Sorghum Diseases in India

King: Was the millet host only Setaria sp, and not
pearl millet? At ICRISAT Center, India, we almost
always find some intermittent streaking
(broken lines) associated with symptoms, especially
at an early stage. Do you usually find
these symproms with SYB in Texas?
Giorda: The millet host was Setaria italica cv
"German strain R". Yes, I usually find streakings
as a few broken lines, which coalesce forming
the typical yellow banding. In addition, a severe
stunting has been observed in plants infected
following mechanical inoculation.
Mughogho: At ICRISAT Center, it is suspected
that the virus is transmitted by the leafhopper
Peregrinus maidis. Have you tried transmission
tests with this insect?
Giorda: I will not discard the possibility of leafhopper
transmission.
Claflin: Since the sorghum yellow banding virus
is a spherical virus, is it vectored by beetles?
Giorda: The fact that SYBV is a spherical virus
does not imply that it has to be vectored by beetles.
For example, the cucunovirus group is isometric
and transmitted by aphids in a nonpersistent
manner. In the luteovirus group,
spherical viruses belonging to this group are
aphid-transmitted in a persistent manner.
Laing: Tolremicity is a characteristic which
could easily be missed by the application of too
high a level of inoculum—a level that could
swamp this resistance mechanism.
Putter: If the characteristic is a qualitative character,
such as a glume being tightly shut, then
the inoculum load will not affect it. However,
with a quantitative characheristic the inoculum
dose levels will be critical to the selection of
tolremicity.
Anahosur: Is it possible to look for tolerance in
systemic diseases such as SDM?
Frederiksen: If you accept the concept of the
ability of the host to produce in spite of disease
and if you consider the host as a population of
individuals/those with a low frequency of systemically
infected plants could be considered as
more tolerant than those with a higher frequency
of disease.
Giorda: You have mentioned resistance and tolerance
in virus; these terms are "defined" based on
symptoms and yield. You have not mentioned the
virus content as another parameter to cosider in
this definition. The spread of the virus depends in
part of the amount of virus present in the host,
since transmission of virus depends on vectors
and this in turn affects resistance. There are different
host-virus-vector interactions in the sorghum
syste, thus the effect of virus content on resistance
will differ. What do we mean by field resistance?
Have you considered escape and nonpreference
as elements of fieldresistance?
Frederiksen: Certainly!
Balasubramanian: Preselective mutants in a
pathogen population increase with the deployment
of matching cvs and decrease with the
withdrawal of the cultivar. Can we utilize deployment
of genotypes in sorghum and millet
disease?
Frederksen: Absolutely!
Anahosur: When we use high inoculum levels
to select for resistance, we may loose minor gene
resistance and we need to look for both major
and minor resistance.
Qhobela: How do you feel about the deployment
of "single vertical resistance" genes that
give phenotypes that could be classified as susceptible
in certain environments, thus allowing
the pathogen to "grow"? Then you are not selecting
for more-virulent races of the pathogen,
Frederiksen: Probably about the same as I feel
about the deployment of any other "single vertical
resistance"gene. Be careful, watch out, and
have a backup system.
King: In India we have a situation where pearl
millet hybrids are breaking down to downy mildew
after 4 of 5 years of intensive cultivation by
farmers. The genetics of resistance is not well
understood, but reports generally indicate that
resistance is dominant, but usually involves
some minor genes as well The genetics of virulence
has not been studied.
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