Ito, D., IVIiller, Z., Menaüed, F., Moffet, M., and Burrows, M. 2012. Re!ative susceptibiüty among a!temative host species preva!ent in the Great P!ains to Wheat streak mosaic virus. Plant Dis. 96:1185-1192.Wild grasses, crops, and grassy weeds are known to host Wheat streak mosaic virus (WSMV) and its vector, the wheat cur! mite (WCM). Their relative importance as a source of WSMV was evaluated. A survey of smaü-grain fie!ds throughout Montana was conducted between 2008 and 2009. Cheatgrass was the most preva!ent grassy weed and the most frequent vira! host, with 6% infection by WSMV in 2008 {n = 125) and 15% in 2009 {n = 358). By mechanically inoculating plants with WSMV in the greenhouse, the highest susceptibiüty was found in rye brome (52.1%), jointed goatgrass (80.9%), and wi!d oat (53.9%). Quackgrass, not previous!y reported as a host, was susceptib!e to WSMV (12.7%). Mite transmission efficiency from susceptib!e grass species was lower than from wheat, and grass species must be a host for both WSMV and the WCM to serve as a virus source. WCM transmission was more efficient than mechanical transmission. Overall, results indicate that grass species can serve as a viral reservoir, regional variation in a weed species' susceptibiüty to WSMV cannot exp!ain geographic variation in epidemic intensity, and crop species and c!ose!y re!ated weeds (e.g., jointed goatgrass) remain the best reservoirs for both WSMV and the WCM.