Introduction to Fungi, Third Edition

472 LOCULOASCOMYCETES
Table 17.2. Selected plant-pathogenic species of Alternaria (from Holliday, 1998).
Species of Alternaria Disease Comments
‘A. alternata’
Widerangeofhostsanddiseases
‘A. alternata’f. sp. lycopersici Tomato stem canker
A. brassicae Brassica greyleaf spot Seed-borne
A. brassicicola Brassica blackleaf spot Seed-borne
A. carthami Safflower leaf spot Seed-borne
A. dauci Carrot leaf blight Seed-borne
A. dianthi Carnationleaf blight Seed-borne
A. linicola Linseed seedling blight Seed-borne
A. macrospora Cottonleaf spot Seed-borne
A. mali Apple core rot
A. porri Onion purple blotch Seed-borne
A. radicina Carrot black rot; also infects celery and parsnip
A. solani Potato early blight and tuber rot
Tomato early blight and fruit rot
Curvularia (Fig. 17.14). The revised genus
Helminthosporium is now rather small, with
about 20 species and H. velutinum as the typespecies
(Fig. 17.12). It has affinities to
Leptosphaeria (Olivier et al., 2000). Drechslera, in
contrast, is the anamorphic state of Pyrenophora
(Fig. 17.16; see p. 477). All these anamorphic
forms produce pigmented (melanized) spores
with only transverse septa, but they differ in
their pattern of conidiogenesis and in the
ultrastructural appearance of their cell walls.
Sporogenesis by Helminthosporium is tretic, i.e.
porospores are formed as described for
Stemphylium by the digestion of a small hole
into the conidiophore cell wall, followed by an
extension of the inner wall to form the conidium
which then lays down its outer wall (Fig. 17.12c).
In contrast to Pleospora, each site can produce
only one conidium, and conidia can develop
apically or laterally on the conidiophore. If the
conidium is produced apically, the conidiophore
cannot grow further. The sequence of conidium
development in Bipolaris and Curvularia is not
of this type. In Bipolaris, the first conidium
always develops apically, and subsequent
conidia are formed either by growing through
the scar left by the first conidium, or by the
conidiophore growing past the first conidium to
form a new apex producing the second conidium
(B. sorokiniana; Fig. 17.13b). Bipolaris is so named
because the conidia germinate by emitting two
germ tubes, one at either end (Fig. 17.13d), and
these grow as extensions of the long axis of the
spore. This is in contrast to Helminthosporium
conidia in which each cell is principally capable
of germination, and the germ tubes grow
perpendicular to the long axis of the spore.
In Curvularia, the conidia are curved because
of an unevenly swollen central cell. The end cells
are usually less strongly pigmented than the
central cell (Figs. 17.14b,c). The first conidium
also develops at the apex of the conidiophore as
a poroconidium extending through a tiny pore,
and then the conidiophore develops a new
subterminal growing point from which a
second conidium initial arises. The process is
repeated so that a succession of new apices, each
terminated by a conidium, is formed (Fig. 17.14a).
The term sympodula has been applied to such
a conidiophore producing conidia sympodially
(Kendrick & Cole, 1968). In some Curvularia spp.,
such as C. cymbopogonis (Fig. 17.14c), the base of
the conidium bears a protuberant hilum.
A feature of the conidia of Drechslera,
Helminthosporium and Bipolaris, but not
Curvularia, is that they are distoseptate
(Luttrell, 1963). This means that the wall separating
adjacent conidial segments is visibly