Sorghum Diseases in India

oospores in the soil (Odvody and Frederiksen
1984a; Odvody et al. 1984; Anahosur and Patil
1980). However, the fungicide is also effective
when applied to the foliage to protect plants
against infection by conidia or to eradicate the
pathogen after infection has occurred (Odvody
and Frederiksen 1984b; Anahosur and Patil
1980). Metalaxyl treatment at rates as low as 0.63
g a.i. kg- 1 of seed provides excellent protection
against SDM (Odvody et al. 1984).
Metalaxyl acts against the pathogen after
penetration of the host has occurred. The chemical
does not affect spore germination or host
penetration, but inhibits further development of
the pathogen after the initial invasion. Metalaxyl's
mode of action is reported to be the inhibition
of the synthesis of RNA in sensitive fungi
(Davidse and de Waard 1984).
Metalaxyl was approved for control of sorghum
downy mildew in Texas because most of
the sorghum hybrids grown in Texas were susceptible
to a new pathotype of P. sorghi (Odvody
et al. 1984). The use of metalaxyl in Texas was
recently expanded by an iatrogenic relationship
between SDM and the herbicide antidote Concep
II ® . Chloroacetanilide herbicides are widely
used to control grasses in sorghum fields in
Texas; sorghum is sensitive to these herbicides,
but can be protected from damage by seed treatment
with a herbicide antidote. In 1984, it was
discovered that the herbicide antidote CGA
92194, marketed as Concep II ® by Ciba-Geigy,
increased the incidence of SDM in sorghum
(Craig et al. 1987). Concep II ® did not affect sorghum
genotypes that had physiological resistance
to P. sorghi, but the disease incidence in
plants of susceptible sorghum from seed treated
with Concep II® was significantly greater than
that in plants from nontreated seed of the same
genotypes. The combination of metalaxyl with
Concep II ® in the treated seed provided adequate
protection against downy mildew (Craig
et al. 1987).
The narrow spectrum of fungicidal activity
exhibited by metalaxyl made it highly probable
that resistant biotypes of the target fungi would
develop. This has occurred relatively quickly.
Metalaxyi-resistant strains have appeared in the
genera Peronospora, Phytophthora, Plasmopara,
Pseudoperonospora, and Pythium. However, no resistant
strains of the graminaceous downy mildew
pathogens have been reported (Cohen and
Samouche 1986; Davidse and de Waard 1984).
The disease systems in which pathogens
have developed metalaxyi-resistant strains are
those in which several cycles of spore production
and host infection occur in one season and
metalaxyl was applied repeatedly under conditions
favorable to multiplication of the pathogen.
Such systems apply extremely heavy selection
pressure for the development and increase
of resistant biotypes of the pathogen.
The disease cycle of SDM in grain sorghum
differs significantly from these systems and is
much less likely to produce metalaxyi-resistant
biotypes of P. sorghi Metalaxyl is usually applied
only once as a seed treatment. Systemic
infection of the plant is required to produce the
oospores needed for survival over the fallow
season. This systemic infection occurs only at
the early stages of plant growth. As a result,
conidia produced on systemically diseased
plants by a metalaxyi-resistant biotype of the
pathogen are unlikely to induce systemic infection
and oospore production in surrounding
plants. Consequently, the buildup of a metalaxyi-resistant
population of P. sorghi would be
appreciably slower than in the systems with several
oospore-producing cycles per season.
These conditions reduce the probability of
producing a metalaxyi-resistant biotype of P. sorghi,
but the possibility remains (Odvody and
Frederiksen 1984a). Practices that increase this
possibility should be avoided. Metalaxyl seed
treatment should not be applied at concentrations
less than required for maximum control.
Where infection is primarily mediated by oospores
in young seedlings, the control goal
should be to achieve as close to 100% control of
SDM as possible to reduce the number of plants
systemically infected with biotypes of P. sorghi
possessing variable tolerance to metalaxyl (Odvody
and Frederiksen 1984a).
The use of metalaxyl to control SDM in forage
sorghum should be avoided. Conidial infection
of the regrowth after each cutting provides
an opportunity for large increases of oospore inoculum
of any metalaxyi-resistant biotype present.
Foliar application of metalaxyl to plants
systemically infected with P. sorghi should be
avoided, because it would select resistant strains
of the pathogen.
A metalaxyi-resistant biotype of P. sorghi
would complicate continued chemical control of
SDM, because no equally effective fungicide is
available. The pathogen is sensitive to other
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