EPIDEMIOLOGY OF ANTHER-SMUT INFECTION OF SILENE-ALBA CAUSED BY USTILAGO-VIOLACEA - PATTERNS OF SPORE DEPOSITION AND DISEASE INCIDENCE

Infection of Silene alba by the anther-smut fungus Ustilago violacea results in development of stamens that produce fungal spores. Insects visit diseased flowers and transmit spores to healthy flowers. During peak flowering, nearly all healthy flowers (97%) in a population with 25% diseased plants in Virginia had spores deposited on them; later in the summer, spores were deposited on 60% of the flowers. The probability of spore deposition was positively related to the number and proportion of diseased flowers in the population but total flower number had little effect. Number of flowers present on a plant did not affect spore deposition per flower. When diseased flowers were introduced into a healthy population, the likelihood of spore deposition decreased with increasing distance from the inoculum; but some spores were moved at least 10 m. The gradient of diseased plants that resulted from infection during the experiment was less steep than the spore dispersal gradient, suggesting that heavy spore deposition near the inoculum, source does not increase disease incidence. Anthur-smut infection can also occur by spores entering vegetative tissue. Experimental studies of the spatial spread of disease by seedling infection resulted in a steep disease gradient, with 88% of infections occurring within 5 cm of the inoculum source. -from Author