Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

and Yr27, the seedling data may not be correct, as in field trials cv. Kauz had
shown a high level of resistance. Kauz also carries Yr9 and Yr27 in
combination. Multiplication of a breeding line derived from a cross of Kauz
and Opata 85, and hence carrying Yr27, was suspended in 2002 due to its
susceptibility, indicating that virulence for Yr27 should be present in Iran but
not in combination with Yr9. Kauz has remained resistant in West and Central
Asia; however, during 2003 we observed stripe rust on a Kauz plot in the
International Disease Trap Nursery planted in Kyrgyzstan, indicating that the
combination of Yr9 and Yr27 is not effective in the area. A different selection of
Attila that does not carry Yr9 has shown moderate susceptibility in Tajikistan,
indicating virulence for Yr27. Combined virulences for the Yr9 and Yr27 genes
have been present in Mexico since 1996, and also occur in Ecuador.
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Incorporation of durable resistance to stripe and leaf rusts
in PBW 343 and Inqilab 92
Durable resistance to stripe rust and leaf rust are based on the interaction of
minor, slow-rusting resistance genes that have small to intermediate, but
additive, effects. If we look at the disease progressions for susceptible and
slow-rusting cultivars (Figure 1), we find that as more minor genes are
accumulated in a line the progress of rust becomes slower. Accumulating 4 to 5
such minor genes leads to near-immunity in most environments (Singh,
Huerta-Espino and Rajaram, 2000). Following the observation that two megacultivars—PBW
343 and Inqilab 91—are moderately susceptible in Mexico, in
1999 we initiated incorporating durable resistance genes into these two
important cultivars using the single-backcross selected-bulk breeding
approach described below.
% Stripe or leaf rust
00
0
10 20 30 40 50
Days data recorded
Susceptible
1 to 2 minor genes
2 to 3 minor genes
4 to 5 minor genes
Figure 1. Relationship between the progress of stripe or leaf rust and the number of
minor genes present in a wheat cultivar.