The epidemiology of Wheat streak mosaic virus in Australia: case histories, gradients, mite vectors, and alternative hosts.
Wheat streak mosaic virus (WSMV) infection and infestation with its wheat curl mite (WCM; Aceria tosichella) vector were investigated in wheat crops at two sites in the low-rainfall zone of the central grainbelt of south-west Australia. In the 2006 outbreak, after a preceding wet summer and autumn, high WCM populations and total infection with WSMV throughout a wheat crop were associated with presence of abundant grasses and self-sown 'volunteer' wheat plants before sowing the field that became affected. Wind strength and direction had a major effect on WSMV spread by WCM to neighbouring wheat crops, the virus being carried much further downwind than upwind by westerly frontal winds. Following a dry summer and autumn in 2007, together with control of grasses and volunteer cereals before sowing and use of a different seed stock, no WSMV or WCM were found in the following wheat crop within the previously affected area or elsewhere on the same farm. In the 2007 outbreak, where the preceding summer and autumn were wet, a 40% WSMV incidence and WCM numbers that reached 4800 mites/ear at the margin of the wheat crop were associated with abundant grasses and volunteer wheat plants in adjacent pasture. WSMV incidence and WCM populations declined rapidly with increasing distance from the affected pasture. Also, wheat plants that germinated early had higher WSMV infection incidences than those that germinated later. The alternative WSMV hosts identified at these sites were volunteer wheat, annual ryegrass (Lolium rigidum), barley grass (Hordeum sp.), and wild oats (Avena fatua). In surveys outside the growing season at or near these two sites or elsewhere in the grainbelt, small burr grass (Tragus australianus), stink grass (Eragrostis cilianensis), and witch grass (Panicum capillare) were identified as additional alternative hosts.