Establishment and Dispersal of Puccinia thlaspeos in Field Populations of Dyer's Woad

Dyer's woad rust is being intensively studied as a biocontrol agent for the noxious weed dyer's woad. These studies report on methods to establish this fungus in woad populations and to obtain information about disease incidence and dispersal of the rust subsequent to establishment. Inoculum dosages as low as 1 mg/plant established disease in dyer's woad rosettes. Dosages higher than 1 mg/plant resulted in better levels of infection, and inoculation was equally effective when the inoculum was placed directly on the plants or on the soil adjacent to the plants. The highest incidence of infection occurred when plants were inoculated in the spring, whereas summer or fall inoculations did not result in infections. Large-scale inoculation of woad was effective and resulted in significant levels of disease. However, it was not clear if artificial inoculation was effective in changing disease incidence in populations of woad where the rust was present prior to treatment because there were no significant differences between inoculated and check plots. Woad rust is systemic but apparently it is not seed dispersed, even though some infected plants occasionally produce seed. Two years after inoculation, dyer's woad rust had dispersed an average of 14.6 m from the focus of inoculation. After 3 years, the dispersal distance had not increased and infected plants averaged only 10 m from the point of inoculation. Disease was maintained naturally in woad populations after treatment, but the overall percent infection in the treated plots decreased by 3 years after inoculation.