Introduction to Fungi, Third Edition

648 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
Fig 23.10 Tilletia caries.
(a) Germinating teliospore showing
a non-septate promycelium and
a crown of primary sporidia.
(b) Two detached primary sporidia
showing conjugation. A secondary
sporidium has developed from
one of the primary sporidia.
(c) Primary sporidia attached
to the promycelium showing
conjugation.
1882, when Alexis Millardet discovered its ability
to control downy mildew of vines (see p. 119).
The process of germination of T. caries teliospores
is shown in Fig. 23.10. The diploid nucleus
in the mature teliospore divides meiotically,
and one or more mitotic divisions follow so that
8 or 16 nuclei are formed. The promycelium
is often but not invariably aseptate, and from
its tip narrow curved uninucleate primary
sporidia arise, corresponding in number to the
number of nuclei in the young promycelium
(Fig. 23.10a). The primary sporidia mate in pairs
by means of short conjugation tubes, often whilst
still attached to the tip of the promycelium
(Fig. 23.10c). Detached primary sporidia may also
conjugate. During conjugation, a nucleus from
one primary sporidium passes into the other
sporidium which therefore becomes binucleate.
Each H-shaped pair of primary sporidia develops
a single lateral sterigma on which a curved
binucleate spore develops (Fig. 23.10b). This
spore is projected actively from the sterigma
using the surface-tension catapult mechanism,
and it is sometimes referred to as the secondary
sporidium. Because of its characteristic method
of discharge, Buller and Vanterpool (1933) have
interpreted this spore as a basidiospore. The
secondary sporidium brings about infection of
the host.
Teliospores of T. caries (Fig. 23.9) are viable
for up to 15 years and germinate along with the
seeds if contaminated grain is sown. Secondary
sporidia produce germ tubes which infect the
coleoptiles of the seedlings. Infection is systemic,
the mycelium growing through the tissues of the
shoot, and by suitable techniques it is possible
to isolate the dikaryotic mycelium from infected
host tissues (Trione, 1972). Although infected
plants may grow less vigorously than uninfected
ones, they show no outward sign of
infection until the ears are almost ripe.
Tilletia controversa
This species is closely related to T. caries and
is most damaging on wheat, in addition to
infecting other cereals and grasses. The crucial
difference is that teliospores of T. controversa
germinate at much lower temperatures than
those of T. caries. In consequence, T. controversa
causes bunt mainly on winter wheat, infecting