Research Highlights of the CIMMYT Wheat Program 1999-2000

Durable Resistance to
Yellow (Stripe) Rust in Wheat
R.P. Singh, S. Rajaram, J. Huerta-Espino, and M. William
Yellow (or stripe) rust, caused by Puccinia
striiformis tritici, is an important disease of wheat in
most cool wheat-producing regions. Using resistant
cultivars is the best disease control strategy, since it
comes at no extra cost to the farmer and is
environmentally safe. Historically, race-specific
major genes have been used to breed rust resistant
wheat cultivars. At present 30 resistance genes
have been catalogued (McIntosh et al., 1998). Most
are race-specific in nature, and virulence for several
of them has been identified somewhere in the
world. One of the main objectives of CIMMYT’s
wheat improvement program is to generate
genetically diverse germplasm that has high yield
potential, wide adaptation, and durable resistance
to important diseases such as the rusts.
Durable resistance as defined by Johnson (1978) is
one that has remained effective in a cultivar during
its widespread cultivation for a long sequence of
generations or period of time in an environment
favorable to a disease or pest. Johnson (1978; 1988)
described the presence of durable resistance in
some European wheat cultivars and indicated that
such resistance is quantitative in nature. The
moderate level of adult plant resistance of
CIMMYT-derived North American cultivar ‘Anza’
and good level of resistance in Mexican cultivar
‘Pavon 76’ have shown durability (Rajaram et al.,
1988). During the early 1990s research was initiated
to identify CIMMYT wheats that may carry
durable resistance, understand the genetic basis of
such resistance, and develop selection strategies to
breed such resistance in newer CIMMYT wheats.
Key knowledge developed during the last decade
of research is summarized here.
Yr18 and other minor genes for durable
resistance to stripe rust
Independent results obtained by Singh (1992) and
McIntosh (1992) indicated that the moderate level
of durable adult plant resistance in Anza and
winter wheats such as Bezostaja is controlled in
part by the Yr18 gene. Gene Yr18 is completely
linked with gene Lr34, known to confer durable leaf
rust resistance. The level of resistance it confers is
usually not adequate when present alone. However,
combinations of Yr18 and 2-4 additional slow
rusting genes result in adequate resistance levels in
most environments (Singh and Rajaram, 1994).
Results of genetic analyses indicate that the level of
resistance increases with the increase in the number
of these genes that individually have minor to
intermediate but additive effects. Cultivars carrying
such Yr18 complexes are listed in Table 1.
Genes Lr34 and Yr18 occur frequently in
germplasm developed at CIMMYT and in various
other countries. Using Jupateco 73 near-isogenic
Table 1. Seedling susceptible bread wheats that carry good
adult plant resistance to stripe rust in field trials in Mexico
and other countries.
Usual yellow Additive for
Genotype(s) rust response 1 genes 2 resistance
Jupateco 73S (check) 100 MS Moderately susceptible
Jupateco 73R 50 M Yr18
Parula, Cook, Trap 15 M Yr18 + 2 genes
Tonichi 81, Sonoita 81, Yaco 10 M Yr18 + 2 or 3 genes
Chapio, Tukuru, Kukuna, Vivitsi 1 M Yr18 + 3 or 4 genes
Amadina 30 M 3 genes
Pavon 76, Attila 20 M 3 genes
1
The yellow rust response data from Mexico have two components: % severity based
on the modified Cobb scale (Peterson et al., 1948) and reaction based on Roelfs et al.
(1992). The reactions are M = moderately resistant to moderately susceptible,
sporulating stripes with necrosis and chlorosis; and S = sporulating stripes without
chlorosis or necrosis.
2
Minimum number estimated from genetic analysis.
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