Domsch et al., 1980). Its
teleomorph is Discosphaerina fulvida, a relative of
Mycosphaerella (Yurlova et al., 1999). The fungus
can be readily isolated from leaf washings. It is
pleomorphic, and in culture it forms a rapidly
growing mycelium with wide, septate hyphae
from which intercalary and occasionally terminal
cells give rise to single or clustered hyaline
blastoconidia by a process of budding. Budding is
associated with local lysis of the wall of the
conidiogenous cell, the inner wall of which then
balloons out and forms the wall of the conidium
(Ramos et al., 1975). Repeated conidium development
from the same or closely adjacent conidiogenous
loci results in the formation of
slimy clusters of hyaline conidia (Fig. 17.25a).
Intercalary cells may enlarge and develop
thicker, dark, melanized walls to become
chlamydospores (Fig. 17.25b). Conidia bud in a
yeast-like manner when the fungus is grown
in liquid culture with high inoculum densities.
The yeast cells can continue to bud