Anther Smut Transmission in Silene latifolia and Silene dioica: Impact of Host Traits, Disease Frequency, and Host Density

Abstract

Predicting the dynamics of hosts and pathogens in natural systems requires a thorough understanding of the disease transmission process. It has been argued that, unlike for airborne diseases, the transmission of vector-borne diseases such as the anther smut fungus Microbotryum violaceum is a function of the frequency rather than the density of disease in a population. However, recent models indicate that frequency- dependent transmission of vectored diseases is only expected if the time required for interplant movement (host search) by the vector is short relative to the time spent per visit (handling time). Hence, frequency dependence is only expected to dominate at lower ranges of interplant distances. We test this prediction by analyzing disease transmission of M. violaceum in relation to interplant distance in a natural population of the host species Silene latifolia and Silene dioica. We conclude that (1) The infection probability of uninfected hosts strongly increased with the frequency of diseased hosts within a patch, whereas the number of infected plants within a patch had no effect in either of the host species. (2) In agreement with predictions based on vector behavior, the probability of infection in patches of S. dioica increased more strongly with increasing disease frequency among patches with high average host density (i.e., small interplant distance) than among patches with low host density. This finding is consistent with the idea that transmission should shift from a more frequency- to a more density-dependent process when host search time increases relative to handling time. (3) In addition to local disease frequency and host density, individual host traits (host sex, size, and phenology) explained a significant part of the variation in infection probability within host species. The precise effects of individual host traits depended on local neighborhood characteristics. For instance, in S. latifolia, male hosts were on average more susceptible than female hosts, but the difference diminished with increasing disease frequency. [KEYWORDS: Violacea microbotryum-violaceum; ustilago-violacea; spore deposition; experimental populations; alba; infection; patterns; dynamics; epidemiology; genetics