Japanese isolates of Verticillium dahliae were examined for vegetative compatibility relationships using nitrate-nonutilizing mutants. Four levels of vegetative compatibility were differentiated according to the degree of compatibility between the tester mutants of nit1 and NitM. Wild-type growth with a complementation line greater than 5 mm wide was defined as "strong reaction (-I-+)", i.e., compatible. Ten out of 15 isolates showed compatibility and were separated into three groups, provisionally designated as VCGJ1, VCGJ2, and VCGJ3, depending upon their reactions. This method was used to estimate genetic diversity within a local population of V. dahliae. Another 12 isolates from Gunma Pref. were paired with tester isolates of the three vegetative compatibility groups proposed, Eight Gunma isolates were assigned to VCGJ1 or VCGJ2. Two isolates were incompatible with all testers. The remaining 2 isolates were self-incompatible. Thus, 18 out of 27 Japanese isolates of V. dahliae were assigned to VCGs: 8 to VCGJ1,7 to VCGJ2, and 3 to VCGJ3. VCGJ1 was compatible with both VCGJ2 and VCGJ3, but VCGJ2 and VCGJ3 showed a weak reaction with each other. Japanese isolates of V. dahliae were thus demonstrated to form a VC group comprising three subgroups.

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