Effects of inoculation method on host plant susceptibility to WSMV infection

Controlling the spread of vector-transmitted viral disease requires an understanding of identity and relative impacts of alternate host species on disease risk.  Susceptibility to viral infection is an important component of the potential of a host to serve as a reservoir of disease.  Susceptibility is often estimated following mechanical inoculation, rather than vector inoculation with the assumption that the methods are equivalent.  We tested this assumption using Wheat streak mosaic virus (WSMV) and its vector Aceria tosichella, the wheat curl mite (Acari: Eriophyidae).  WSMV is the most common and damaging viral disease in cereals in the Great Plains and many grasses can serve as reservoirs for both vector and virus.  In a greenhouse experiment, we compared WSMV infection rates following mechanical and mite inoculation in three common weeds, Aegilops cylindrica, Bromus tectorum, and Setaria viridis, and barley and wheat cultivars that are resistant to mechanical inoculation.  In general, infection rates were higher following mite inoculation but differences between inoculations varied among species.  Setaria was immune to WSMV in both inoculation methods.  Infection rates and effects of inoculation methods were similar among wheat, barley and Aegilops.  On average, 8% of plants were infected following mechanical inoculation and risk of infection was four times higher (mean=32%, CI=26-38%) in mite inoculated plants.  The largest difference between inoculation methods was observed in Bromus.  This species was immune to mechanical inoculation but the most susceptible species to mite inoculation (mean infection rate=55%, CI=43-67%), demonstrating that mite vectors must be used to provide accurate estimates of susceptibility to WSMV.