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

Authors: Fisher, Alison; WOODS, DALE - CA DEPT OF FOOD & AGRI; Smith, Lincoln; Bruckart, William

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2007
Publication Date: 7/1/2007
Citation: Fisher, A.J., Woods, D.M., Smith, L., Bruckart, W.L. 2007. Developing an optimal release strategy for the rust fungus Puccinia jaceae var. solstitialis for biological control of Centaurea solstitialis (yellow starthistle). Biological Control. 42:161-171.

Interpretive Summary: Yellow starthistle is an invasive rangeland weed that infests over 14 million acres in California. In 2003 a rust fungus was introduced to California as a biological control for yellow starthistle. Due to the limited number of fungal pathogens used in biological control programs, there is little information regarding optimal strategies for releases. In 2005 and 2006 we conducted a field experiment to determine the optimal conditions to release the rust fungus at two sites in Northern California: the coastal hills and the Central Valley. At both sites, disease symptoms were visible after each release. In the Central Valley, the fungus was able to reproduce and infect new plants over time. In the coastal hills, however, the fungus disappeared by late spring. A year after releases, the rust had established in the Central Valley but not the coastal hills. Our data suggests that local climate factors, such as temperatures and moisture, are likely to determine where the rust will be able to multiply and persist in Northern California.

Technical Abstract: The rust fungus Puccinia jaceae var. solstitialis (P. j. solstitialis) was first approved for release as a classical biological control agent for Centaurea solstitialis (yellow starthistle, YST) in California in 2003. Because it is difficult to produce large quantities of this obligate pathogen, it was necessary to develop an optimal release strategy for the efficient use of spores. In 2005-2006 we conducted a field experiment in two distinct habitats types, coastal hills and Central Valley, CA, to determine the optimal month for introductions, and to determine if tenting plots at the time of inoculation was necessary to achieve high levels of infection. All releases resulted in infected plants at both sites in both years. At the Central Valley site near Woodland CA, disease incidence was high, compared to the coastal hills, tenting had no effect on infection, and the pathogen persisted throughout YST’s growing season. One year after the 2005 release, P. j. solstitialis had reappeared in most Central Valley plots, regardless of when they were inoculated, although early season releases resulted in higher severity than late season releases. In the coastal hills near, Napa, CA, tenting improved incidence and severity after the January and May releases, perhaps by holding in moisture, but the pathogen did not persist in all plots until senescence and there was no establishment at this site. The rust did not significantly impact plant mortality, biomass, or flower production at either location. Our results show that infection can be achieved from January to June, without the use of tents, however, establishment is likely to be limited by local environmental conditions.