Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

• To investigate the effectiveness of the functional Yr gene(s) over sites,
seasons and pathotypes.
• To gain knowledge regarding the diversity of the pathogen population and
its sources.
This paper focuses primarily on yellow rust monitoring studies in the
country, with special emphasis on changes in the effectiveness of Yr genes and
their utilization in the commercial wheat cultivars over space and time.
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Methodology
In order to investigate the virulence spectrum of yellow rust races and to
assess the effectiveness of different yellow rust resistant gene(s) in different
agro-climates, trap nurseries were planted in different wheat production
zones. The nurseries comprised isogenic and near-isogenic lines of Yr genes,
and old and current commercial cultivars. Furthermore, to monitor the status
countrywide, the wheat disease situation and susceptibility of commercially
grown wheat cultivars were assessed at research establishments (provincial
and federal) and in farmer fields throughout the country.
For the assessment of different rust-resistance genes and their effectiveness
over sites and seasons, and to understand the prevalence of pathotypes in
different areas, the composition of the trap nurseries is very important. For this
purpose, the nursery was constituted using World, European and Australian
stripe rust differential sets (Habgood, 1970; Johnson et al., 1972). Three testers,
viz. Oxlay (Yr6), a resistant selection of Avocet (YrA), and susceptible check
Corona W195, were used from Australian sources. In addition, various entries
were taken from the Central and West Asian Yellow Rust Trap Nursery
(CWAYRTN) to form a nursery consisting of 100 entries. During 2003/04 a
new Yr differential comprising lines in an Avocet background was added. The
purpose of this extended yellow rust nursery was to identify potential sources
of resistance from genotypes possessing similar Yr genes (Table 1)
The nursery was planted at selected locations throughout Pakistan
representing different agro-ecological zones and disease hot-spots, where the
conditions are favourable for rust development (Figure 1). Each entry of the
nursery was planted as a 1.5 m row, 30 cm apart. A single row of rustsusceptible
spreader cv. Morocco was planted after every 10 entries in the
nursery, and the entire nursery was bounded by two rows of cv. Morocco as
spreader. Observations were recorded at different crop stages upon natural
occurrence and first appearance of rust infection on susceptible checks, and
final observations were recorded after 100% rust infection on the spreader.
Observations on yellow rust reaction were recorded according to Loegering
(1969). The severity was recorded as percentage of rust infection on the