Asian soybean rust was first discovered in the continental United States in November 2004. This foliar disease is caused by the fungus Phakopsora pachyrhizi. Soybean rust is capable of destroying a soybean crop if it becomes established in a field early and weather conditions favor the spread of infection. The fungus is an obligate parasite, which means it can grow and reproduce only in living plant tissue. The fungus uses nutrients manufactured in soybean leaves to support its growth and reproduction. As a result, this drain on soybean plants can substantially reduce yields.