Artificial inoculation and colonization of dyer's woad (Isatis tinctoria) by the systemic rust fungus Puccinia thlaspeos

Dyer's woad plants were inoculated successfully with the systemic rust fungus, Puccinia thlaspeos, using a dew chamber. At least 94% of plants became diseased when whole plants were inoculated, but only 28% became diseased when single leaves were inoculated. The plants were asymptomatic for at least 10 weeks after inoculation, but some remained asymptomatic for as long as 9 months. Polymerase chain reaction was used to study asymptomatic colonization by the rust. After inoculation at the leaf tip, P. thlaspeos moved through the leaf at a rate of about 0.25 cm per week. By 10 weeks, the fungus had grown down the leaf, through the petiole, and into new leaves produced by the plant. The fungus was detected in most leaves and in the roots of an overwintering rosette that was infected, but asymptomatic. P. thlaspeos likely invades woad through its leaves and moves into the meristematic areas and roots where it overwinters. Second-season plants, including the roots and asymptomatic shoots, were completely colonized by P. thlaspeos after bolting. Seed produced by diseased plants germinated as well as seed from controls