Damage from the wheat streak mosaic virus is widespread and severe in New South Wales (NSW) this season.  Plant pathologist with NSW Department of Primary Industries, Dr Gordon Murray, reports damage early in the season to crops in the north of the state and more recently in the Forbes, Orange and Cowra regions.

“Were it not for the severe drought conditions we’re experiencing this outbreak would be creating greater concern,” he says.

With the backing of the Grains Research and Development Corporation (GRDC) a team of scientists has been investigating the development of the virus disease and seeking ways to counter it.  NSW DPI plant pathologists and agronomists are working with the University of Melbourne and Charles Sturt University to better understand the disease.

“We’ve established the importance of a green summer bridge in proximity to a crop for the establishment of the disease,” Dr Murray said. “We are learning more about its spread both by the wheat curl mite and by grazing animals. While it’s vital to get rid of that green bridge, the timing of the removal can be critical in either stopping or actually hastening the spread of the disease.”

Dr Murray highlighted a situation in the Orange district this season where in adjacent paddocks, volunteer wheat plants were sprayed out before and after the crop was sown. Where the volunteers were removed before sowing the subsequent crop was disease free but its neighbour was heavily infected.

“The mites carrying the virus move away from a stressed plant,” Dr Murray said. ”Pretty obviously, spraying with glyphosate is a good way to stress a plant. In this case the mites simply moved onto the germinating crop.

“We’re also seeing severe infections in grazed crops and we’re investigating the role of animals in transmitting the disease.  We’ve established trials in the Galong and Coolah areas using seed we know to be contaminated by the virus, and we’re anxious to see whether or not the virus can be spread in the absence of mites by grazing animals.”

On a positive note Dr Murray said that while chemical treatments had proved ineffective against the mite at least some cereals show resistance to the disease. While resistance is present in oats, barley and triticale, only a few varieties of wheat and durum show any resistance. However, partial resistance may be available in these cereals.

“What we need to establish now is what form the resistance is taking, whether it is resistance to the virus or to the mite,” Dr Murray said. “That will provide a clue to further research directions.

“We really need to know what the primary source of inoculation is. Dr Roger Jones in Western Australia was the first person in the world to prove that the virus could be seed borne and the very rapid spread of the disease throughout the Australian wheat belt suggests that contaminated seed has been the primary cause. But, once it is in the district, is contaminated seed or the work of the mite more important?”

Offering some practical advice on control, Dr Murray said the presence of the over-summer green bridge was critical to the spread of the disease.

“That’s got to be knocked out before the crop is sown,” he said. “Removal after the crop has been sown will make the situation worse.

“Grain growers sowing an early crop for grazing face a difficult problem. They face possible contamination from green grass weeds along roadsides, drainage ditches and the like. These areas are outside their control but we know that mites cannot travel far and that the source of contamination must be close to the crop. Growers should take that into account when choosing paddocks for early sown crops.”