Introduction to Fungi, Third Edition

476 LOCULOASCOMYCETES
Fig17.15 Toxins produced by Cochliobolus spp. a.The sesquiterpene prehelminthosporol produced by C. sativus.b.Thechlorinated
pentapeptide victorin C produced by C. victoriae. c.The dominant form of T-toxin, a polyketide produced by C. heterostrophus.
d.The major form of HC-toxin, a tetrapeptide produced by C. carbonum.
Fig17.16 Pyrenophora tritici-repentis. (a) Immature pseudothecial stroma on overwintered barley stubble, with conidia produced
on the setae and on separate conidiophores. (b,c) Conidia of Drechslera tritici-repentis mounted in lactic acid (b) showing the typical
‘peas-in-a-pod’ appearance, and mounted in water (c). (b) and (c) to same scale.
mildew (see p. 408) are highly susceptible
to C. sativus. A hypersensitive response which
effectively suppresses biotrophic pathogens may
thus actually enhance pathogenicity by others
with a necrotrophic potential.
A similar feature was observed with
C. victoriae, which became pathogenic on the
Victoria cultivar of oats bred for resistance
against the crown rust, Puccinia coronata
(Meehan & Murphy, 1946). The main toxin
produced by C. victoriae, victorin C (Fig. 17.15b),
is one of several related cyclic pentapeptides
which are absolutely essential and sufficient for
pathogenesis, i.e. the purified toxin can reproduce
the disease symptoms which consist of
widespread leaf necrosis, and strains of
C. victoriae not synthesizing this toxin do not
cause the disease. It now seems that victorin
triggers a form of hypersensitive response, i.e.
widespread host cell death (Navarre & Wolpert,