Genetics of resistance in lettuce to races 1 and 2 of Verticillium dahliae from different host species

Race 1 resistanceagainst Verticillium dahliaein lettuce was originally shown in the cultivar La Brillante to be conditioned by a single dominant gene ( Verticillium resistance1, Vr1). Multiple, morphologically diverse sources of germplasm have been identified as resistantto race 1. In this study, allelism tests indicated that resistancein these different lettuce cultivars is closely linked or allelic to the Vr1 gene. The Vr1 gene is defeated by race 2 isolates of V. dahliae. Only partial resistanceto race 2 isolates is available in a few plant introductions (PIs). Greenhouse and field experiments conducted with these PIs demonstrated partial resistanceto V. dahliaerace 1 as well as race 2 isolates from lettuce. Cultivars resistantto race 1 and PIs with partial resistanceto race 2 were challenged with several race 1 and 2 isolates originating from hosts other than lettuce. This indicated that cultivars resistantto race 1 and the breeding lines derived from them would also be resistantto race 1 isolates from other hosts; similarly, the partial resistancewould be effective against race 1 and 2 isolates from hosts other than lettuce. Nevertheless, there were specific interactions that warrant further study. Although race 1 currently predominates in the major lettuce production area of the Salinas Valley, CA, breeding lettuce for resistanceto V. dahliaeshould take both races into account.