Introduction to Fungi, Third Edition

CLAVICIPITALES
351
Fig12.25 The homothallic life cycle of
Claviceps purpurea growing on rye (Secale
cereale). A sclerotium fallen to the ground
will produce a perithecial stroma after
overwintering. In the perithecia,
plasmogamy (P), karyogamy (K) and
meiosis (M) give rise to filamentous
ascospores, each with numerous haploid
nuclei (small open circles). Ascospores
germinate on the stigmata of the rye
ovary, and mycelium penetrates to the
ovary stalk (rachilla). Infections develop
both as conidial regions and, lower down,
as a sclerotium initial.The developing
sclerotium pushes the ovary upwards,
andthisremainsasacapabovethe
conidial stroma.The stroma contains
cavities lined by phialides which produce
phialoconidia of the Sphacelia segetum
type.These accumulate in beads of a
sugary liquid (honeydew).Conidia are
carried by insects to fresh rye stigmata
and initiate secondary infections. Later in
the season, the sclerotia enlarge and
become visible as ergots. Some images
redrawn from Luttrell (1980).
with binucleate penultimate segments. The
penultimate cell elongates to form the ascus,
and fusion between the two nuclei occurs. There
are numerous asci in each perithecium, each
containing a bundle of eight filiform ascospores.
The ascus bears a conspicuous perforated cap at
its tip (Fig. 12.27c).
Successful infection of rye (Secale cereale) from
cultures derived from a single ascospore show
that C. purpurea is homothallic. Despite this,
genetic recombination is possible through
heterokaryosis and parasexual reproduction
(Tudzynski, 1999). Curiously, sclerotia are
frequently formed from heterokaryotic mycelia,
indicating multiple infection of the grass flower.
Although common on rye and some other
cereals in Europe and North America, C. purpurea
is not usually troublesome on cereals in Britain.
In the occasional years in which its incidence is
high, there is a correlation with high relative
humidity and low maximum temperature in
June, which probably prolongs the period during