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The gene ym3 which is found in both Ea52 and Haganemugi is recessive and shows resistance to all Japanese strains of BaYMV (Iida et al. 1992). It was first found in a mutant line Ea52 (Ukai and Yamashita 1980). The chromosomal location of ym3 has not been identified yet.

 The gene ym4 found in Tokushimamochihadaka, which carries also ym3, is recessive and is linked to the gene n on chromosome 1 (Fukuoka et al. 1991). A breeding line "Shimakei 1 " which carries only ym4 has been developed from a cross involving Tokushimamochihadaka. This line has resistance to BaYMV strains I and II (Makino, personal communication). The resistance reaction of ym4 to strain III has not been tested yet.

The first source of resistance Mokusekko 3 was crossed with lines having a high malting quality at the Tochigi Prefectural Agricultural Experiment Station. As a result, two resistant malting barley cultivars Misato Golden (Seko et al. 1986) and Mikamo Golden (Yoshida et al. 1988) were released in 1985 and 1987, respectively. Another resistant cultivar Nishino Gold developed from a cross involving Mokusekko 3 was released from the Fukuoka Prefectural Agricultural Experiment Station in 1986 (Itoh et al. 1987). The resistance reaction of Misato Golden and Nishino Gold to BaYMV strains I, II and III is R, R and S, respectively (lida et al. 1992, 1993). They probably inherited only Ym5(t) from Mokusekko 3 (Table 3). As malting barley breeders made extensive use of the Mokusekko 3 gene(s), most of the current promising lines are resistant to BaYMV.

In the six-rowed barley breeding program, the mutant line Ea52 has been used and promising lines have been developed. Ea52 was induced from the variety Chikurin Ibaraki 1 by treatment with gamma rays (Ukai and Yamashita 1980). Chikurin Ibaraki 1 is a six-rowed hulled barley with a winter habit. The mutant, Ea52, was originally selected as an early heading mutant in the M3 generation after irradiation. Irradiation involved application of 250 R of gamma rays to plants at the vegetative stage. In a field severely infected with BaYMV, all the early heading mutants except Ea52 showed varying degrees of leaf yellowing. Ea52 is a rare example of an artificially induced mutant resistant to a virus disease. A new six-rowed hulled barley cultivar Masakadomugi whose resistance gene to BaYMV was inherited from Ea52 was released from the National Agriculture Research Center in 1989 (Table 3) (Makino et al. 1994).

Haganemugi, described above as a gene source for straw stiffness, has been also used as a gene source for improving resistance to BaYMV. An allelism test proved that the resistance gene of Haganemugi was identical with the gene ym3 of Ea52 (Makino, personal communication). Kyushu National Agricultural Experiment Station released the first BaYMV resistant two-rowed non-malting barley cultivar Ishukushirazu from a cross involving Haganemugi in 1981 (Tsuru et al. 1983). Ishukushirazu inherited the gene ym3 from Haganemugi (Kawada 1988) and shows the resistance to all Japanese strains of BaYMV (Table 3).

No cultivar has been developed from a cross involving Mihorihadaka having Ym2 or Tokushimamochihadaka having ym3 and ym4 (Table 3).


3) Breeding for resistance to barley yellow dwarf virus (BaYDV): Barley yellow dwarf was first reported in Japan in 1983 (Kojima et al. 1983). The virus which is carried by aphids also attacks rice (Kojima et al. 1983). Consequently where double cropping of barley and rice is adopted, the occurrence of this disease may increase. Breeding for resistance to BaYDV was just started using the barley cultivars Atlas 68 and CM 67 as gene sources at the Shikoku National Agricultural Experiment Station. These two cultivars carry a resistance gene Yd2 which is probably linked on chromosome 3 with the lnt gene which confers a small number of tillers (Schaller et al. 1964; Sogaard and Wettstein- Knowles 1987). The gene lnt may be a useful marker for the selection of resistance to BaYDV.

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