An unpublished paper by Jennifer K. Balch, of the Penn State geography department, shows that fires occurred four times as often in cheatgrass landscapes in the West as in all other types of ground cover combined. The paper, which has been accepted by the journal Global Change Biology, says that cheatgrass was a factor in nearly 25 percent of the 50 largest Western fires in the 1990s.

For decades, scientists have been trying to stop its advance, to little effect. Now Dr. Meyer and a few other scientists are exploring biological warfare. The new weapons, all fungi, are being tested here in Skull Valley, a place so thick with cheatgrass that a stubbly, bleached-blond carpet of it stretches to the hills on all sides.

The experiments depend on a fine-grained knowledge of the weed’s behavior. James McIver, of Oregon State University, who heads an interagency federal research effort on restoring sagebrush ecosystems, explained that cheatgrass — so called by farmers whose wheat yields dropped when it gained a foothold — “gets into interstices in the sagebrush plant, grows right under the sagebrush.”

Cattle, Dr. McIver said, prefer native perennial grasses, eating them when possible and leaving cheatgrass alone. In addition, said Rory Reynolds of Utah’s natural resources department, the plant’s natural rhythms give it an advantage in arid regions. Its seeds germinate in the fall instead of spring, and it begins to grow as the snows melt, getting to the precious moisture long before native plants and shrubs.

This is a crucial advantage in the high desert areas of Idaho, Nevada, Oregon, Utah and Washington, where native plants depend on scarce water resources later in the spring. It is less of a factor in moisture-rich areas to the east. Cheatgrass grows throughout the contiguous United States, but poses no threat to native vegetation outside the arid West.

The black fingers fungus, which like cheatgrass is native to the Eurasian steppe, attacks the cheatgrass seeds before they germinate. Robust cheatgrass plants can cover a square yard with as many as 25,000 seeds. “If we can get that down to 300 seeds, we consider that a successful bio-control,” Dr. Meyer said.