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Hydrophobic exudate from roots of sorghum [Sorghum bicolor (L.) Moench. #3 SORVU ‘IS 8768’] contain four p-benzoquinones which in the dihydroquinone form are active as germination stimulants of witchweed [Striga asiatica (L.) Kuntz. # STRLU]. The three minor p-benzoquinones were partially characterized and found to be structurally similar to sorgoleone, the major p-benzoquinone of this exudate. Herbicidal activity of the hydrophobic exudate was due to concentration- and pH-dependent inhibition of root elongation in some but not all weeds tested. Witchweed has apparently adapted these “defense” compounds of sorghum as host-specific germination stimulants.