QTL mapping of resistance to bacterial fruit blotch in Citrullus amarus.

KEY MESSAGE: Six QTLs were associated with affected leaf area in response to inoculation with Acidovorax citrulli in a recombinant inbred line population of Citrullus amarus. Acidovorax citrulli, the causal agent of bacterial fruit blotch (BFB) of cucurbits, has the potential to devastate production of watermelon and other cucurbits. Despite decades of research on host-plant resistance to A. citrulli, no germplasm has been found with immunity and only a few sources with various levels of BFB resistance have been identified, but the genetic basis of resistance in these watermelon sources are not known. Most sources of resistance are plant introductions of Citrullus amarus (citron melon), a closely related species that crosses readily with cultivated watermelon (Citrullus lanatus L.). In this study, we evaluated a recombinant inbred line population (N = 200), derived from a cross between BFB-resistant (USVL246-FR2) and BFB-susceptible (USVL114) C. amarus lines, for foliar resistance to A. citrulli in three replicated greenhouse trials. We found the genetics of BFB resistance to be complicated by strong environmental influence, low heritability and significant genotype-by-environment interactions. QTL mapping of affected leaf area identified six QTL that each explained between 5 and 15% of the variation in BFB resistance in the population. This study represents the first identification of QTL associated with resistance to A. citrulli in any cucurbit.