Monitoring Puccinia jaceae var solstitialis for biological control of yellow starthistle (Centaurea solstitialis) in California

The rust fungus Puccinia jaceae var. solstitialis (P. jaceae) was first released as a classical biological control for yellow starthistle (YST) in California (CA) in 2003. At that time, a monitoring program was initiated to 1) determine the best time to release the agent, 2) determine if P. jaceae can complete its full life cycle in CA, and 3) determine if the fungus is likely to establish at two field sites in CA. In 2005-2006 P. jaceae was released monthly from January to June near Napa, in the coastal hills and near Woodland, in the Central Valley. All releases resulted in infected plants at both sites in both years. Urediniospores (infecting spores) were short lived, and remained viable less than three weeks in the field. Teliospores (overwintering spores) emerged as YST senesced and germinated as YST seedlings emerged in the winter. Pycnia were observed shortly after teliospore germination indicating the rust can complete its full life cycle in CA. Reemergence, the appearance of the rust one year after inoculations, occurred only at the Central Valley site. Our results suggest that P. jaceae is likely to persist in California, although establishment is expected to be limited by local environmental conditions.