JUST in time to complicate spring planting, the federal government is preparing to issue what agriculture officials call the most sweeping restrictions on the shipment of nursery plants ever undertaken in the United States, to try to prevent the spread of a virulent disease that has killed tens of thousands of oaks and other species along the West Coast. The restrictions, expected to be issued in early January, will affect millions of plants grown in California, Oregon and Washington, about one-third of the country's nursery plant supply. They will require inspection, sampling and possibly testing of all plants that could...