Sorghum Diseases in India

Singh (1964) was the first to report that sorghum
ergot sclerotia germinate to produce clear
stipe and capitulum, but he did not describe the
perfect stage. Sexual germination of sclerotia
was confirmed by Kulkarni et al. (1976), and
Sangitrao and Bade (1979b) in India, and by
W.A.J. de Milliano and coworkers (W.AJ. de
Milliano, SADCC/ICRISAT Sorghum and Millets
Improvement Program, Bulawayo, personal
communication 1987) in Zimbabwe. In India,
sclerotia appeared to have a 3- to 4-month dormancy
period. Ascospores obtained from germinated
sclerotia, when inoculated repeatedly on
panicles of male-sterile plants, produced honeydew
and sclerotia (Singh 1964). Most reports indicated
that very few sclerotia germinate, but
Kulkarni et al. (1976) obtained 80% sclerotial
germination. Details of the methodology used in
these investigations were not reported.
Mode of infection and colonization
Information in this section is based on unpublished
data. Conidia germinate on stigma to
produce short germ tubes that penetrate stigmatic
papillae. Sundaram (1970) reported that conidia
germinate to produce secondary conidia,
which then produce long germ tubes that penetrate
stigma. This has not been confirmed. Occasionally,
conidia can also germinate and
penetrate style and ovary wall, but further route
of invasion inside the host tissue is not known.
Infection hyphae passes intercellularly through
the style and ovary wall, and attaches itself to
the vascular bundles at the base of the ovary 3
days after inoculation. Bundles of hyphal strand
then grow rapidly along the inner layer of the
ovary wall and envelop the ovule, without colonizing
the ovule tissues. At the same time mycelial
strands grow along the outer epidermis of
the ovary wall, and then within the tissues of the
ovary wall. Ovule tissues are the last to be colonized.
The ovary is gradually digested by the
fungal stromata, and in 5 to 7 days the ovary has
been displaced by deeply involuted fungal
stroma. Internal stromatic tissues possess locules
in which conidia are produced on short
conidiophores. They are also produced on the
surface of the stromata. Conidia are released
from the honeydew that oozes from the infected
spikelet. Histological changes associated with
the transformation of stroma into sclerotia have
not been studied.
A more detailed histopathological study,
such as those on rye (Luttrell 1980) and on wheat
(Shaw and Mantle 1980a,b), is required to understand
the host-pathogen relationship in sorghum
ergot.
Flowering Biology and Environment
in Disease Development
Flowering biology
Since C. sorghi infects and colonizes only the gynecia,
knowledge of the inflorescence is essential
to understand the disease. Flowering of the
spikelets has not been studied in detail, but is
thought to be a factor in susceptibility of
genotype.
Flowering behavior of sorghum was described
by Stephens and Quinby (1934) and
again by Quinby (1958). It is known to vary in
different environments and genotypes. For example,
almost all spikelets flower during the
night and pollen loses viability after 5 h when
sorghum flowers at 26-39 °C and 25 to 60% RH
(Stephens and Quinby 1934); but at 10-30 °C and
30 to 95% RH, anthesis occurs only after sunrise
and pollen remain viable for longer periods
(Sanchez and Smeltzer 1965).
The spikelet parts most critical for infection
and colonization are its stigma and anthers. Nor*
rnally, a stigma is pollinated soon after it is exposed,
pollen germinates within 30 min, and
fertilization occurs within another 2-12 hours
(Stephens and Quinby 1934; Artschwager and
McGuire 1949). On the other hand, conidia require
8-12 hours for germination on the stigma,
and 36-48 hours to reach the base of the ovary
(R. Bandyopadhyay, unpublished data). Therefore,
under normal conditions, pollen germinates
earlier than conidia and reaches the
embryo sac faster than the colonizing infection
hyphae. However, the times for these events
vary from floret to floret.
It is generally believed that ovaries resist
infection and colonization after fertilization
(Fuentes et al. 1964; Futaell and Webster 1965).
However, Puranik et al. (1973) reported that ovaries
could be infected 5 days after pollination. It
is not clear if infected ovaries were fertilized at
all, because not all ovaries are fertilized by hand
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