Got crop pest problems? Spread some mustard on them — the plant, that is, not the condiment.

Agricultural Research Service scientists are growing stands of cultivated mustard and other Brassica species as a possible alternative to using chemical fumigants to rid crop fields of nematodes, weed seeds and other soilborne pests.

The mustards' “biofumigant” effect is attributed to isothiocyanates, chemical byproducts of the plants' decomposition that make the soil toxic to nearby pests. Indeed, farmers in parts of the United States and Europe have sought to exploit this phenomenon by preceding their crops with stands of mustard, rapeseed and other Brassica species.

But there's still much to learn about how these biofumigant plants control pests, the conditions Brassicas prefer and their cumulative effects on the soil environment, according to Rick Boydston, an agronomist in ARS' Vegetable and Forage Research Unit at Prosser, Wash.

Since 2000, Boydston has led a team of scientists in monitoring the mustards' biofumigant effects in greenhouse and field studies. Eventually, the resulting information could lead to new cropping systems. For example, scientists are checking the sprouting ability of redroot pigweed seed that has been dug out from beneath stands of white mustard, sorghum-sudangrass, winter wheat or an oat-hairy vetch mixture. Results thus far indicate delayed germination only. In contrast, 99 percent of the redroot pigweed seeds from fumigated plots didn't germinate at all.

In greenhouse studies, scientists monitored the effects of crushed seedmeal from brown mustard and field pennycress on potted irises and three pests: chickweed, prickly lettuce and root-knot nematodes. The irises suffered no ill effects, but more than half of the weeds failed to sprout, and nematode numbers fell by 70-80 percent.