Effects of the rust Puccinia jaceae var. solstitialis on Centaurea solstitialis (yellow starthistle) growth and competition.
The rust fungus Puccinia jaceae var. solstitialis was introduced to California in 2003 for biological control of yellow starthistle (Centaurea solstitialis). To test its effectiveness under field conditions, we examined biomass production, chlorophyll levels, and seedhead production of yellow starthistle (YST) infected with P. jaceae. We also evaluated the effect of P. jaceae on the competitive ability of YST grown with wild oat (Avena fatua), a widespread winter annual grass commonly associated with YST infestations. Chlorophyll levels were reduced by over 50% in severely rust-infested YST leaves. P. jaceae had no effect on growth or reproductive variables in monoculture plots, but caused a modest reduction in YST performance in the competition experiment. In this study, infected plants had fewer leaves than uninfected plants and slightly reduced rosette diameters. P. jaceae also decreased YST biomass from 3.18 kg m-2 in non-inoculated competition plots to 2.62 kg m-2 in inoculated plots over both years of the experiment. Although not significant, there was a trend towards reduced seedhead production in the inoculated plots. These results indicate that P. jaceae may have some negative effects on YST growth, especially under conditions of interspecific competition. However, the effects of P. jaceae appear to be of minor biological significance and are unlikely to cause major declines in YST populations statewide.