ICARDA annual report 2004

ICARDA Annual Report 2004
32
stored for further Amplified
Fragment Length Polymorphism
(AFLP) analysis. DNA fingerprints
were produced by up to 60 primer
combinations, resulting in more
than 100 polymorphic DNA fragments
or markers. This allowed
each isolate to be characterized
with a high degree of confidence.
Only two pathotypes were
found in 2002. These differed
slightly in their ability to infect the
range of host varieties used in the
tests. Both pathotypes were also
found in 2003, in addition to a third
which occurred at a low frequency.
One of the pathotypes displayed
a number of virulence traits commonly
found in Mediterranean isolates;
the other two shared a previously-undescribed
virulence phenotype.
There was no obvious
aggregation of specific pathotypes
on specific wheat varieties, suggesting
that most wheat varieties in
Eritrea lack yellow-rust resistance.
However, the resistance of Eritrean
wheat varieties has not yet been
formally tested. The results indicate
that resistance breeding programs
could greatly improve the control
of yellow rust in wheat in Eritrea.
Understanding the evolution
of the barley scald
pathogen
Scald, caused by the fungus
Rhynchosporium secalis, is an economically
important disease of barley.
Cultivated barley is, genetically,
extremely vulnerable to scald and
breeding for scald resistance is difficult
because the fungus evolves
quickly. In addition, the climatic
conditions and cultural practices
found in many barley-growing
areas encourage scald development.
Knowledge of R. secalis’s evolutionary
potential is vital to the
development of sustainable resistance
breeding strategies. An under-
standing of its genetic structure has
provided useful insights into the
evolutionary processes that affect
R. secalis population genetics.
However, specific hypotheses
about the pathogen’s evolution
have not been tested in controlled
and repeatable field experiments.
In collaboration with the
Phytopathology Group at ETH-
Zurich, ICARDA is using an innovative,
replicated, mark–release– recapture
field experiment to quantify the
relative impacts of sexual reproduction,
asexual propagation, immigration,
and selection on the genetic
structure of an experimental random
population of R. secalis in Syria.
More than 1500 scald isolates collected
from field plots were assayed
for eight individual microsatellite
loci (Fig. 10). Researchers found
eight isolates that differed markedly
in their ability to infect, compete
and reproduce on individual barley
genotypes and barley mixtures.
Significant differences were
observed in the frequencies with
which the inoculants were found,
over time, on the different hosts
tested. In addition, a considerable
number of new R. secalis genotypes
were found, based on their multilocus
haplotype.
These results showed that use of
the mark–release–recapture strate-
Fig. 10. Chromatogram of a representative Genescan gel illustrating microsatellite
DNA fragment analysis of R. secalis.
Culturing scald in order to
understand pathotypic and
genotypic relationships.