Sorghum Diseases in India

orne conidia (asexual spores) of Peronosclerospora
sorghi, the causal organism of SDM. Conidial
showers, blown by wind from infector
rows onto test materials, provide the inoculum.
Two especially important aspects of the successful
employment of this technique are establishment
of the disease in infector rows, and
favorable temperatures and humidities for
abundant conidial production by plants in the
infector rows. Screening should be conducted,
therefore, at a location known to be favorable for
the disease, as is the Dharwad installation in
India.
Source and off-season maintenance of
inoculum
Most of the downy mildew diseases have
evolved in close association with a number of
noncultivated grasses. The DMs have become a
concern only when extensive sowings of susceptible
genotypes are established in areas where a
DM is introduced and/or endemic on a native
grass species. In general, grass species that are
hosts for DMs are native to the southeastern
Asia, and generally of the tribes Andropogoneae
and Maydee. In North America, johnsongrass
(Sorghum halepense) and shattercane (a feral
S. bicolor) can become infected naturally in the
field by P. sorghi. Since oospores and conidia are
produced in both species, these spp often serve
as sources of primary inoculum. Perennial wild
sorghums are common in borders of cultivated
fields and in drainage and irrigation ditches. The
sources of primary conidial inoculum used for
establishment of screening techniques at three
locations were:
Dharwad, India. During May 1981, two volunteer
plants of a sorghum cv with systemic disease
were located. We used these two plants as
conidial sources, and increased the inoculum by
use of the sandwich inoculation procedure. The
conidia inoculum in systemically infected plants
was carried over in the off season. Just before
the main growing season (rainy season), this inoculum
was increased for use in establishing infector
rows.
Matopos, Zimbabwe. The primary inoculum
was collected from systemically infected ratoons
of cultivated sorghums at Kadoma. Infection
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(plants with systemic disease) on some entries
was as high as 60%. These ratoons surely were
infected by conidia blown from other infected
plants, that had been infected by either oospores
or conidia from weedy and wild sorghums. This
inoculum was used to infect 20 sorghum lines,
using the sandwitch inoculation procedure and
some 20 plants with systemic infection were
obtained (Pande 1987); these in turn were used
to establish infector row screening during the
1986/87 season (Singh 1987) using modified
sandwich inoculation. Natural temperatures
were found to be congenial for sporulation and
infection.
Golden Valley, Zambia. During 1986/87, a severe
outbreak of sorghum downy mildew was
observed at Golden Valley and some of the entries
showed as high as 80% systemic infection
(B.N. Verma and W.A.J. de Milliano, personal
communication). During the last week of November,
few systemically infected weedy and
wild sorghums (S. halepense) were observed near
the irrigation canal. After the first rains, we collected
thousands of systemically infected S. halepense
and shattercane plants from many
locations around Golden Valley Inoculum from
these plants was used to establish the infector
rows.
Establishing infector rows
The most important and vital component of the
ICRISAT SDM screening technique is the infector
rows. To establish these, we sow germinated
and downy mildew-infected seeds of a highly
susceptible cultivar (DMS 652, IS 643, Marupantse,
or Sugardrip). On these cvs, the pathogen
produces abundant conidia. Pregerminated
seeds were inoculated following the sandwich
inoculation technique.
In sandwich inoculation, germinated seeds
are incubated by placing them on the adaxial
surface of a piece of systemically infected leaf,
and covering them with another piece of systemically
infected leaf, so that the germinated
seeds are sandwiched. The procedure is carried
out in petri dishes lined with moist filter paper;
dishes are incubated in darkness at 18-20 °C for
12 to 16 h. By this time, the seedlings are covered
with a mycelial mat of the fungus, indicating
that the fungus has sporulated and conidia