The Eleventh Regional Wheat Workshop For Eastern ... - Cimmyt

Assessment ofpathogenic variability in stripe rust - Wellings et al.
distinctive forma specialis, although the geographic distribution outside the USA remains
unclear.
A potentially new form of P. striiformis was described in Australia by WeI lings et al. (2000).
The pathogen was closely associated with weedy Hordeum species, showed broad avirulence
on the wheat differential testers and appeared to contrast at one isozyme locus with Pst. The
pathogen, temporarily referred to as barley grass stripe rust (BGYR), was observed to cause
disease on certain barley cultivars naturally infected in the field.
2. Pathotype variation within P. striijormis f. sp. tritici
Pathotype variability has traditionally been studied using defined sets of host genotypes
inoculated with pathogen isolates in controlled environments. Such studies have almost
entirely focussed on greenhouse testing of seedling plants and, when most effective, have
attempted to relate the results to resistance genes deployed in commercial agriculture. The
various sets of differential genotypes employed in the study of Pst were reviewed by
McIntosh et al., (1995). The set of Johnson et al. (1972), as modified by Wellings and
McIntosh (1990), have been used for continuing studies of pathotype variability in the cereal
rust laboratory at PBI Cobbitty since the first introduction of Pst to Australia in 1979. During
this period, pathotype evolution arising from progressive single step mutation has resulted in
the detection of over twenty variants. The results are summarized in Figures 1 and 2. While
the majority of new pathotypes have represented a progressive increase in virulence, several
reversions to avirulence have also been detected. With the exception of pathotype 64 El A-,
all new pathotypes have only varied by one gene for virulence compared to a previously
detected pathotype. On the basis of these observations, it can be concluded that there has
been no evidence of further exotic introduction ofPst into Australia.
Similar studies of Pst have been conducted by various groups around the world. The most
notable contribution was that of the late R.W. Stubbs at the Research Institute for Plant
Protection (IPO), Wageningen, The Netherlands. Although Stubbs work was focussed on
Europe, he was able to accept Pst isolates from around the world under an agreement with
CIMMYT, and so was able to gain some insight into global variability. Investigations of
recent stripe rust epidemics in Central Asia discovered that The Institute of Genetics,
Tashkent, Uzbekistan, has conducted similar Pst surveys for the former Soviet Union over
many years, although the data has remained essentially unavailable to the Western scientific
community.
As a result of the recurring epidemics noted in the above regions, and in the absence of a
consistent monitoring program for pathotype change in Pst over large areas of the world's
wheat growing areas, an alternative method employing near isogenic lines in trap nurseries is
being evaluated. This has become a joint project between CIMMYT, ICARDA and PBI
Cobbitty, with some funding provided by the Australian Center for International Agricultural
Research. The method provides advantages in being less reliant on specialist facilities and
expertise, and can potentially sample pathogen populations over the period that the nursery
remains viable in the field.
The nurseries distributed as four cohorts have resulted in the collection of 58 data sets.
Several of these collections represent multiple observations across several years at a single
site. Low disease responses on the recurrent parent Avocet S has indicated poor and uneven
infection or the presence of highly avirulent pathotypes at several sites. In these situations
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