Two independent mapping populations; a 113-line recombinant inbred line population and a 258-line association population, were used to map loci associated with sensitivity to the CF at the seedling stage. A major QTL on chromosome 4 was identified at the same locus using both populations. Mapping in the association population defined a 400 kb region that contained the sensitivity locus. By comparing CF-sensitivity of the parents of the RIL population with that of the F1 progeny, we determined that the sensitivity allele was dominant. No relationship was observed between CF-sensitivity in seedlings and SLB susceptibility in mature plants; however, a significant correlation (− 0.58) was observed between SLB susceptibility and CF-sensitivity in seedlings. The activity of the CF was light-dependent and was sensitive to pronase, indicating that the toxin was proteinaceous.