Sorghum Diseases in India

pathogen virulent to these hybrids will appear
in the near future. The variability for virulence
found in P. sorghi presents a problem for the control
of SDM with host-plant resistance.
To date, five races of P. sorghi have been identified
in the Americas, three in Texas (Frederiksen
1980), one in Brazil (Fernandes and Schaffert
1983), and one in the Honduras (C. D. H.
Meckenstock, Soil Crops Department, Texas
A&M University, College Station, Texas, personal
communication). These five races represent
only a small portion of the variability
present in the pathogen. In a study of virulence
in P. sorghi, Pawar tested 75 sorghum cultivars
for reaction to a collection of isolates of the
pathogen from Asia, Africa, Central America,
and North America (Pawar 1986). Differential
reactions of the test cultivars identified each of
the 16 isolates as a different pathotype. The pathotypes
from Africa and India had a much
wider range of virulence among the 75 cultivars
than did pathotypes from the Americas. Ten of
the 75 sorghum cultivars were resistant to all of
the pathotypes.
There is no shortage of sorghum cultivars resistant
to one or more races of P sorghi. In a
recent test I screened 2000 accessions in an Ethiopian
sorghum collection, and 17% of these were
resistant to the three Texas pathotypes of P. sorghi.
These tests identify resistant genotypes but
provide no information about the range of genetic
diversity for SDM resistance represented
by these resistant cultivars. Diversity of resistance
genes, rather than numbers of resistant
cultivars, is the relevant factor in the use of hostplant
resistance for control of SDM.
Recent research conducted in Texas on the
modes of inheritance of SDM resistance in the
sorghum lines SC 414-12 and QL 3 indicated that
resistance in SC 414-12 was conferred by a dominant
gene (Craig and Schertz 1986; Sifuentes
1985). In QL 3, resistance was conditioned by
each of two independent dominant genes; neither
at the same locus as the resistant factor in
SC 414-12. An earlier study of SDM resistance in
QL 3, conducted in India (Bhat 1981), found that
the mode of inheritance was relatively complex
and involved six genes. The discrepancy between
results obtained in India and those reported
in Texas could have been caused by
differences in the methodology of testing, pathogen
biotypes, or the QL 3 selections.
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Our experience in Texas indicates that monogenic
physiological resistance will not provide
horizontal, or even durable, resistance to SDM,
We seem to have embarked on an endless cycle
of development, deployment, breakdown, and
replacement of SDM resistance. The oligogenic
resistance found in QL 3, and possibly present in
some other cultivars, may provide the mechanism
for durable resistance to SDM. Selections
of QL 3 were resistant to P. sorghi at several geographically
disparate test sites for many years
(Mughogho 1983). In addition, QL 3 was resistant
to an international collection of 16 pathotypes
of P. sorghi in greenhouse trials (Pawar
1986). However, translation of these past performances
of QL 3 into agronomically desirable
sorghum hybrids is yet to be reported.
Another approach to durable host-plant resistance
that should be given attention is the use
of minor, nonspecific genes to reach an acceptable
level of resistance to SDM. Presumably,
these minor factors exist and account for differences
in SDM incidence between sorghum cultivars
when neither possesses major genes for
physiological resistance to P. sorghi. The task of
identifying these minor genetic factors and increasing
their frequencies in sorghum selections
will be difficult, but success in this endeavor
would provide stable resistance to SDM.
Chemical Control
Until the advent of acylanilide fungicides in the
1970s, there was no satisfactory chemical control
for systemic downy mildews of cereals. Other
types of contact and systemic fungicides were
partially effective against local lesion infection
by downy mildew pathogens, but only as protectants
against further infections (Cohen and
Coffey 1986; Schwinn 1980). The acylanilides
have a narrow spectrum of activity, restricted to
the Peronosporales and the Hypochytridiomycetes.
The acylanilides are taken up by the
roots and leaves, and are primarily translocated
acropetally to become systemic (Cohen and Coffey
1986). The acylalanines include metalaxyl,
furalaxyl, and benalaxyl. Of this group, metalaxyl,
marketed by Ciba-Geigy as Apron ® , is the
only fungicide used to control sorghum downy
mildew. Metalaxyl's principal use is as a seed
treatment to protect plants against infection by