Research studies were conducted in Japan and the United States under a 3-year cooperative project in order to select an international set of rice varieties for differentiating the pathogenic races of Piricularia oryzae. Initially, the 39 varieties comprising the former United States, Japan and Taiwan differentials were used. As the studies progressed, 12 candidate varieties were selected. A total of 59 races were differentiated through use of the 12 varieties. Eight of the 12 candidate varieties were selected as the final set of international differentiating varieties: Raminad Str. 3; Zenith; NP-125; Usen; Dular; Kanto 51; Sha-tiao-tsao-S; and Caloro. Among P. oryzae isolates from several rice-producing countries, 32 international races were characterized.
1. The paper gives the results of experiments on the nuritional requirements of P. oryzae in media for abundant sporulation and stable stock cultures. 2. A semi-synthetic medium, designated as the modified basic agar medium, was much more suitable for sporulation and stock culture with consistency for all the isolates of P. oryzae tested than the other agar media which have been commonly used. The constituents of the modified basic agar medium are as follows: Yeast extract powder 0.4% MgSO4⋅7H2O 0.06 Glucose 0.4 CaCl2 0.10 Tween 80 0.06 FeCl3⋅6H2O trace K2HPO4 0.06 Agar 2.0 3. The nutritional requirements seem to differ with the kind of biologic species identified on the basis of the appressorium type. The modified basic agar medium and potato sucrose agar seem to be favorabl for isolate No.8 CG-1, a homocaryon belonging to the biological species C-1, and some biologic species belonging to the mutant type. The former medium was suitable, in general, for the biologic species belonging to the wild type. 4. Further fundamental studies on the nutritional physiology of each homocaryotic biological species of P. oryzae will be necessary for finding the medium or media suitable enough for uniform and abundant production of conidia and for stock culture stability. 5. In conclusion, the writer again wishes to emphasize that all the basic studies on P. oryzae, in which sexual reproduction has not been found, absolutely need the homocaryotic biologic species with genetic stability-a need he has strongly expressed for many years.