Seasonal Infection of the Weed Dyer's Woad by a Puccinia sp. Rust Used for Biocontrol, and Effects of Temperature on Basidiospore Production

Potted dyer's woad rosettes exposed to natural rust inoculum at field sites became infected when exposed from late April through early July, depending upon the location. The latent period between exposure and symptom expression varied from 9 to 54 weeks. The length of this latent period was unrelated to either the age of plants at exposure or the exposure period itself. The age of rosettes at the time of exposure did not affect the incidence of infection. Fall infection of potted rosettes occurred, but the incidence was low. When naturalized stands of woad were inoculated with teliosori, either fresh or dried, the incidence of infection was 58 to 76%, compared with 2 to 7% incidence in noninoculated plants. Basidiospores were readily produced from intact teliosori when suspended over water agar, with the highest rate of production between 3 and 6 h of incubation, at 10 to 20°C. The optimum temperature for basidiospore production over a 24-h period was 15°C, but they were produced at temperatures as low as 5°C, although not at 25°C. These lower-than-expected temperatures for spore production corroborate the field evidence that dyer's woad rust most actively infects in springtime, when temperatures are comparatively low and rainfall is more frequent.