RA 00015.pdf - OAR@ICRISAT

materials in a sick-plot (i.e. a field in which the
soil contains resting spores of the DM causal
fungus) is unreliable and restrictive. A new
technique was required which (i) would allow
even exposure to inoculation of many thousands
of segregating lines, (ii) would allow flexibility in
size and location of the testing site, and
(iii) could be used efficiently in the postrainy
season as well as in the rainy season. Evidence
from two field experiments during the rainy
season clearly indicated that the sporangia
(ephemeral asexual spores produced in millions
on infected plants each night) played a major role
in the early infection of plants and buildup of the
disease (development of the epidemic) in the field
when high humidities prevailed. This information
was used to develop a screening technique
based on sporangia produced by "infector
rows" (a known high-susceptible hybrid) planted
in every third row 15 days before seeding of the
test rows. Inoculum is provided for initiating
infection in the infector rows by strategically
placed pots of infected plants, and the necessary
humidity is provided by fine-mist irrigation in the
evening hours. The test materials are planted
when the infector rows show rapid disease development
(15 to 21 days after seeding of the
infector rows) and the mist irrigation is continued
twice weekly until the test materials reach
heading stage. Throughout the screening area,
after every 10 test lines, a known high-susceptible
hybrid (indicator) is planted at the same time as
the test lines in order to measure the success of
the inoculation process. This technique was
highly effective during both the rainy and postrainy
seasons in 1976, and details of the screening
results are presented below and in the breeding
subprogram report.
Screening Activities at ICRISAT Center
Using the infector-row system, 4 hectares of
breeding material were screened at ICRISAT
Center for resistance to DM in both the rainy
and postrainy seasons. This was a joint activity
between the breeding and pathology staffs. In the
population-improvement materials, selfing and
sibbing for the next cycle of selection was carried
out in the disease nursery utilizing only D M -
resistant plants. A measure of the advance
achieved in this process is presented in Table 22.
In addition to the ICRISAT breeding materials,
230 millet lines were tested in the disease
nursery for scientists working in the All-India
Coordinated Millet Improvement Project.
Multilocational Screening
The International Pearl Millet Downy Mildew
Nursery (IPMDMN). One of the features of
certain types of resistance is that it "breaks
down" through the ability of the pathogen to
overcome the resistance. In an attempt to find
Table 22. Comparison of downy mildew susceptibility in four populations after one generation of selection
under heavy disease pressure during rainy and postrainy seasons at ICRISAT Center, 1976.
Lines with < 5 % incidence
Maximum incidence
Population Rainy' Postrainy Rainy Postrainy
G A M 73 S 2 's
G A M 73 FS
G A M 75 S 1 's
Late Composite
(%)
3.1
. 9.0
0.0
33.3
(%)
50.0
16.7
9.5
68.4
(%)
76.59
69.35
87.75
59.57
(%)
50.0
41.4
69.7
31.0
a Resistant plants were selfed in rainy season 1976 and the progeny retested during the postrainy season.
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