Regulars at Rusty's restaurant near this town outside St. Louis have a saying: Take a helping of horseradish sauce "even if you don't eat it."

The pungent root that adds a kick to roast beef and is a staple at Passover seders grows in bigger quantities in the shadow of the St. Louis Arch than anywhere else in North America. Locals don't miss a chance to help their growers.

But a soil-borne fungus is cutting into the farmers' yields and threatening their tradition.

Although verticillium disease affects the crop everywhere, it's particularly bad in Illinois, where horseradish is grown on 1,800 acres, experts say. Those acres produce up to two-thirds of North America's horseradish each year.

If resistant varieties aren't developed soon, Illinois growers will have to find other land that is free of the fungus, said Mohammed Babadoost, a researcher at the University of Illinois.

"This could stop horseradish production in Illinois," he said.

Farmer George Willaredt is searching for other areas to raise horseradish. But he hopes to remain for many years on the family farm, where he rotates horseradish with other crops such as corn, soybeans and wheat. The other crops aren't affected by the disease, and they're also far easier to grow.

"Anyone can raise soybeans, but you have to be really hands-on to raise horseradish," said Willaredt, 68.

There are no seeds for root crops. Instead, offshoots the size of carrots are separated from the thick part of the horseradish by hand, then replanted in the loose, rich soil of this stretch of Mississippi River bottom land.

"I have friends who ask me why I want to work this hard," said Barry McMillan, president of the Illinois Horseradish Growers Association and a third-generation grower. He said he starts at 5:30 a.m. and quits around 8:30 p.m. on busy days.

"You've got to have horseradish in your blood to work like this," he said.

The crop is profitable, at about 35 cents a pound, or $2,800 for a typical acre, growers say. It is sold to processors to make horseradish sauce, shrimp cocktail sauce and other products. Corn, by comparison, brought around $300 an acre for Illinois farmers last year.

Willaredt is helping researchers search for varieties resistant to verticillium, which has forced him to cut the size of his crop by about 20 percent in recent years, he said.

Babadoost, the plant pathologist, said killing the fungus is particularly difficult because growers replant cuttings from previous crops in a limited area. It's like humans reproducing from a small gene pool: Birth defects and disease become more common.

The Willaredts have started growing horseradish around Sikeston, Mo., another section of rich soil along the Mississippi River. It's a trend that Willaredt believes will continue, as the fungus flourishes on his family's land.

"It's a sad thing for this area," Willaredt said. "It's our heritage."

Illinois farmer George Willaredt holds a flat of hybrid horseradish plants that are being grown to combat the fungus verticillium.Willaredt peels off the muddy hide of a horseradish root. The verticillium fungus causes the normally white root to turn brown.