ORIGINAL ARTICLE Impact of disease frequency and host density on pollination and transmission of an African anther-smut fungus Helen R. Curran • Le ´anne L. Dreyer • Francois Roets Received: 4 June 2012 / Accepted: 9 July 2012 / Published online: 3 August 2012 Ó Springer-Verlag 2012 Abstract The vast majority of flowering plants rely solely on insects for pollination. A number of pathogens have evolved mechanisms to exploit these close associa- tions and use pollinators as vectors of infective propagules. Factors that affect pollinator movements and successful pollination may in turn also influence successful trans- mission of fungal spores. Here we investigate the effect of host density and the frequency of diseased Oxalis lanata individuals infected by the anther-smut fungus, Theca- phora capensis, on the likelihood of receiving pollen and fungal spores. Specifically, we determined the numbers of spores and pollen grains deposited on stigmatic surfaces of selected flowers under natural and standardized conditions where host density and disease frequency varied. The effect of host flower density and diseased flower frequency on pollen and spore transfer was variable under natural con- ditions and these factors interacted significantly. However, an increase in host density and disease frequency signifi- cantly influenced pollen and spore deposits under stan- dardized conditions. The effect of host density was, however, not linear and an optimal flower density for pollen and fungal spore transmission was found. Similar to other systems of vector-borne disease, the transmission of anther-smut of Oxalis lanata is more frequency-dependent than density-dependent. This study represents a first step towards understanding the disease transmission process of T. capensis on Oxalis and lays the foundation for future comparative studies between this and other systems to develop and test general hypotheses of disease dynamics in vector-borne disease transmission systems. Keywords Thecaphora capensis Á Oxalis lanata Á Spore transmission Á Microbotryum violaceum Á Anther-smut Introduction Most angiosperm species rely on insects for transfer of pollen among individuals for sexual reproduction. Pollin- ators are usually attracted to these plants through floral display and odour, and are rewarded with pollen and nectar (Pacini et al. 2003). This mutualistic interaction has resulted in the explosion of flowering plant numbers in the recent evolutionary history of the planet (Pellmyr 1992; Johnson 1996; Barrett 2002). Although pollinators are essential for plant reproduction, they provide a similar service to flower-sporulating pathogens by dispersing their reproductive propagules (Roy 1994). Some of these pathogens aid their own dispersal by contributing towards floral displays (Shykoff et al. 1997), altering flowering phenology (Jennersten 1988), and enhancing olfactory cues and nectar rewards (Shykoff and Bucheli 1995) of their hosts. This specialised mode of spore transmission is thought to be due to larger dispersal distances and more effective transmission using insects as vectors rather than relying on passive dispersal by wind and rain (Alexander 1990; Roche et al. 1995). In a recently described system a smut fungus, Theca- phora capensis, was found to be transmitted between Oxalis flowers by flower visitors in the southwestern Cape of South Africa (Roets et al. 2008; Curran et al. 2009). This systemic pollinator-transmitted disease sterilizes its host by H. R. Curran Á L. L. Dreyer Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa F. Roets (&) Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch, South Africa e-mail: fr@sun.ac.za 123 Planta (2012) 236:1677–1685 DOI 10.1007/s00425-012-1727-6