Abstract
Isatis tinctoria (Brassicaceae), dyer’s woad, a plant of Eurasian origin, has become invasive after its introduction to western North America. Traits that may contribute to its invasiveness include prolific seed production, a rapid growth rate, an extensive two-layered rooting pattern, high phenotypic plasticity, and assumed allelopathic properties. Integrated weed management methods have shown some success, but currently I. tinctoria is not considered controlled. Puccinia thlaspeos ‘woad strain,’ a rust fungus native to North America, was discovered on I. tinctoria and later developed as a registered mycoherbicide. We quantified the presence and incidence of P. thlaspeos ‘woad strain’ in natural populations and its effect on the reproductive output of I. tinctoria at the individual plant and population level, and on a larger geographical scale. P. thlaspeos ‘woad strain’ may have synergistic effects when combined with other management practices and/or potential classical biological weed control organisms currently studied in Europe.