The influence of plant spacing on density-dependent versus frequency-dependent spore transmission of the anther smut Microbotryum violaceum

The anther smut fungus Microbotryum violaceum is a pollinator-transmitted p lant disease. As for other vector-borne diseases, frequency-dependent trans mission patterns are predicted, in contrast to the density-dependent transm ission of passively spread diseases. Frequency dependence will, however, on ly arise if vectors compensate for varying plant spacings. To test this ass umption, we set up experimental populations of the host plant, Silene latif olia, with varying disease density (number of diseased plants per plot) and frequency (proportion of plants diseased), and three different plant spaci ngs. We measured spore deposition on healthy flowers in these plots on two dates. Spore deposition decreased considerably from the first to the second census, perhaps related to the concomitant decrease in inflorescence sizes of diseased plants. At our first census, spore deposition rates varied wit h disease frequency, and the effect of frequency depended on plant spacing. While spore deposition was positively frequency dependent at the 1.5-m int er-plant spacing, no effect of disease frequency was found at a spacings of 0.5 m or 3 m. Nor was there an effect of disease density on spore depositi on at the first census. At the later census, on the other hand, spore depos ition increased almost significantly with increasing disease density (P = 0 .08). This difference in deposition pattern together with a significant dec rease in spore receipt indicates changes in pollinator spectrum and/or acti vity. The correlation of spore numbers among flowers within plants, an indi cation for intra-plant moves by vectors, was significant at 0.5 m and 1.5 m but not at 3 m. Floral traits and sex of individual plants influenced the number of spores they received. On the first census date, spore deposition increased with increasing inflorescence size in female but not in male plan ts. On the second census date, neither sex nor number of open flowers had a n effect on spore receipt. None of the experimental plants became infected, however, probably because of the unusually hot and dry weather.