The present invention relates to a method for detecting a quantitative trait locus (QTL) associated with resistance to Botrytis cinerea in tomato, comprising 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 said 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 said donor tomato plant in said one or more offspring plants, and assigning to a QTL the contiguous markers on said map that are linked to a reduced disease incidence and/or a reduced lesion growth rate.