Faba bean viruses - Disease threat needs constant vigilance

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 by viruses.

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 percent infection.

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 pea.

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 craccivora).

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 remain necessary.

More common than expected: Faba bean damaged by subterranean clover stunt virus.

For more information:
Joop van Leur, NSW Agriculture, joop.vanleur@agric.nsw.gov.au
Angela Freeman, Department of Primary Industries Victoria, angela.freeman@nre.vic.gov.au
Safaa Kumari, International Center for Agricultural Research in the Dry Areas, s.kumari@cgiar.org.