The Population of f. sp. in California and Arizona.

Fusarium wilt of lettuce, caused by f. sp. , is now found in all major lettuce producing regions in California and Arizona. The population structure of f. sp. in California and Arizona was characterized based on somatic compatibility and sequences of the translation elongation factor 1-α gene (EF-1α) and rDNA intergenic spacer region (IGS). In this study, 170 isolates were tested for somatic compatibility based on heterokaryon formation, using complementary nitrate nonutilizing () mutants. Five subgroups (A to E) of somatic compatibility group 0300 were identified. Isolates associated with the same subgroup had a strong complementation reaction, whereas reactions between isolates of different subgroups were weak or delayed. An isolate from the first known infestation of Fusarium wilt of lettuce in California was associated with subgroup A, which predominated among isolates in our collection. Isolates representative of each subgroup were confirmed to be associated with race 1, based on the reaction of differential lettuce cultivars. It is possible that somatic compatibility subgroups B to E of f. sp. were derived from subgroup A, as a consequence of somatic mutations affecting compatibility. If so, subgroups of f. sp. may represent an intermediate step in divergence that will lead to clearly separable compatibility groups. Sequences of EF-1α and IGS were both identical for 58 isolates of f. sp. that represented all somatic compatibility subgroups and locations from which isolates were obtained, indicating that subgroups were derived from the same clonal lineage (VCG 0300).