THE EFFECT OF A VECTOR-BORNE DISEASE ON THE DYNAMICS OF NATURAL PLANT-POPULATIONS - A MODEL FOR USTILAGO-VIOLACEA INFECTION OF LYCHNIS-VISCARIA

1. A mathematical model was used to study how a parasitic castrating f ungus affects the population dynamics of its host plant. The model is based on the interactions between the smut fungus Ustilago violacea an d its host plant Lychnis viscaria. The model also includes the pollina tion process as this plays a central role in the transmission of the f ungal spores between plants. 2. Analysis of the model, with the diseas e absent, revealed that the pollinators have a limiting effect on the growth rate of the population. The model did not include any other den sity-dependent processes. 3. A condition was calculated which had to b e satisfied to enable the disease to invade the host population and it is shown that the disease must possess a minimum transmission potenti al to be able to invade the host population. The condition was entirel y dependent on the demographic processes of the host plant. The higher the recruitment rate of the host, the easier it is for the pathogen s uccessfully to invade the host population. 4. When the disease was abl e to invade the host population, the disease incidence varied with the transmission potential of the fungal spores. If the disease incidence rose above c. 50%, the host population was driven to local extinction . This was mainly because, at high disease incidence levels, the fract ion of pollinators that carried pollen was almost non-existent. This l ead to the transmission of the disease to all plants in the population and thus to the extinction of the entire host population.