Submitted to: Western Society of Weed Science Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: February 3, 2006
Publication Date: N/A
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 a field experiment was initiated to determine the optimal conditions and time of year to release the rust fungus. Experimental plots were established in Northern California the coastal hills and the Central Valley. The fungus was released on a monthly basis from January to June. Tents were used on half of the plots to maintain high humidity the night after inoculations. 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 May in plots inoculated earlier in the season. Tenting increased the success of inoculations when night temperatures dropped below 0ºC. Therefore, tents may protect release plots from the detrimental effects of cold. Our results show that the rust fungus can be released over most of yellow starthistles growing season, and we expect high rates of infection at sites where the fungus can reproduce, for example in the Central Valley.
Technical Abstract: The rust fungus Puccinia jaceae var. solstitialis (P. jaceae) was first released as a biological control for yellow starthistle in 2003. Due to the limited number of fungal pathogens used in biological control programs, there is little information regarding optimal strategies for releases. A field experiment was initiated in 2005 to determine the optimal time of year for P. jaceae introductions and to determine if tents were necessary to achieve high levels of infection after plants were inoculated in the field. Permanent experimental plots were established outside the cities of Napa, Napa County and Woodland, Yolo County. Six blocks, each comprised of seven permanent plots, were installed at each site. Within each block, one plot was repeatedly inoculated every four to five weeks from January to June, five plots received single inoculations, (one plot for each inoculation date), and one plot was an un-inoculated control. Half of each plot was tented overnight to determine if tents substantially increase disease incidence and/or severity. Disease incidence ranged from 10% to 80% in Woodland. Disease incidence ranged from 0% to 50% in Napa. Severity remained relatively constant from January to July in Woodland; in Napa, severity increased during the summer months of June and July. Tenting did not have an effect on disease incidence or severity at the Woodland site. In Napa, incidence was higher in tented plots in both January and May. In January, the temperature outside tents dropped to 0oC, while inside temperatures remained above freezing. Therefore, tents may protect release plots from the detrimental effects of cold. Our results show that infection can be expected after inoculations with P. jaceae during most of yellow starthistle’s growing season. In addition, while tenting is not necessary for infection, tents may increase the likelihood that plants develop symptoms in cool temperatures.