Hokuriku National Agricultural Experiment Station, Joetsu, Niigata, 943-0193, Japan

Background and objectives
Barley scald caused by Rhynchosporium secalis has been a very serious problem for winter barley production in the Hokuriku district in Japan. Breeding for resistance to R. secalis is now a feature of barley breeding programs, especially for the Hokuriku district. Pathogenic races of R. secalis have been reported in USA, Australia, as well as in several other countries [l]. In Japan, using 10 differential cultivars, 10 races(J-1 to J-10) were differentiated in 39 collected isolates of R. secalis, some 30 years ago [2]. This study was surveyed the extent of pathogenic variation in R. secalis in Japan.

Material and methods
Barley leaves infected R. secalis were collected from throughout the barley growing areas of Hokuriku district and other district in Japan, during 1989 to 1996. Inoculum of the various isolates was produced on corn meal agar(CMA) plates in the dark at 15C. The spore concentration was adjust to 200,000 ;spores/ml of distilled water with Tween-20. Barley seedlings were inoculated when they had two fully emerged leaves. Inoculum was sprayed to plants at a rate of 0.25 ;ml per seedlings. Scald reaction was rated on a five-point (0-4) scale similar to that used by Goodwin et al.[1]. Pathotypes of R. secalis were characterized on two sets of differential barley cultivars. Set A comprised 10 barley cultivars used by Kajiwara and Iwata[2], plus cv. Minorimugi and cv. Kashimamugi, the widely growing cultivars of Japan. Set B were 24 standard differential cultivars for R. secalis suggested by Goodwin et al.[1].

Results and conclusions
A total of 124 isolates of R. secalis were grouped into five races based on the reactions of a set A. These races were identified J-4, J-5, J-7, J-9 and J-4a (tentative name). Race J-4a was the most prevalent and comprised 70% of R. secalis isolates with a narrower virulence spectrum than other races. Races J-7 and J-9 were found in breeding fields of barley in Tohoku NAES, Iwate, Japan. Minorimugi was susceptible to all isolates of R. secalis, leading cultivar of six-rowed barley in Japan. Using standard differential cultivars [l], isolates of race J-4a and J-9 were identified pathotype 10202001 and 32202431. This study indicates that R. secalis is pathogenically variable in Japan and presents a risk to the use of single gene resistance in barley cultivars. The continuous monitoring of pathogenic variation is an essential means for use of the scald resistant cultivars.

1. Goodwin SB, Allard RW, Webster RK, 1990. Phytopathology 80, 1330-1336.
2. Kajiwara T, Iwata Y, 1963. Bulletin of the National Institute of Agricultural Science C15, 1-73.