1. Theory has shown that sterilizing diseases with frequency-dependent transmission (characteristics shared by many sexually transmitted diseases) can drive host populations to extinction. 2. Anther-smut disease (caused by Microbotryum sp.) has become a model plant pathogen system for studying the dynamics of vector and sexually transmitted diseases: infected individuals are sterilized, producing spores instead of pollen, and the disease is spread between reproductive individuals by insect pollinators. We investigated anther-smut disease in a heavily infected population of Dianthus pavonius (alpine carnation) over an eight-year period to determine disease impacts on host population dynamics. 3. Over the eight years, disease prevalence remained consistently high (>40%) while the host population numbers declined by over 50%. 4. The observed rate of vector transmission to reproductive, adult hosts was inadequate to explain the high disease prevalence. Additional density-dependent aerial transmission to highly susceptible juveniles, indicated from experimental field and greenhouse studies, is likely to play a key role in maintaining the high disease prevalence. 5. Epidemiological models that accounted for the mixed transmission mode predicted an eventual decline in disease. 6. Synthesis: Our results demonstrate that high prevalence of a sterilizing disease does not necessarily drive host populations towards extinction and also highlights the importance of demographic studies for establishing the presence of alternative transmission modes.
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