REPORT - Search CIMMYT repository

sound scientific approach, incorporate and
utilize it in the breeding program.
Each season several hundred crosses are
made between superior CIMMYT dwarf phenotypes
and materials from the International
Spring Wheat Rust Nurseries (ISWRN) which
have been found to be not only resistant in
Mexico but also in several other parts of the
world. Mexico, and especially the summer
nursery site in Toluca, is ideal for screening
for stem rust and stripe rust and to a lesser
degree for leaf rust resistance. Although leaf
and glume blotch Septoria tritiei and Septoria
nodorum occur in Mexico, especially in the
summer Toluca nursery, it seldom reaches
epiphitotic proportions. Consequently, it is impossible
to select effectively for res'istance
under Mexican conditions. Other diseases,
such as scab(Fusarium or Giberella head-blight)
and powdery mildew (Erisyphe graminis var.
tritiei) are rare and never epidemic. In these
cases too it is necessary to bring heavy selection
pressure to bear on populations segregating
for resistance if resistant varieties are to
be ~eveloped. However, CIMMYT being an
international research institute, and directly
or indirectly cooperating with many countries,
is in a strong position to assist in the development
of lines or varieties combining resistance
to all of the aforementioned diseases.
Materials reported as being resistant to Septoria,
scab and mildew in national programs in
other parts of the world are crossed widely
with CIMMYT dwarfs in Mexico. The F 2 bulk
materials derived from these crosses are sent
for screening to cooperating countries in other
parts of the world. Seed of any outstanding
lines identified through these procedures again
are sent back to CIMMYT for further crossing
and up-grading of the gene pool.
Within the past year a CIMMYT Rust Research
Laboratory, under the direction of Dr.
S. Rajaram, has been established at Chapingo.
Research at this laboratory is done to identify
the genes for resistance in basic and advanced
generation lines· of bread and durum wheats
and in triticales. The data supplement those
from the field. The summarized data from the
past year appear below.
Identification of Races: Three hundred (300)
collections of stem rust were made from
Sonora, Toluca, Chapingo, Roque and Huamantia
areas during 1969 and these were identified
on the set of International Differentials.
The most prevalent major races found were 11,
12, 15, 113 and 151. These races are further
defined in terms of their pathogenecity genes
on genetic stocks.
In 1969, 112 genetic wheat stocks were received
from Dr. I. A. Watson (Australia) for use
in analyzing the Mexican stem rust flora as
a part of the International Virulence Gene
Survey. The results have given a good idea
about the effectiveness of the various stem
rust genes. On the basis of these tests 7 lines
with different genes are now used as supplemental
rust differentials. These genes are internationally
used in various breeding programs,
and listed as follows:
1) Sr 6 (Me Marachy); 2) Sr 11 (Yalta); 3) Sr 9b
(Gamenya); 4) Sr n. (Mengavi); 5) Sr 8 (Mentana);
6) Sr 17 (Selkirk), and 7) Yuma.
The definition of the pathogenecity of the
standard races, in terms of precise genes has
permitted a more conclusive and realistic analysis
of the CIMMYT breeding materials. It
has also made possible the detection of new
genes in the program.
Based on the analysis of stem rust races
from last summer's collection in Toluca, Chapingo,
Roque and Huamantla it is clear that
no single gene would give very effective protection.
The summarized results are given in
Table W6.
Six strains of stem rust having different
combinations of genes for pathogenecity were
TABLE W6. Analysis of the total stem rust collection
made from Toluca and Chapingo, Mexico,
during the summer of 1969, showing virulence in
relation to different tester genes, Mexican cultivars
and other resistant genotypes.
Me Murechy
V.It.
G.meny.
Mengevi
Ment.n.
Siete Cerros
Pitlc 62
Sonor.64
Tobarl66
Nor_Ie 66
Nortefio 67
Selkirk
Vuma
W.ldron
Chris
Crim
Genotype
Sr6
Sr11
Sr9b
SrTt
SrI
(Sr6 + Sr11)
(Sr11 +)
(Sr11 +)
(SrI + Sr9b +)
(Sr6 + Sr17)
Percent.ges of
Isol.tes Sh_ing
Virulence
(on seedlings)
57
51
66
33
11
32
71
22
1
o
1
o
44
22
65