Surveys for faba bean viruses in 2003 showed an unusually high incidence
of subterranean clover stunt virus (SCSV), a potentially serious disease.
With 11 viruses reported on faba bean in Australia, it is essential that
regular surveys are made to monitor virus incidence and to set priorities
for virus research.
Such surveys require diagnostic tests because different viruses can cause
similar symptoms. Also, nutrient deficiencies, herbicide damage and water
stress can produce symptoms almost indistinguishable from those caused
The 2003 survey was based on a large number of plants in random paddock
sampling, and testing for a wide range of viruses. In the southern region,
testing of samples by ELISA (enzyme-linked immunosorbent assay) was used,
but for the northern region (where virus incidences are generally higher)
plants were tested individually using Tissue Blot Immuno Assays (TBIA).
The TBIA membranes were processed in the virology laboratory of the International
Centre for Agricultural Research in the Dry Areas (ICARDA, Syria) as part
of a GRDC-funded collaborative project.
Preliminary reports show that both the magnitude of virus infection as
well as the range of viruses detected were larger than assumed. The most
frequently found – and likely most damaging – virus in 2003
was Bean yellow mosaic virus (BYMV), while incidences of the destructive
Bean leafroll virus (BLRV) were low.
The unique feature in 2003 was widespread infections by SCSV. On faba
bean this closely resembles BLRV; yellowing, thickening and rolling of
leaves and stunting of the plant – hence the need for serological
tests to differentiate.
However, the SCSV infection pattern was unlike that normally observed
for BLRV. While BLRV is usually randomly distributed through a field,
the SCSV-affected plants were clustered. This suggests that the aphid
vectors of BLRV fl y into the crop, feed and fl y off, whereas the SCSV
vectors fl y into the crop, feed and move to neighbouring plants.
This makes SCSV potentially more dangerous than BLRV at lower infection
levels. Single stunted plants can be overgrown by healthy plants, but
clusters of virus infected plants will remain a source of infection until
the end of the season.
SCSV infection on faba bean in 2003 appeared to be widespread in eastern
Australia. Incidences in the surveyed paddocks in northern New South Wales
were below one percent in all but a single paddock near Moree with nine
Although SCSV infection levels in most faba bean crops in Victoria were
below one percent, a number of paddocks around Horsham showed very high
incidences (20-80 percent) and severe yield losses (up to 100 percent).
SCSV is indigenous to Australia and has not been reported anywhere else
in the world. It is normally thought of as a destructive pathogen of subterranean
clover, but is known to severely affect faba bean, french bean and field
Studies during the 1970s on the epidemiology of this virus in subterranean
clover demonstrated huge year-to year fluctuations, caused largely by
the population dynamics of its main vector, the cowpea aphid (Aphis
Causes for the higher than normal infection levels in faba bean were
not investigated, but are likely related to the exceptionally high aphid
populations observed throughout eastern Australia early in the season.
Whether the infection originated from local sources or was the result
of large-distance aphid migration is not clear.
In Victoria, a build-up of aphids was observed on volunteer pulse crops.
Volunteers were abundant after heavy February 2003 rains, as many paddocks
were not harvested in 2002 due to the drought. Aphid populations moved
from the volunteers into newly sown pulse crops soon after germination,
and could have picked up SCSV from infected medic and/or clover paddocks.
SCSV-infected clover and medic plants were found in paddocks adjacent
to the severely infected faba bean crops in Victoria.
Comparing SCSV infection levels between varieties in yield trials in
an experimental site in northern NSW indicated the existence of large
genotypic differences. Established varieties like Fiord, Fiesta and Icarus
had around one percent infection levels, but some experimental lines showed
over 10 percent SCSV-infested plants.
The presence of highly susceptible germplasm among advanced breeding
lines is a reminder to breeding programs to continuously test material
not only for major but also for (perceived) minor pathogens.
Incidental epidemics of SCSV are likely to happen again and continuous
monitoring of viruses in experimental and commercial pulse crops will
More common than expected: Faba bean damaged by subterranean
clover stunt virus.
For more information:
Joop van Leur, NSW Agriculture, firstname.lastname@example.org
Angela Freeman, Department of Primary Industries Victoria, email@example.com
Safaa Kumari, International Center for Agricultural Research in the Dry
GRDC RESEARCH CODE DAV411, program 3