Dispersal and Disease Gradients of Anther-Smut Infection of Silene Alba at Different Life Stages

Given the increased recognition of the importance of spatial aggregation of infected individuals on disease spread, we used a field experiment to examine spore dispersal and disease transmission at increasing distances from an inoculum source in a well-studied system, the herbaceous plant Silene alba, infected by the anther-smut fungus Ustilago violacea. Two different life stages (vegetative and floral) were examined. We measured spore deposition on flowers and, as an analog of dispersal to vegetative plants, in small vials on the ground. Spore and disease gradients (number of spores or proportion of plants infected as a function of distance) were best fit with the classic power law, Y = aD-b. The slope parameter, b, was similar for spore dispersal gradients of vegetative (b = -2.38) and flowering plants (b = -1.91). However, at every distance measured, the proportion of flowers with spores was higher than the proportion of ground vials with spores, indicating overall higher relative spore dispersal to flowering plants. We also determined that disease gradients for flowering plants were more shallow (b-0.07) than spore dispersal gradients, and vegetative infection was more spatially restricted than floral infection (no infection beyond 3.2 m for vegetative plants while floral infection was detected up to 11.2 m). The relationship between spore dispersal and disease incidence was linear for vegetative plants and curvilinear for flowering plants, with limiting returns on disease after a low threshold number of spores deposited. Overall, our study suggests that the limited dispersal of spores is likely to be important in the aggregation of diseased plants in nature ; such aggregation of inoculum in turn can increase or decrease disease spread depending on how much inoculum is needed to cause infection