Identification of a New Wheat Yellow Mosaic Virus Strain with Specific Pathogenicity towards Major Wheat Cultivars Grown in Hokkaido.

Yellow mosaic of wheat was first described in Japan in 1927. Its causal agent was identified as wheat yellow mosaic virus (WYMV)1, 2, 5). In Canada, the virus respon sible for a similar disease of wheat was designated as wheat spindle streak mosaic virus (WSSMV)6). Later, Usugi and Saito11) reported that WYMV and WSSMV are strains of the same virus differing only slightly in their serological and pathological characteristics. Both WYMV and WSSMV have filamentous particles with two modal lengths of 250-300 and 550-650 nm, and are transmitted by the soil-inhabiting fungus Polymyxa graminis6, 10, 11). WYMV is widespread in Japan, and causes, often in co-infection with soil-borne wheat mosaic virus (SBWMV), serious damage to winter (autumn-sown) wheat crops. However, soil-borne viruses have never been recorded on wheat in Hokkaido region; major autumn-sown wheat cultivars grown in Hokkaido were reported to have resistance against both WYMV and SBWMV3). Nevertheless, in 1991 yellow mosaic was first found in winter wheat in Eniwa-city, Hokkaido. Here, we report on the occurrence and distribution of WYMV in Hokkaido, comparisons of pathological and sero logical properties of a Hokkaido isolate of WYMV (WYMV-H) with a Japanese type isolate of WYMV (WYMV-T) from Ibaraki Prefecture (Kanto Region)10) and a Canadian WSSMV isolate6) and field experiments on the effect of WYMV infection on different wheat cultivars. During 1991-1994, field surveys were conducted in major winter wheat-growing areas in Hokkaido. Leaf samples with suspected virus symptoms were collected from April to June and tested by ELISA9) with anti serum to WYMV-T, which was newly produced using