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Wheat streak mosaic virus and the wheat curl mite

Wheat streak mosaic virus (WSMV) affected around 5000 ha of wheat in the high rainfall zone of NSW in 2005, with new outbreaks also detected throughout the growing season in 2006. Agronomist, Paul Parker (NSW DPI), says growers should again be on the lookout for the virus. Areas most at risk include wheat crops in the slopes regions of NSW, particularly paddocks with a history of WSMV.

Plants infected with WSMV initially have light green streaks on the leaves which later develop into yellow stripes running parallel to the leaf veins. These symptoms can sometimes be confused with nutritional, environmental and chemical damage. Affected plants can die prematurely, become stunted or fail to grow. Heads on infected plants can be sterile and contain no seed, or can contain small shrivelled grain.

The vector for WSMV is the wheat curl mite (Aceria tosichella). Adult mites are wingless, cigar-shaped, about 0.2 mm long and have two pairs of legs located at the front. The mite’s small size and secretive habits make it extremely difficult to detect even with a microscope. The wheat curl mite can directly damage young growth through feeding, and is also responsible for the transmission of another important disease of wheat, the high plains virus.

Chemical control of the wheat curl mite is believed to be largely ineffective as they live (and are protected) within leaf whorls. The mites typically colonise the youngest tissue of a wheat plant and acquire WSMV when feeding on infected plants. They are widely distributed in south-eastern Australia and can survive on plants other than cereals. Alternate hosts identified in Australia include barley grass, great brome, annual ryegrass, cocksfoot, black oats, prairie grass, hairy panic, soft brome, wild oats, winter grass and rat’s tail fescue. At this stage, controlling these host plants (which can provide a “green bridge” between seasons) is likely to be one of the most practical methods to reduce the build-up of mite numbers and the risk of WSMV.

In areas that are at risk of WSMV, it is recommended that growers sow diversified crops. Although oats, barley and triticale can be infected with WSMV, they are usually not severely affected. Growers should also consider which paddocks are high risk and low risk. High risk paddocks would be those with a history of WSMV or those paddocks located near an infected crop.

Last season, symptoms were apparent from early-winter onwards. It is unclear whether the mild weather conditions experienced this autumn will favour the build-up of mite numbers and place crops at greater risk. If you observe any symptoms that resemble WSMV, contact your local agronomist.

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