Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

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Adult plant resistance effective against new strains of wheat
stripe [yellow] rust
J. Sthapit and E.A. Milus
Department of Plant Pathology, University of Arkansas, Fayetteville, Arkansas, USA
Stripe [yellow] rust, caused by Puccinia striiformis West. f.sp. tritici, has been an
important disease of soft red winter wheat in eastern USA since 2000, when a
new strain of stripe rust with virulence on resistance gene Yr9 and enhanced
aggressiveness and adaptation to warmer temperature appeared. Although the
new strain was devastating when it first appeared in 2000, stripe rust severities
and losses have been low since 2006. Many cultivars and lines have low
severities even in inoculated and irrigated nurseries, indicating that these may
have effective adult-plant resistance (APR). The objective of this research was
to characterize APR to stripe rust in contemporary wheat cultivars and
breeding lines. Seedlings of 50 lines with low to moderate levels of stripe rust
in fields infected with the new strain were evaluated for resistance to races Pst-
3 (isolate AR90-01) and Pst-100 (isolate AR00-05) that are representative of the
old and new strains, respectively. Nineteen lines that were susceptible to both
races and one line with Yr9 that was susceptible to Pst-100 but resistant to Pst-3
at the seedling stage were selected for adult-plant experiments. To determine
the effect of race and post-inoculation temperature on the expression of APR,
the 20 lines (including susceptible and very susceptible checks) were
inoculated with each race at heading stage and incubated in growth chambers
at low (10 to 18ºC) and high (12 to 28ºC) temperatures, gradually changing
temperature regimes. To characterize resistance on flag and flag-1 leaves, the
time from inoculation to the first sporulation was recorded to estimate latent
period, and the infection type (0 to 9 scale) and percentage of leaf area diseased
were recorded 21 days after inoculation. The same 20 lines were planted in two
fields at University Farm in Fayetteville during October 2008. The
experimental design in each field was a randomized complete block with six
replications. At jointing stage, one field was inoculated with Pst-3, and the
other field was inoculated with Pst-100. Lines were rated several times for
infection type. Many infections on resistant lines produced few or no spores,
indicating that latent period may not be an appropriate variable for
characterizing such resistance, and no latent period results will be reported. In
the field, the susceptible checks had high infection types to both races; six lines
had higher infection types with race Pst-3 than with Pst-100, and the other lines
had low to intermediate infection types with both races. Under the low
temperature regime, both races produced high infection types on the
susceptible checks, and all except two lines had a higher range of infection