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September 2017
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Wheat rusts continue to evolve:
New leaf rust races detected in Western Cape
L
eaf rust, stem rust and stripe rust are important fungal
diseases of wheat in South Africa. Stem rust and leaf rust
commonly occur on dryland wheat in the winter rainfall
wheat growing regions such as the Western Cape and in
areas where wheat is grown under irrigation. Stripe rust is more im-
portant in the cooler wheat production areas.
Any of these three types of rust can cause more than 50% yield
loss on susceptible cultivars under favourable weather conditions
and early onset of disease.
Wheat rusts are usually controlled using resistant cultivars. The
main challenge in using resistance is that new strains (races) of
wheat rusts have the tendency to frequently evolve and overcome
resistance in existing cultivars, especially when cultivar resistance
is based on a single gene. New rust races develop mostly through
mutation of local races, but data generated with molecular mark-
ers strongly indicate that in South Africa, introduction of new and
aggressive races from neighbouring countries occurs. The stripe
rust pathogen, for example, established itself in South Africa after
introduction in 1996.
In an effort to deal with this continual evolution of rust pathogens,
breeders and pathologists at ARC-Small Grain and the University
of the Free State (UFS) have been continuously monitoring wheat
rusts during the past three decades. This has helped in the timely
detection of new races, determining their impact on commercial
cultivars and advanced breeding lines and in the identification of ef-
fective resistance sources.
This information is valuable to wheat breeders as they need to
use the best resistance sources against the most virulent races in
their programmes. Surveys, as part of ongoing surveillances, were
conducted during the 2016/2017 season to determine the identity
and frequency of stem rust, leaf rust and stripe rust races in
South Africa.
The surveys covered over 17 experimental sites and 56 commercial
wheat fields across different localities in the major wheat growing
regions of South Africa (Free State, KwaZulu-Natal, Western Cape
and Eastern Cape). Races of the respective rusts were identified
based on their virulence profile on standard differential wheat lines.
Leaf rust was observed at all experimental sites visited in the West-
ern Cape where up to 70% severity was recorded at some of the
localities. Only about 55% of the producers’ fields surveyed in the
Western Cape showed signs of leaf rust and infections in those
fields did not exceed 10% of leaf tissue infected.
Leaf rust was detected only in three out of seven experimental sites
monitored in the Free State (Bethlehem, Meets and Reitz). Many en-
tries in Bethlehem had up to 20% severity, whereas most entries in
Reitz and Meets had <10% severity. In KwaZulu-Natal, low infection
rates of leaf rust were seen late in the season at Bergville.
Similar to leaf rust, stem rust occurred at all experimental sites in
the Western Cape, but a significant level of infection only occurred
late in the season after the soft dough stage of crop development.
Out of 56 producers’ fields surveyed in the Western Cape, seven
showed stem rust infections. Stem rust was found also in Free State
at one experimental site west of Bethlehem. There was no stem rust
at any of the localities surveyed in KwaZulu-Natal. The lower levels
of leaf- and stem rust in producers’ fields compared to experimental
plots can be attributed to the application of fungicides by commer-
cial wheat growers.
Stripe rust was detected in most localities surveyed in the Free State
(Bethlehem, Clarens, Ficksburg, Meets and Reitz), with infection lev-
els mostly being higher than 70%. This high stripe rust infection can
be attributed to the moist conditions (rainfall) experienced towards
the end of the 2016 wheat season in the Eastern Free State. No stripe
rust was found in any of the localities visited in the Western Cape
and KwaZulu-Natal during 2016.
Generally, the severity of wheat rusts in the major wheat growing
regions of South Africa appeared to be higher in 2016 than in 2015.
Although factors such as prevailing environmental conditions, culti-
var susceptibility, and timeous application of chemicals by produc-
ers play a significant role in rust occurrence, the increase in rust
prevalence, especially leaf rust in the Western Cape, will have to be
closely monitored in the 2017 season.
According to the results of rust race analyses, a few leaf rust isolates
collected in 2016 from three localities in the Western Cape (De Vlei,
Riviersonderend and Piketberg) showed virulence profiles different
from races previously reported in South Africa. Therefore, these
isolates were considered as belonging to a new race, designated as
3SA10.
In addition, four leaf rust isolates collected from two localities (Bre-
dasdorp and Riversdal) in the Western Cape were pathotyped to
another new race named 3SA38. Researchers from UFS have also
received leaf rust samples from Napier in the Western Cape dur-
ing 2016 and identified three new leaf rust races of which two had
the same virulence as 3SA10 and 3SA38 and the third was named
UVPt26.
Except for increased virulence on the leaf rust resistance genes
Lr24
and
Lr20
, the new races appeared similar to 3SA145, a race that
was first detected in the Western Cape in 2009 and eventually domi-
nated the leaf rust population in South Africa (
Photo 1
) on page 79.
Scientists at UFS are currently busy with DNA marker work to deter-
mine whether the new races can indeed be grouped with 3SA145.
Subsequent seedling screening of commercial wheat cultivars and
advanced breeding lines from all three breeding programmes in
South Africa, have confirmed that UVPt26 is the most threatening of
the new leaf rust races, especially when compared to 3SA145.
Spring wheat entries were more affected by the new races with
10% of entries found susceptible to UVPt26 as compared to 3SA145.
Field evaluation of commercial cultivars and advanced breeding
lines will be performed in the coming season and the leaf rust rat-
ing of cultivars affected, based on their adult plant field reaction,
will be adjusted for inclusion in the 2018 production guidelines of
ARC-Small Grain. In the meantime, wheat producers need to monitor
the rust situation in their wheat fields during 2017 and the following
seasons as the new leaf rust races could result in increased rust in-
fection on susceptible cultivars.
FOCUS
Integrated pest control
Special
DRS TAREKEGN TEREFE,
ARC-Small Grain,
Bethlehem ,
WILLEM BOSHOFF
and
PROF ZAKKIE PRETORIUS,
University of the Free State