Unapparent virus infection and host fitness in three weedy grass species

Abstract

Summary The effects of pathogens on natural plant populations are poorly understood, in part because many cause few overt symptoms. I investigated the correlations between the incidence of barley yellow dwarf viruses (BYDVs) and total panicle length, a fitness proxy, in three apparently unaffected hosts, Bromus hordeaceus (Pooideae), Setaria viridis and Setaria lutescens (Panicoideae). Whereas B. hordeaceus individuals did not differ in fitness with respect to infection, average fitness of infected S. viridis was approximately half that of uninfected individuals, and in one out of two years, infected S. lutescens had approximately 25% higher fitness. Variability among hosts appears to contribute more than variability among BYDV serotypes to the pattern of infection outcomes. These correlations suggest that the relationship between BYDVs and these co‐occurring host species ranges from antagonism to mutualism. If so, BYDVs can act as agents of selection in natural populations in which the symptoms of infection are unapparent. The results also demonstrate that infections can be unapparent due to large trait variance rather than small mean effects. Because high trait variances are widespread in many natural populations, unapparent infections may be common in wild plant communities.