The ban follows discovery of scattered areas of late potato blight fungus in western Montana in 1995, says Barry Jacobsen, a Montana State University plant pathologist. Montana's certified seed potato crop is mainly grown in eight western and central counties. However, the 300 million pounds yielded represent one of the most profitable per-acre crops in Montana. Montana certified seed potatoes are sold to many states, including Idaho, where they are used to produce commercial crops of potatoes.
Infected plants develop spores, which are the seeds of the fungus.
Spores can be carried for 10 miles or more by wind, so even a few infected plants in a private garden can be the source of wind-borne fungus spores that infect crops miles away. Since 1992 when a new type of potato blight was found in the United States, it has caused hundreds of millions of dollars in lost crops and increased production costs. The blight is famous for causing the Irish potato crop failures of the mid-1840s that led to famine there. New strains of late blight fungus are resistant to the most effective fungicide and could be capable of sexual reproduction and possibly long-term survival in soil and crop residues. Before the new strain of blight fungus appeared, the blight could only survive between crop seasons on living plants or tubers.