Establishment of the Yellow Starthistle Rust in California: Release, Recovery, and Spread.

The rust fungus Puccinia jaceae var. solstitialis is the first pathogen released for biological control of yellow starthistle (Centaurea solstitialis). From 2004 to 2006, the pathogen was released at 176 sites in 40 counties throughout the state of California. Release sites were evaluated 1 to 3 months and 1, 2, and, in some cases, 3 years after releases to monitor for reemergence. At 1 to 3 months after inoculation, 58 to 93% of sites had rust infection, depending on the year. After 1, 2, and 3 years, the percentages declined to 19 to 21, 9 to 10, and 3% respectively. Spread was detected at 19% of the sites with rust infection, with an average distance of 21 m (±13.3 standard error). The greatest spread occurred at a site in Sonoma County. At this site, the rust spread to over 37 acres 1 year after it was released and has remained in this area for three seasons. Reemergence 1 and 2 years after inoculations was more likely in Northern (above 40°N) compared with Southern California (below 36°N). In general, reemergence was more likely at lower elevations when release sites were within 150 km of the coast. Overall, the rust has not demonstrated a strong record of persistence based on these observations.