Sorghum Diseases in India

higher densities and apply fertilizer. Otherwise,
yields of the improved varieties will not be
higher than the maicillos they are to replace. Under
such conditions, as in the experimental
plots, improved cultivars will have more disease
problems with gray leaf spot and zonate leaf
spot than do the landrace cultivars. If a farmer
sows susceptible materials year after year in the
same field, it may eventually lead to increased
initial inoculum levels, creating potentially serious
problems.
It was not possible to test the reaction of improved
cultivars to oval leaf spot, so it is not
known if they possess adequate levels of resistance,
or if the absence of this disease in the
improved cultivars sampled is due to other reasons.
However, 2 of 16 maicillos tested were resistant
to oval leaf spot (Table 8). Until resistant
materials are identified, these can be used in
areas where oval leaf spot is likely to be detrimental
to production.
Table 8. Sorghum landrace cultivars resistant
to oval leaf spot at Santa Irene, Choluteca, the
Honduras, 1984.
Cultivar Source
San Miguel 1
MC3
CENTA
RRNN
Mean severity
(%)
3.04
5.46
If it were true that the improved varieties
now being used do possess resistance to both
AW and oval leaf spot, they could eventually
become reinforced with gray and zonate leaf
spot resistance; and step by step, yield losses
caused by several diseases can be prevented and
transformed into higher yields.
Sources of resistance were found for AW,
gray leaf spot, SDM, oval leaf spot, and zonate
leaf spot. The variety Sureno (VG 146), already
released, was found resistant to SDM and to
gray leaf spot. The maicillos San Miguel 1, Las
Lajas, and Pom pom were resistant to both species
of Cercospora. In addition, San Miguel 1 was
resistant to oval leaf spot. These materials can be
recommended for use where these diseases may
be a problem. In the future these and other resistance
sources may serve as donors for breeding
programs designed to incorporate this resistance
into superior cultivars.
Currently, it is feasible to grow maicillos, improved
varieties, or hybrids already released
that possess disease resistance as a means of reducing
losses. In SDM areas, for instance, the
hybrid Catracho and the improved varieties
Sureno and Dorado can be recommended; the
maicillos MC 97, MC 177, MC 122, and MC 136
could also be recommended for farmers who
prefer to sow the maicillos, particularly in sorghum-maize
intercropping.
The sorghum accessions that were resistant to
SDM in Comayagua should be evaluated for
their reaction to downy mildew in El Paraiso.
Two isolates of Peronosclerospora sorghi, from Comayagua
and from El Paraiso, were shown to
have differing virulence (Pawar et al. 1985).
More recently, different pathotypes have been
observed to occur in Comayagua (Fernandez
and Meckenstock 1987). Therefore, it is important
that SDM resistance-screening in the Honduras
be carried out in both Comayagua and El
Paraiso. When sources of resistance at both locations
are identified, they can be used to incorporate
resistance into future releases—improved
varieties and hybrids or landraces—by backcrossing
and retesting at both locations. One can
also use sources of resistance identified elsewhere,
such as any of the 10 cultivars found
resistant to isolates from different parts of the
world (Pawar et al. 1985). However, those sorghum
materials already adapted to climatic conditions
in the Honduras would be the most
desirable sources of resistance.
Ideally, different sources of resistance should
be used. Relying on resistance from the same
genetic background for all varieties and hybrids
must be avoided. Several approaches to the production
of reliable SDM-resistant cultivars are
possible. Various approaches of resistance could
be combined into one cultivar. A different resistance
source could be incorporated into each of
several cultivars grown in the same region. If
heterogeneity is acceptable in a cultivar, as it is
the case of maicillos, then various sources of disease
resistance can be introduced without having
to combine these in the same plants.
The fact that two resistance sources are of
different origin is no guarantee that they are
truly different. Mechanisms of resistance constitute
a better criterion for selecting truly different
sources of resistance. There are differences, for
MS