RA 00110.pdf - OAR@ICRISAT

Introduction
Downy mildew of pearl millet (Pennisetum americanum)
is incited by Sclerospora graminicola (Sacc.)
Schroet., which is the type species of the genus.
Although S. graminicola has been recognized as
an important pearl millet pathogen since the early
part of this century, it received relatively little attention
until the early 1960s. Even today international
comparative research on the biology and epidemiology
of this pathogen and the disease it causes is
needed to expand control techniques.
Comprehensive reviews of research on the disease
up to 1976 are available (Nene and Singh 1976,
Safeeulla 1976b), but information on the inoculum
sources, climatic influence, and the disease cycle has
been misunderstood so that the epidemiology of the
disease has been seriously misinterpreted. In this
paper, current knowledge on the origin and distribution,
survival of the infective propagules, sporulation,
dispersal and infectivity, dual culture, and
cytology of the pathogen are presented and discussed.
Geographic Distribution
S. graminicola is widely distributed in the temperate
and tropical areas of the world, including Europe,
the Middle East, Africa, and Asia. Although the
fungus has been reported as occurring in the USA, it
has not been found in pearl millet in that country.
Since its discovery by Schroeter (1879), it has been
recorded in more than 20 countries (Safeeulla 1976b).
Thus far there is no report of the fungus in South
America and Australia. The principal host of S.
graminicola is pearl millet, which originated in
Africa (Harlan 1975, Brunken et al. 1977). Information
on the most frequent sources of resistance to 5.
graminicola (Williams and Singh 1979) supports a
hypothesis of African origin with dissemination to
other countries possibly by seedborne inoculum.
However, it is also possible, but less likely, that the
pathogen originated in indigenous Indian grasses,
and moved to pearl millet when this crop was introduced
from Africa. The present widespread occurrence
in Africa would then be explained by its introduction
long ago on plant products from India, with
the many resistance sources among African pearl
millet merely reflecting a long period of coevolution
in regions with a wide range of host variability (Williams
1984). On the contrary, Shaw (1981) suggested
that S. graminicola has a temperate origin because it
is circumpolar on species of Setaria, and that it has
become adapted to plants in tropical habitats, particularly
pearl millet. He also believes 5. graminicola
to be not only primitive, but to have been circumpolar
on the Paniceae since Pleistocene times because it
has coevolved in many locations with species of
Setaria, Panicum, Chaetochloa, and Pennisetum.
Shaw also said that S. graminicola has occurred in
Africa since that continent was much more temperate,
evolving and adapting to the changing climate
along with its host.
Inoculum Sources
Pearl millet is an annual crop, but the nature of the
perennation of the pathogen during the off-season
was not clearly understood. The obligate nature
rules out the possibility of it perennating on the
straw, and the fragile nature of the sporangium
makes it unsuitable for survival for more than a few
hours. Logically, wild hosts, the seed, and soilborne
oospores are the sources of inoculum for the recurrence
of the disease year after year.
Wild Hosts
About 14 species of graminaceous plants belonging
to eight genera are known to be susceptible to 5.
graminicola (Safeeulla 1976b). The role of wild hosts
in offering sporangial inoculum each growing season
to the fresh crop is not yet clear. However,
Suryanarayana (1965) made some observations on
5. graminicola occurring on Setaria verticillata
(Linn.) P. Beauv, a common annual grass found
during the rainy season. He made a detailed study
and observed abundant production of oospores and
sporangia on this common Indian grass, which
raises the possibility of S. verticillata serving as an
alternative host for downy mildew of pearl millet.
148