Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

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The segregation ratios of the hybrids varied according to parental genotype
(Tables 1 and 2).
Table1. Segregation ratio in F 2 hybrids
Hybrid
Estimated no. of hybrids
Ratio
R MR MS S R:MR:MS:S R:S
Ravi × Sanzar 8 (R/MS) 17 22 20 41 3:3:3:7 7:9
CIMMYTUZ × Sanzar 8 (R/MS) 5 30 45 20 1:6:6:3 7:9
Yr7 × Sanzar 8 (R/MS) 5 25 30 40 1:3:6:6 4:12
F19 × Sanzar 8 (R/MS) 14 3 5 78 2:1:1:12 3:13
F15 × Saykhun (R/R) 15 13 17 55 1:3:3:9 4:12
F34 × Saykhun (R/R) 6 27 30 37 1:3:6:6 4:12
F19 × Marjon (R/S) 100
The yellow rust resistance screening included samples from different
germplasm sources and nurseries. We made crosses between entries from
various CIMMYT germplasm nurseries (ESWYT, HTWYT, SAWYT, YR/LR).
These nurseries focus on different traits, such as high yield (ESWYT),
resistance to high temperature (HTWYT), or resistance to yellow and leaf rusts.
In Table 1, parents Ravi, CIMMYT UZ, Yr7 are from the LR/LF nursery, and in
combinations with Sanzar 8, Ravi×Sanzar 8 (R/MS), CIMMYT UZ×Sanzar 8
(R/MS), Yr7×Sanzar 8 (R/MS), segregation ratios were 7:9, 7:9 and 4:12,
respectively. In combinations of Sanzar with parents from another nursery, the
F19×Sanzar 8 (R/MS), F15×Saykhun (R/R), F34×Saykhun (R/R) ratios were
3:13, 4:12 and 4:12, respectively. From the F19×Marjon (R/S) cross all hybrids
were susceptible, with the ratio of susceptible hybrids increased over
combinations between Sanzar 8 and YR/LR nursery varieties. So, genotypes
with certain traits inherited resistance traits with specificity according to their
trait. Here, certain traits may influence resistance negatively or positively. We
can conclude that in inheritance of yellow rust resistance the role of interaction
between genes is significant.
Usually in traditional resistance breeding to get a resistant hybrid one
backcrosses to the resistant parent. We tested this with susceptible parents.
Interesting results were observed after backcrossing hybrids with
susceptible parent Sanzar 8. Resistant variety Ravi was used as a resistant
parent. Backcrossing hybrids Ravi×Sanzar 8, CIMMYT UZ×Sanzar 8 again
with Sanzar 8, the number of resistant hybrids with durable resistance
increased instead of the expected susceptible hybrids. From Table 2 it appears
that in combinations Ravi×Sanzar 8, CIMMYT UZ× Sanzar 8 and
YR7×Sanzar 8, the ratio of the resistant and susceptible hybrids are 6:10, 7:9
and 4:12, respectively, and here the quantity of susceptible hybrids is almost