Does grazing of infected wheat by sheep result in salivary transmission of Wheat streak mosaic virus?

Abstract

Research is reported probing the concern of some wheat producers that grazing of early sown, dual-purpose wheat for winter forage may accentuate the spread of Wheat streak mosaic virus (WSMV). In experiments with housed sheep, we investigated whether there were any grounds for this concern. In the first experiment, sheep were allowed to graze heavily virus-infected wheat in trays, followed over a period of 24 h by a series of test trays of healthy wheat. The grazed plants were allowed to recover and new leaves were tested for symptoms and the presence of virus. In total, 2352 test plants were negative for WSMV, assessed through symptoms, ELISA, and RT-PCR. In the second experiment, no WSMV particles (assayed with ELISA) or RNA (assayed by RT-PCR) were detected in any saliva samples collected from sheep 0.5, 7.5, and 24.5 h after being fed heavily virus-infected wheat. Furthermore, these saliva samples, when inoculated onto test wheat seedlings under optimal conditions, failed to transmit the virus. In a third experiment we showed that the urea concentration in sheep saliva is at least two orders of magnitude lower than that required to render WSMV non-infective, and therefore is not responsible for the failure of sheep to transmit the virus. Our data provide no support for the suggestion that grazing sheep spread the WSMV between plants in a grazed wheat crop as a consequence of the grazing process itself.