Epidemiology of Wheat streak mosaic virus in wheat in a Mediterranean-type environment

Abstract

In a region of Australia with a Mediterranean-type climate Wheat streak mosaic virus (WSMV) infection of wheat occurs in localised epidemics following extended periods when no grass or cereal hosts are present. To achieve a greater understanding of the epidemiology of WSMV in this environment, diverse components of the disease cycle were examined, especially the importance of seed transmission in initiating epidemics. The role of seed transmission in providing WSMV inoculum sources for these epidemics was examined by determining WSMV seed transmission rates and susceptibilities in cereals, and in both annual and perennial grasses that occur locally. Also, examined were the patterns of spread of WSMV from simulated seed-borne infection sources external or internal to wheat plantings. When 11 wheat, 11 oat and seven barley cultivars, and six annual and five perennial grass species, were inoculated with WSMV, all cereal cultivars and annual grass species became infected. In contrast, none of the perennial grass species became infected despite inoculation by both infective sap and wheat curl mite (WCM, Aceria tosichella) vector. When seeds from all WSMV-infected plants from these inoculations to cereals and grasses were germinated, seed transmission was only found in wheat (0.03 to 0.1 %). These findings suggest seed transmission of WSMV is limited to wheat only. In three field studies involving wheat plantings where both simulated WSMV seed-borne infection in wheat and the WCM vector were present, WSMV infection was clustered in plants <4 m from infector plant infection sources. Regardless of whether WSMV infector plants were placed internally or externally, and killed by herbicide or left alive, there were no differences in WSMV spread to wheat plants. In addition, when internal WSMV infection sources were present, the rate and amount of WSMV spread in wheat were directly related to the number of infector plants introduced. Thus, when increasing levels of internal simulated seed-borne infection sources (0.1–1.4 %) were present final WSMV incidence increased (17–97 %). At a natural field infection site, after a long dry summer fallow, volunteer wheat seedlings emerged following autumn rains and were not removed before nine wheat cultivars were sown. All cultivars became infected (78–99 %) and seed transmission rates of 0.03–0.06 % were found in six cultivars. In a Mediterranean-type climate where there is an absence of WSMV infection sources during the hot, dry summer conditions between successive wheat crops, seed-infected wheat seedlings growing as volunteers or from sown infected wheat seed stocks play a critical role in initiating WSMV epidemics.