2005 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative Agreement between ARS and the California Department of Food and Agriculture. Additional details of research can be found in the report for the parent CRIS 1920-22000-031-00D Evaluation and Characterization of Exotic Plant Pathogens for Biological Control of Introduced, Invasive Weeds, under NP 303. The objective of this cooperative research is to achieve successful establishment of the rust fungus, Puccinia jaceae var. solstitialis as a biological control agent on yellow starthistle (YST) in California and determine its effectiveness in controlling field populations of YST. The rust pathogen was released under USDA APHIS and CDFA permits on a small experimental scale plot in Napa, CA in July 2003. The major accomplishment for 2005 was the distribution and release of the rust fungus at 75 locations in 37 counties of California. Cooperators with the California Department of Food & Agriculture organized meetings with representatives of County Agricultural Commissioner offices. At each meeting, the problem (YST) and an anticipated solution (release of the rust fungus) were described. This was followed by distribution of a sample of the spores, a misting bottle, wetting agent, and a plastic tent. Participants then observed a demonstration of the protocol to inoculate plants in the field. CDFA cooperators verified reports of infection by site visits of field locations. In addition, CDFA cooperators used inoculum delivered from FDWSRU in 2003 to produce more inoculum in their greenhouse, so there was sufficient inoculum for YST field inoculations in 2005. Determination of the parameters affecting establishment of the biological control pathogen and its effectiveness in controlling the invasive weed will aid the State of California in developing and implementing effective management strategies in the long term. Control of yellow starthistle will lead to restoration of rangeland, natural areas and endangered habitats to productivity and their natural state.