The present invention relates to a method for detecting a quantitative trait locus (QTL) associated with resistance to Botrytis cinerea in tomato, including the steps of crossing a Botrytis-resistant donor tomato plant with a non-resistant, or Botrytis-susceptible, recipient tomato plant, contacting one or more offspring plants with an infective amount of Botrytis, quantitatively determining the disease incidence and/or the rate of lesion growth in the one or more offspring plants, establishing a genetic linkage map that links the observed disease incidence and/or rate of lesion growth to the presence of chromosomal markers of the donor tomato plant in the one or more offspring plants, and assigning to a QTL the contiguous markers on the map that are linked to a reduced disease incidence and/or a reduced lesion growth rate.