QTL mapping for target leaf spot resistance in a Japanese-type cucumber

Target leaf spot (TLS), caused by Corynespora cassiicola, is among the most economically important diseases of cucumber (Cucumis sativus L.). Because of the rising incidence of disease problems, much effort has been put toward resistance breeding. Here, we investigated the inheritance of high TLS resistance in a Japanese-type inbred line. QTL analyses were performed using F₂ and F_2:3 families derived from a cross between the resistant and a susceptible (Beit Alpha-type) inbred line. We evaluated disease severity in F_2:3 families in four independent assays and mapped a total of five TLS-resistance QTLs on chromosome (Chr.) 2, Chr. 5, and Chr. 6. The effect of the two major QTLs on Chr.5, Tls5.1 and Tls5.2, was confirmed through additional assays and verified in different genetic backgrounds. Fine mapping using recombinants in several generations narrowed down Tls5.2 to a 496.8-kb region with 69 genes, several of which are potential candidates involved in plant disease resistance. The newly identified resistance loci may confer more durable resistance when combined with previously characterized resistance genes or loci. Furthermore, molecular markers linked to TLS resistance derived from the Japanese-type inbred line could be effectively used to accelerate breeding for this trait.