Greenhouse and laboratory studies were conducted to determine the allelopathic potential of two sweet potato [Ipomoea batatas L. (Lam.)] cultivars, ‘Regal’ and ‘SC 1149-19’. Yellow nutsedge (Cyperus esculentus L. # CYPES) and alfalfa (Medicago sativa L.) plants grown in soil from sweet potato field plots accumulated less dry matter than plants grown in soil from adjacent weedy plots. Growth of yellow nutsedge and alfalfa plants was also reduced when grown in a potting medium containing decomposing sweet potato plants in comparison to plants grown in potting medium alone. When the sweet potato potting medium mixture was incubated at 25 C and tested weekly using an alfalfa growth bioassay, inhibition was high initially but decreased over time and was not observed after a 12-week incubation. Aqueous methanol (50%)-soluble extracts of sweet potato periderm were inhibitory to yellow nutsedge root growth in vermiculite and alfalfa seed germination on filter paper. Regal extracts were inhibitory to yellow nutsedge root growth at a concentration of 2.5 mg periderm extracted/ml, but SC 1149-19 extract was inhibitory only at a concentration eight times higher. Similar differences between cultivars were observed with the alfalfa germination bioassay. Preliminary separation of the Regal periderm extract by paper chromatography indicated the presence of phenolic compounds with a wide range of polarities. Several of these UV-visible compounds were inhibitory to alfalfa germination.

Benzyl isothiocyanate (BITC) was extracted from mature papaya (Carica papaya L.) seeds and applied to imbibed velvetleaf (Abutilon theophrasti Medic. ♯3 ABUTH) seeds. Complete inhibition of germination occurred at a concentration of 6 × 10-4 M BITC, and only 28% (compared to controls) of the seeds germinated after 4 days exposure to the 4 × 10-4 M treatment. Corn (Zea mays L. ‘DeKalb XL-66′) was unaffected even at higher concentrations; however, 72% of imbibed soybeans [Glycine max (L.) Merr. ‘Tiger’] germinated in 6 × 10-4 M BITC. When this natural inhibitor was applied to etiolated velvetleaf seedlings at 4 × 10-4 M, 100% died in 2 days.