Field studies of a population of Arisaema triphyllum affected by the systemic rust Uromyces ari-triphylli were conducted to examine dynamics of disease transmission and to identify factors influencing plant susceptibility to rust infection. Of undiseased plants that developed foliar infections during the growing season, 55% possessed active resistance to the rust disease, and were able to prevent the foliar infection from becoming a systemic one. Flowering increased the susceptibility of plants to both Mar and systemic infections. A phenotypic selection study was carried out to estimate selection on five plant traits and to determine the role of foliar infections of U. ari-triphylli in mediating selection. Multivariate path analysis suggests that both leaf diameter and a general size factor are under positive directional selection. Pathogen-mediated selection coefficients are small, but comparable in magnitude to overall selection coefficients (15%, on average), indicating the potential importance of diseases as selective agents in nature.

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