Dyer's woad rust (Puccinia thlaspeos 'strain woad') is used as a pesticide to control the spread of dyer's woad, an invasive weed in the dry open areas of eight western states. Rusts are a group of fungi that infect only plants, and are often very selective about their hosts. Despite extensive testing of related plants, dyer's woad is the only known plant host for this rust. When used according to label directions, pesticide products containing dyer's woad rust present no known risks to humans, non-target plants, wildlife, or the environment.
Isatis tinctoria (dyer's woad) http://en.wikipedia.org/wiki/Isatis_tinctoria
Via Kamoun Lab @ TSL