The genetic relationship between the vegetative compatibility groups (VCGs) and between physiological races of Fusarium oxysporum f. sp. lactucae (FOL), the causal pathogen of lettuce root rot, was determined by analyzing the intergenic spacer (IGS) region of its ribosomal DNA. A total of 29 isolates containing a type strain were tested: 24 Japanese isolates, 2 Californian isolates, and 3 Italian isolates. Three races (races 1, 2, and 3) were found in Japan, and race 1 was also distributed in California and Italy. Races 1, 2, and 3 each belonged to a distinct VCG: VCG-1, VCG-2, and VCG-3 (VCG-3-1, VCG-3-3), respectively. Phylogenetic (neighbor-joining) analysis of the IGS sequences revealed that races 1, 2, and 3 coincided with three phylogenetic groups (PG): PG-1, PG-2, and PG-3, respectively. These results indicate that the three races are genetically quite different and have a strong correlation with VCGs and phylogenetic groupings.
Selective media without pentachloronitrobenzene were developed for quantitative assays of Fusarium oxysporum in soils. Media Fo-G1 and Fo-G2 were effective for naturally infested soils, Fo-W1 and Fo-W2 for wildtype isolates in soils containing a nitrate-nonutilizing (nit) mutant, and Fo-N1 and Fo-N2 for nit mutants. Selective media were made using ammonium citrate dibasic, L-sorbose, econazole nitrate, 25% iminoctadine triacetate solution and 50% tolclofos-methyl wettable powder for soil dilutions of 100-fold or more (Fo-G1, FoW1 and Fo-N1) and 10-fold (Fo-G2, Fo-W2 and Fo-N2
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