Preferential Grazing by Molluscs of Plants Infected by Rust Fungi

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Abstract
In the field infection by pathogenic rust fungi was observed to increase invertebrate herbivory of a number of plant species. This phenomenon was investigated in detail in Allium ursinum L. infected by Puccinia sessilis Schroter and Tussilago farfara L. infected by Puccinia poarum Niels and Coelosporium tussilaginis (Pers.) Berk. In the field, 57% of P. sessilis pustules on A. ursinum were grazed and the area consumed by herbivores was significantly greater in rusted than control leaves in nine out of the ten host populations studied. Examination of herbivores associated with rusted A. ursinum suggested that much of the observed grazing could be attributed to several species of mollusc. In a controlled perference test the slug Arion ater L. consumed 6-23 times more rusted tissue than controls. Aeciospores of P. sessilis were present in the faeces of 44 .+-. 4% of the snail Trichia striolata Pfeiff. 48% of P. poarum-infected T. farfara leaves showed evidence of selective grazing. Such leaves had 29.25 .+-. 3.52 rust pustules of which 4.57 .+-. 0.66 (ca. 17%) were grazed. The slugs Arion hortensis Fer. and A. fasciatus Nilss. consumed significantly greater amounts of P. poarum-infected T. farfara than uninfected tissues but another slug Deroceras reticulatum Muell. showed no significant preference. Grazing of T. farfara by A. fasciatus was also significantly increased after infection by C. tussilaginis.

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