At the University of Idaho’s (UI) Parma Research and Extension Center, onion specialist Brad Geary planted the yellow mustard IdaGold in a half-acre research field this August. The mustard was plowed under as soon as it flowered -- sometime in late October or early November -- and its damaged tissues will begin releasing naturally toxic compounds underground.

Scientists call the process “biofumigation.” Jack Brown, the UI plant breeder who released IdaGold in 1998, says the variety is especially high in substances called glucosinolates that will break down primarily into p-hydroxybenzyl isothiocyanates. That mouthful of biochemistry should pack a fistful of fumigants against the fungi that cause pythium, pink root, and fusarium diseases in onions, Brown says.

The compounds are also likely to kill weed seeds. “Glucosinolates themselves are harmless, but we’ve known for a long time that their by-products kill things,” says Brown. “What we don’t know are the quantities necessary to kill crop pests, the types of glucosinolates that are most effective, and the best times to apply them to the field.”

Geary says agricultural scientists are interested in the potential role biofumigants can play in pest control because these substances leave no long-lasting residues, can be significantly cheaper than chemical alternatives, and may be less damaging to beneficial soil organisms. The chemical fumigants used by onion growers to kill disease organisms and nematodes also kill beneficial mycorrhizae.

According to Terence McGonigle, an Idaho State University soil biologist who will participate in the research, mycorrhizae are naturally occurring associations of soil fungi and root tissues that form on many crops, although not on mustards. The fungi take sucrose from the host plants and give them phosphorus in return.

The effects of glucosinolates on mycorrhizae aren’t yet understood, McGonigle says, although some scientists suspect they’re potentially damaging. The glucosinolates’ impact on mycorrhizae is exactly what McGonigle will measure in the onion research. “We expect the impact of biofumigants to be less than the impact of chemical fumigants,” he says.

At Parma, UI Extension crop management specialist Brad Brown will examine the biofumigant-treated onions for their ability to take up phosphorus. “If the mycorrhizae are destroyed, we very likely will need to maintain higher concentrations of phosphorus in the soil to maintain the plant,” he says.

Brown will also examine the glucosinolates’ effects on the soil bacteria that convert nitrogen into forms usable by plants. Results of the first year of the three-year trial should be available by fall 2002, after next spring’s onions are harvested.