DISEASE SPREAD AND POPULATION DYNAMICS OF ANTHER-SMUT INFECTION OF SILENE ALBA CAUSED BY THE FUNGUS USTILAGO VIOLACEA

SUMMARY (1) The anther-smut fungus, Ustilago tiolacea, sterilizes its dioecious host plant Silene alba by transforming plant reproductive parts into stamen-like structures that produce and disperse spores. To examine the dynamics of the plant-fungus interaction, a population of S. alba in Virginia, U.S.A. was mapped, and followed over two years, to study the demography of healthy and diseased individuals and spread of the disease. Additional field experiments explored the processes of spore dispersal and floral infection. (2) The number of spores deposited on male, but not female, flowers was related to the number and proportion of diseased flowers close-by. (3) Within a flowering season, both natural levels of floral infection and infection levels resulting from any inoculation experiment were about 20%/o. Observation of marked plants revealed, however, that only 4'%o of plants healthy in one year were diseased the following year, indicating that floral infection does not always lead to successful systemic infection. (4) A deterministic model was used to explore the effects of plant recruitment and disease-spread on the fate of infected populations. Either elimination of the fungus, coexistence of plant and fungus, or local extinction of both organisms could occur depending on circumstances.