Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

336
factor is cultivation of moderately susceptible to susceptible cultivars over a
large area. The third factor is the pathogen and its pathogenicity.
The most common mechanisms driving evolution in the yellow rust
pathogen are mutation and migration, either natural or anthropogenic. For
example, the first appearance of yellow rust pathogen in Australia was in 1979,
probably due to human agency or migration (O’Brien et al., 1980). Virulence for
Yr27, which was widely deployed in cultivars selected from CIMMYT
materials, such as Attila 50 (Chamran), apparently spread rapidly through the
West Asia and North Africa (WANA) region and arrived in Iran in 2003
(Afshari, Torabi and Malihipour, 2004). Monitoring of pathogen virulence
factors and their changes provides basic information for the development of an
early warning system. To monitor yellow rust virulence and virulence changes,
this study was carried out at 28 sites in wheat growing areas in Iran.
Materials and methods
This study comprised 45 lines that included a standard set of stripe rust
differentials, supplemental lines, and near-isogenic lines developed by Dr
Wellings from Sydney University. Entries were sown in 2-m rows for each line
in cropping seasons 2004–2008 (Table 1). At the flag leaf stage, when the
infection and severity under natural infection on the susceptible control was
high, field assessments were done for disease severity according to the
modified Cobb scale of Peterson, Campbell and Hannah (1948) and disease
reaction according to Roelfs (1978).
Results and discussion
Over the five years of this study, stripe rust developed in most nurseries. The
results are summarized for four main regions of Iran (west, north, east and
central) and presented in Table 1. According to the results, virulence on Heines
Kolben (with gene Yr2), Kalyansona (Yr2), Lee (Yr7), Avocet R (YrA),
Federation*4/Kavkaz (Yr9), Yr6/6*Avocet “S”, Yr7/6*Avocet “S”,
Yr9/6*Avocet “S”, Yr17/6*Avocet “S” and TP 1295 (Yr25) and YrSU was
common during the study (Table 1). The frequencies of virulence on plants
with Yr2, Yr6, Yr7, Yr9 and YrA were up to 70%. Virulence for Yr1 was
common in Central Asia and China, rare in most parts of the Middle East and
absent in Iran (Afshari, unpublished). No virulence was observed on plants
with Yr1, Yr3, Yr4, Yr5, Yr10, Yr18, YrND or YrCV genes in our trap nurseries.
Ma and Singh (1996) noted that Yr18 might not provide adequate protection
when deployed alone in a susceptible background. According to them, the
preferred option for achieving durable stripe rust control is to have