Sorghum Diseases in India

green and slightly shriveled, in contrast to dark
green and round appearance of a healthy, fertilized
ovary. Within 2 days, superficially visible
white mycelial stromata appear in the base of
the ovary and gradually extend upwards. The
ovary is converted into fungal stromata with
shallow folds. The first external symptoms, clear
to pinkish drops exuding from infected ovaries,
appear 5-10 days after inoculation. The name,
'sugary' disease, for sorghum ergot originates
from this sticky sweet fluid. It is also called honeydew
and contains numerous conidia. Under
humid conditions, a saprophyte Cerebella volkensii
(Syn. C. sorghi-vulgaris) grows on honeydew
and converts it into a matted, black mass. However,
warm and dry conditions after the formation
of honeydew will dry it, forming an easily
removable, hard, white crust on the panicle. Finally,
the fungal stromata are transformed into
the hard, resting structure (sclerotia) that may or
may not be concealed by the glumes.
Causal organism
The fungus is best known by its imperfect stage,
Sphacelia sorghi McRae, but the perfect stage,
Claviceps sorghi, has been described by Kulkarni
et al. (1976). The imperfect stage is associated
with the honeydew phase of symptoms. Closely
packed pallisade-cell-like conidiophores are produced,
either in the interior of folded stromata
that replace the ovary or on the surface of the
stromata. Numerous apical, hyaline, unicellular
conidia are borne one at a time on the short conidiophores,
possibly by a constriction mechanism.
Conidia are oval or elliptical or oblong,
and often have distinct vacuole-like bodies at
the ends. Conidia are 5-8 x 12-20 µm in size
(Mantle 1968; Kulkarni et al. 1976) and are released
in the honeydew exuding from infected
ovaries. More complete description of the asexual
stage is required, considering the existence
of two distinct conidial types (macroconidia and
secondary conidia) of the pathogen (see Frederickson
and Mantle this publication).
Abundant honeydew with conidia have been
produced in culture on modified Kirchoff's medium
(Nagarajan and Saraswathi 1975). Trace elements
affect growth and sporulation of the
sphacelial stage (Chinnadurai 1972). Conidial
shape and size was reported to vary with nitro­
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gen content of the culture substrate (Chinnadurai
and Govindaswamy 1971b).
The fungal stromata transform into mature
sclerotia within 4 weeks (Futrell and Webster
1965) to 2 months (Sangitrao and Bade 1979b)
following inoculation. The shape and size of mature
sclerotia depends on host genotype, environment,
and nutritional factors (Sangitrao and
Bade 1979b). Sclerotial development is often
hampered by fungal contaminants that grow on
honeydew and developing sclerotia. Velvety olive
to black growth of Cerebella spp is the most
common contaminant. Fusarium moniliforme,
F. roseum f. sp cerealis, and Cladosporium spp may
also be found (Futrell and Webster 1966). Most
reports (Futrell and Webster 1966; Sangitrao and
Bade 1979b) consider Cerebella as a parasite, but
proof of parasitism is lacking.
Langdon reported in 1942 that Cerebella inhabiting
sclerotial formation is a saprophyte
nearly always associated with honeydew.
The perfect stage of Claviceps is initiated
when the sclerotium germinates to produce two
or three stipes bearing stromatal heads containing
embedded flask-shaped perithecia with
slight protrusion at the ostiolar region. Perithecia
measure 66.4-124.5 x 132.8-232.4 µm and
contain hyaline cylindrical asci 2.4-3.2 x 56-112
µm in size, bearing hyaline caps at their apices.
Each ascus contains eight aseptate, filiform ascospores
measuring 0.4-0.8 x 40-85 µm. This description
of the perfect stage of Claviceps is based
on the single report to date (Kulkarni et al. 1976).
Additional descriptions of the Claviceps stage are
required of isolates from different regions to determine
if morphological and pathogenic variations
occur in this pathogen.
Several researchers have reported varying
degrees of success in attempts to germinate sclerotia
in the laboratory. In some instances sclerotia
did not germinate at all (Sundaram 1970;
Nagarajan and Saraswathy 1975). In others,
grayish bulges appeared from the cortex that
broke through the rind, but instead of producing
a clear stipe and capitulum, the bulge continued
to proliferate to form a callus-like, spherical
growth with mycelial tuft (Mantle 1968; R. Bandyopadhyay,
unpublished). Mantle suggested
the possibility of lack of infection by heterothallic
strains as the reason for the lack of
production of sclerotia that can germinate to
produce ascospores.