Periodic host absence can select for higher or lower parasite transmission rates

This paper explores the effect of discontinuous periodic host absence on the evolution of pathogen transmission rates by using R-0 maximisation techniques. The physiological consequence of an increased transmission rate can be either an increased virulence, i.e. there is a transmission-virulence trade-off or ii) a reduced between season survival, i.e. there is a transmission-survival trade-off. The results reveal that the type of trade-off determines the direction of selection, with relatively longer periods of host absence selecting for higher transmission rates in the presence of a trade-off between transmission and virulence but lower transmission rates in the presence of a trade-off between transmission and between season survival. The fact that for the transmission-virulence trade-off both trade-off parameters operate during host presence whereas for the transmission-survival trade-off one operates during host presence (transmission) and the other (survival) during the period of host absence is the main cause for this difference in selection direction. Moreover, the period of host absence seems to be the key determinant of the pathogen's transmission rate. Comparing plant patho-systems with contrasting biological features suggests that airborne plant pathogens respond differently to longer periods of host absence than soil-borne plant pathogens.