| Summary: | In vegetables of <i>Brassica rapa</i> L., <i>Fusarium oxysporum</i> f. sp. <i>rapae</i> (<i>For</i>) or <i>F. oxysporum</i> f. sp. <i>conglutinans</i> (<i>Foc</i>) cause Fusarium yellows. A resistance gene against <i>Foc</i> (<i>FocBr1</i>) has been identified, and deletion of this gene results in susceptibility (<i>focbr1-1</i>). In contrast, a resistance gene against <i>For</i> has not been identified. Inoculation tests showed that lines resistant to <i>Foc</i> were also resistant to <i>For</i>, and lines susceptible to <i>Foc</i> were susceptible to <i>For</i>. However, prediction of disease resistance by a dominant DNA marker on <i>FocBr1</i> (Bra012688m) was not associated with disease resistance of <i>For</i> in some komatsuna lines using an inoculation test. QTL-seq using four F<sub>2</sub> populations derived from <i>For</i> susceptible and resistant lines showed one causative locus on chromosome A03, which covers <i>FocBr1</i>. Comparison of the amino acid sequence of <i>FocBr1</i> between susceptible and resistant alleles (<i>FocBr1</i> and <i>FocBo1</i>) showed that six amino acid differences were specific to susceptible lines. The presence and absence of <i>FocBr1</i> is consistent with <i>For</i> resistance in F<sub>2</sub> populations. These results indicate that <i>FocBr1</i> is essential for <i>For</i> resistance, and changed amino acid sequences result in susceptibility to <i>For</i>. This susceptible allele is termed <i>focbr1-2</i>, and a new DNA marker (focbr1-2m) for detection of the <i>focbr1-2</i> allele was developed.
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