Studies of the structure-activity relationship (SAR) directed to evaluate the effect of several sesquiterpene lactones (SL) as germination stimulants of three Orobanche spp. (O. cumana, O. crenata, and O. ramosa) have been achieved. Results are compared with those obtained in the same bioassay with an internal standard, the synthetic analogue of strigol GR-24. A high specificity in the germination activity of SL on the sunflower parasite O. cumana has been observed, and a relationship between such activity and the high sunflower SL content is postulated. Molecular properties of the natural and synthetic germination stimulants (GR-24, GR-7, and Nijmegen-1) and SL have been studied using MMX and PM3 calculations. Consequently, comparative studies among all of them and their activities have been made. SL tested present similarities in molecular properties such as the volume of the molecule and the spatial disposition of the carbon backbone to the natural germination stimulant orobanchol. These properties could be related to their biological activity.