What is it?
WSMV is an important disease of wheat. It is known to exist in many parts of North America, Europe and the Middle East. In 2003 it was found in SA, VIC, NSW and QLD, followed by WA in 2006. Testing in mid-2007 by the Department confirmed its presence in Tasmania.
How does it spread?
WSMV is primarily spread by its vector, the wheat curl mite, as it moves from infected to healthy plants within a crop while feeding. These mites are too small to be seen by the naked eye. They are dispersed by the wind or by attaching themselves temporarily to other insects, animals and humans.
Recent research has shown that the virus is seed-borne and this is the most likely explanation of how it entered Australia and then Tasmania. Transmission by seed is at very low levels and therefore is not considered a health risk to a current crop, but it is a source of virus that can build up, via the vector, and eventually find its way to volunteer cereals, weeds and grasses - thereby threatening future crops.
What does it look like?
Wheat streak mosaic virus in wheat
Photo: CSIRO Canberra
Note the 'mosaic' appearance
Photo: CSIRO Canberra
In wheat, the first signs are light green streaks in the leaves and then, as the disease progresses, these become yellow marks resembling a 'mosaic' look. These signs can vary somewhat in other susceptible plant species.
Normally, the temperature needs to be above 10o
C for the signs to show up, so the disease may not be visible during the winter months.
WSMV symptoms are very similar to those of other health problems, so laboratory diagnosis involving sensitive tests is needed for confirmation.
What plants are hosts?
WSMV, as the name suggests, is primarily a disease of wheat, but it can infect a range of other cereals and grasses. While these other species don't appear to be greatly affected by WSMV, they are significant as sources of virus and hosts of the vector. These other plants are:
- Cereals - oats, barley, maize, rye, sorghum, millet
- Grasses - brome, annual rye, wild oats, couch, phalaris, barley grass
How can WSMV be managed?
The disease is best managed by reducing the wheat curl mite population. The mite can only survive on live, green plants and depends on the presence of these green plants in cropping paddocks over the summer, the so-called "green bridge". The key step in breaking the disease cycle is therefore to destroy the "green bridge" that the wheat curl mite needs for survival over the summer. This entails controlling volunteer cereals and grasses between harvest and the next sowing. You should do this at least four weeks before sowing the new crop.
Grasses and grassy weeds around the edge of newly sown crops can also harbour the mites so these should be controlled as well.
Is WSMV serious?
Losses in diseased crops are typically 4% or less when management practices are in place.
Evidence from overseas, where the disease has been around longer, is that serious crop damage only occurs where there is heavy subsequent infection of early sown crops, and this only happens where there has been a "green bridge" to enable the wheat curl mite to survive in significant numbers from a previous crop.
In areas or seasons where the summer is dry and the sowing break is late, there might be little or no impact.
Tasmania is considered at risk because our summers are milder and moister than cropping areas of the mainland and, therefore, a greater risk of the "green bridge".
There were serious epidemics of WSMV in wheat crops in NSW in 2005 and 2006 causing crop failure of 5,000 and 20,000 hectares respectively.
Is it a notifiable disease?
All State governments are developing a better understanding of this disease and its incidence around Australia. If you think you have WSMV, you are encouraged to take suspect plants to NRE Tas for identification and advice. It's important you place any suspect plants in a sealed bag to prevent spread of the disease.