Horseradish crops in Illinois, which produces some two-thirds of North America's output each year, are under threat from a particularly savage attack of verticillium disease. The strong-tasting root is a staple accompaniment to roast beef and an important crop in Illinois, where it is cultivated on 1,800 acres. Growers are so concerned about the fungus that they may have to find other land free of it if they are to continue to grow horseradish in current quantities. The fungus is depressing yields and is particularly difficult to fight as growers replant cuttings from previous crops in a limited area, thus facilitating the spread of the fungus, as crops are effectively reproduced from the same 'gene pool'.