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utilized to evaluate the 550 lines of the 1969
A CIMMYT Screening Nursery. Their pathogenecity
is described below in Table W7.
With the help of these strains all the lines
of the Screening Nursery were grouped into
5 categories:
1) Resistant to all races. Thirty seven lines
of this group are given in Table W8. It is
suggested that these lines carry either an additional
gene or a combinaion of genes or both.
They are a good source of resistance to stem
rust; 2) Lines with the genes Sr 6 and Sr 11
with or without minor genes; 3) Lines with
the gene Sr 11 with or without minor genes;
4) Lines with the resistance similar to that
found in Sonora 64, and 5) Lines susceptible
to at least 5 strains of stem rust.
All the 550 lines were raised to adult stage
and inoculated with two strains 15-2, 4, 7 (#
13-1), and 151- 1,2, 3, 5 (# 64) separately by
spraying. All the lines which were resistant
to these races in the seedling stage were also
found to be resistant in the adult stage. However,
21 lines which were classified as susceptible
in seedling to both the races, turned
up resistant to both in adult stage, and the
results are presented in Table W9.
Twenty eight (28) lines included in multiplication
plots in 1969-1970 were screened for
resistance to 5 stem rust strains (15-2, 4, 7;
151-1, 2, 3, 5; 158; 12-1, 3, 5, 6; and 12-1, 3,
4, 5) and the results are given in Table W10,
together with adult plant reaction to 15-2, 4, 7
and 151-1, 2, 3, 5. The lines which are susceptible
in the seedling stage to the later
mentioned races are resistant in adult plant
stage to the same races, cfnd were found also
resistant to the most prevalent races in Sonora
1969-1970 in field condition.
Resistance in the Durum Breeding Program
Apparently the materials in the Crossing
Block and Durum Screening Nursery are highly
resistant to stem rust. This may be more apparent
than real. Since most of the "wild"
inoculum of stem rust is produced on bread
wheat varieties, it is likely that these races
will generally not be pathogenic on the durums.
Consequently, races virulent on durum constitute
only a small percentage of the total wild
rust population. They are consequently exerting
very little selection pressure in the durum
breeding nurseries in Mexico. Therefore, unless
adequate and proper testing techniques
are used now whereby the various sources of
resistance are identified and recombined, the
new durum wheat varieties are likely to meet
the same fate as has too often happened to
the bread wheat varieties in the past once
they are in large scale commercial production.
Recently an analysis has been made using
aces 15-2, 3, 4, 7 (221-1); 12-1,3,5,6; (15B);
High yielding triple dwarf wheats, 26-30 inches
in height, have been developed and will 500n be in
large scale commercial production among farmers.
TABLE W7. Mexican Strains of Stem Rust Used to
Evaluate the Resistanc of the Wheat Lines Included
in the Second 1969 A CIMMYT Bread Wheat Screening
Nursery.
2 3 4 5 6 7
~ IV
5train Race .s~, 5rll 5r9b SrTt 5r8 :iii E
1j ::>
III >-
# 13-1 12-1,3,4,5,6 R R R R R R
# 13-1 15-2,4,7 R 5 R 5 R R
# 64 151:1,2,3,5 5 5 5 R 5 R R
# 221 15B R 5 R 5 R R
# 158 12·1,3,4,5,6 S R 5 5 5 5 ~
#7 12-1,3,4,5 S R S S 5 R R
# 122 12.1,2,3,5 S 5 5 R S R R
"