Coming: A new crop of organic pesticides.

Because many plant pathogens hide out in the soil, growers have learned to counter the pests' dirty tricks by pumping toxic chemicals into the ground shortly before planting. Now, scientists report promise for an alternative--fumigating soils with a living mulch.

Plants in the brassica family--which includes cabbages, mustards, and the rapeseed plants from which growers harvest canola oil--produce compounds called glucosinolates. Enzymes in the soil or in other organisms can transform the glucosinolates into compounds toxic to a wide range of creatures. Brassicas themselves contain such enzymes, which are released when the plant is chewed.

Matthew J. Morra of the University of Idaho in Moscow and his colleagues now show that as they grow, brassica plants infuse the soil with these toxins. If farmers then plow the plants into the soil, the pesticidal chemicals flood the root zone. These toxins tend to degrade to harmless chemicals within 3 days. Morra's team describes its findings in the just-published September JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY.

Most of a brassica's glucosinolates develop in its stems and leaves, notes coauthor James B. Gardiner, now of the University of Vermont in Burlington. However, his Idaho data show that even when the entire plant is plowed under, only the root glucosinolates give rise to measurable toxins in the soil. Gardiner also found another surprise. Rapes in the field release a different mix of toxins than do those produced in indoor experiments.

As brassica mulches stabilize soil, they offer "a softer, greener approach" to pest control, says John A. Kirkegaard of the Commonwealth Scientific and Industrial Research Organization in Canberra, Australia. He has linked improved wheat yields to toxins released by an earlier crop of rape. Yet "it's not a silver bullet," he says, since some pests are immune to the toxins--at least in the concentrations now made by rapes.

Ironically, Gardiner notes, breeders have traditionally worked to reduce a rape's glucosinolates, since they can render it unpalatable as a forage and impart a pungent flavor to cooking oils. With a potential market for high-glucosinolate brassicas, Jack Brown at the University of Idaho has been crossing distantly related species of winter-hardy rapes with glucosinolate-rich mustards.

He's just created two fertile hybrids. Unlike mustards, the new species are hardy to well below freezing, allowing their use as an erosion-controlling winter cover, he says. Field tests are just getting under way to compare their crop-protection value against that offered by traditional rapes and commercial soil fumigants.