Intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth & D.R. Dewey) (synonyms: Agropyron intermedium [Host] Beauv.; Elytrigia intermedia [Host] Nevski) is widely grown as a forage crop and is also used to control soil erosion. In a seed production field of cv. Rush in Washington County, ID, more than 80% of the plants were found affected by a disease with leaf stripe symptoms. The lesions were initially elongated, water-soaked, and translucent, later developing into brown, necrotic, interveinal stripes that often coalesced. Frequently, yellowish, dried, granular, or flaky exudate was present on the lesion surfaces. Microscopic examination of cut pieces of symptomatic tissue showed profuse bacterial streaming. Isolations on nutrient agar and King's medium B agar consistently yielded smooth, circular, butyrous, yellow, raised bacterial colonies. The bacterium was rod-shaped, Gram-negative, oxidase-negative, aerobic, and did not reduce nitrate. Substrate utilization profiles (Biolog Inc.), and cellular fatty acid analysis (Analytical Services Inc.) identified the bacterium as a pathovar of Xanthomonas translucens (syn: X. campestris pv. translucens). For pathogenicity tests, 3- to 5-week-old greenhouse-grown seedlings were injected in the whorl with a water suspension of 24-h-old culture (approximately 107 cfu/ml) of the bacterium. Control plants were injected with sterile distilled water. The plants were incubated at 25 to 28°C and observations were recorded after 6 to 10 days. The bacterium was pathogenic (causing water-soaked lesions, often with bacterial exudate) to T. intermedium cvs. Rush, Tegmar, PI 547316, and PI 380636; wheat cvs. Stephens, Vandal, FF 301, and FFR 525; barley cvs. Galena, Lud, and Steptoe; oat cvs. Boone, Clinton, Erban, Marion, Mohawk, Nemaha, Olena, and Tama; rye cvs. Florida 401, Hazel, Musketeer, Oklon, Rymin, Wintermore, and Wrens 96; Agropyron cristatum cv. Ephraim; A. cristatum × desertorum cv. Hycrest; Bromus arvensis; B. briziformis; B. catharticus; B. inermisssp. inermis; B. inermis ssp. pumpellianus; B. japonicus; B. marginatus; B. popovii; B. rigidus; B. tomentellus; Dactylis glomerata cvs. Paiute and Potomac; Elymus repens; Leymus mollis; L. angustus cv. Prairieland; Lolium arundinaceum cv. Fawn; L. perenne cv. Zero Nui; and Psathyrostachys juncea cv. Bozoisky. It was only weakly pathogenic (with small, chlorotic or water-soaked lesions and no exudation) to Phleum pratense cv. Climax and Pseudoroegneria spicata ssp. spicata cv. Goldar. It was not pathogenic to Andropogon gerardii cv. Pawnee; Festuca ovina; Oryza sativa cvs. Cypress, Newbonnet, and M201; or Schizachyrium scoparium cv. Camper. Based on the pathogen's natural host and its wide host range among cereals and grasses as verified by inoculation, the bacterium was identified as X. translucens pv. cerealis. This is the first report of natural occurrence of this pathogen on T. intermedium. A sample (105 g) of seed used for planting the affected field was found contaminated with 7.5 × 104 cfu/g of the pathogen, and seed to seedling transmission was observed in greenhouse tests. Contaminated seed, thus, may serve as a source of primary inoculum to intermediate wheatgrass, which in turn may serve as an inoculum source to other susceptible cereals and grasses growing in the vicinity.