Developing an optimal release strategy for the rust fungus Puccinia jaceae var. solstitialis for biological control of Centaurea solstitialis (yellow starthistle)

The rust fungus Puccinia jaceae var. solstitialis (P. j. solstitialis) was first approved for release in California in 2003 as a classical biological control agent for Centaurea solstitialis (yellow starthistle, Asteraceae). It is difficult to produce large quantities of this obligate pathogen so it was necessary to develop an optimal release strategy for the efficient use of urediniospores. In 2005–2006 field experiments were conducted in two distinct habitats types, the coastal hills and Central Valley, CA, to determine the optimal month for introductions, and to determine if enclosing plots in tents at the time of inoculation was necessary to achieve high levels of infection. All releases resulted in infected plants at both sites for both years. At the Central Valley site near Woodland CA, disease incidence was higher than at the coastal hills, tent enclosures had no effect on infection, and the pathogen persisted throughout the growing season. One year after the 2005 release, P. j. solstitialis had reappeared in most Central Valley plots, although early season releases in 2005 resulted in greater severity in 2006 than the late season releases. In the coastal hills near Napa, CA, tent enclosures improved disease incidence and severity after January and May releases, perhaps by retaining moisture, but the pathogen did not persist in all plots until plant senescence, and there was no reinfection (the following spring) at this site. The rust fungus did not have a negative affect on plant mortality, biomass, or flower production at either location. Our results show that infection can be achieved from January to June, and tent enclosures sometimes appeared to increase infection; however, reinfection is probably limited by local environmental conditions.