1. Plant Biotechnology Institute Ibaraki Agricultural Center, Ago, Iwama, Nishi‐Ibaraki 319–02 Japan,
2. Sugadaira Montane Research Center, The University of Tsukuba, Sanadamachi, Nagano 386–22, Japan,
3. Laboratory of Plant Pathology, Faculty of Agriculture, Tohoku University, Sendai 981, Japan