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PATHOLOGY
Resistance to Stunt Disease
(A Cooperative Study with INIA)
To build up populations with a high degree
of resistance to the stunt disease, a new
cycle of recombination of previously selected
materials was planted early in 1969, at Cotaxtla,
Veracruz, Mexico.
A total of 238 selected families were grown
in a triple lattice experi.ment; with one replicate
in an isolated field employing the modified
ear-to-row technique used for the improvement
of the level of resistance and yield of this
population.
Of the 238 families tested, 136 came from
the composite "Republica Dominicana", and
102 from the composite Cubano x Rep. Dominicana".
The new population derived from a
recombination of the best 20 percent of these
238 families will be known as Composite A.
Three commercial double crosses and the
original population were included as controls
in the test. The results show an average reduction
of 16 percent and 8 percent in stunt
index for the selected families, when compared
with the average for the controls and
the original population, respectively. In yield,
there was an average gain of 7.1 percent over
the original population (no significant difference
with the mean of the hybrids). In number
of ears per plant, the selected families
showed an average gain of 13 percent over
the original population and the hybrids. Maturity
was two percent later than the original
population, and three percent earlier than the
mean of the commercial hybrids.
In another attempt to isolate sources of
resistance to the corn stunt disease, a series
of 8Q lines were selected from among highly
susc~ptible and highly toLerant 8 1 lines from
the composite "Cubano x Rep. Dominicana",
These 8:! lines were inoculater by means of
infectious leaf hoppers Dalbulus maidis, with
three insects allowed to feed for 48 hours on
each seedling. The most resistant and susceptible
plants were then selfed to advance
the lines to 8;j. The cycle was repeated to
obtain highly homozygous lines (resistant and
susceptible) for future studies on the inheritance
to the virus disease.
The resulting resistant and susceptible lines
were inoculated in the manner described
earlier. The findings indicate some breakdown
in resistance in the 8 3 generation; within
some lines which were highly resistant in
both 8 1 and 8 2 ,
The highly resistant 8 3 Jines are now being
used in a study to determine whether the
resistance could be due to a resistance to the
insect vector, rather than to the virus itself.
The most resistant and susceptible 8 3 lines
have been advanced to the 8. and a composite
made up of 25 of the more resistant lines.
A preliminary screening of 125 8 1 ' lines
from the Cuban Composite, artificially inoculated,
indicate that some of these lines may
prove to be good sources of resistance to this
disease.
Ear Rot Resistance
In a search for sources of resistance to the
ear rot caused by Diplodia maydis, 140 varieties
were tested at five locations in the tropics
of Mexico where the disease is endemic.
At one location, several hundred plants
were artificially inoculated, and selfed. 8ixty
five of the resistant 8 1 lines were recombined
(C 1 ) using a modification of the ear-te-row
technique. The female rows were again inoculated,
as were several hundred plants in the
male rows which, in turn, were selfed to develop
8 2 lines.
In the summer of 1969, a new cycle (C 2 )
was planted for recombination among the nine
most highly resistant 8 1 lines. The 8 2 lines
also were planted and artificially inoculated.
The relative gains in resistance (one year's
data) are shown in Fig. 1 where the mean
of the Co (Composite of 32 varieties), 8 1 's
(balanced composites of resistant 8 1 lines),
and C 1 (first cycle of recombination of selected
8 1 lines) are compared.
%
Disease
Free
90
80
70
60
50 ~---t----+--I------
Co s,'s C 1
Cycles of Selection
The average gain in resistance can be
estimated as 15 percent above the original
popUlation, which, as expected, showed a
relatively high degree of resistance.
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