Effect of Introduced Biological Control Organisms on the Density of Chondrilla juncea in California

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Abstract
(1) Following the intentional introduction of the exotic biological control organisms Puccinia chondrillina, Cystiphora schmidti and Aceria chondrillae into northern California, reductions in the density of Chondrilla juncea (rush skeletonweed) at three monitoring locations varied from 56.3% to 87.2%. (2) The best correlation between change in plant density and the number of plants attacked by control agents was between the incidence of P. chondrillina on rosettes in early spring and change in rosette number the following year. As in Europe and Australia, P. chondrillina appears to be the organism most damaging to populations of C. juncea in California. (3) While C. schmidti may have local impact on plant density, there were no significant correlations between attack by A. chondrillae and plant decline.