Population Differentiation in Agrostis Tenuis Sibth.

Summary In some of thirty‐seven populations of Agrostis tennis and A. stolonifera sampled by tillers and grown as spaced plants, the incidence of infection by the fungal parasite ‘Choke’, Epichloe typhina, has been found to be remarkably high. These populations are from environments in which vegetative growth is important owing to the incidence of heavy grazing, exposure, etc. A comparison of the growth of infected and non‐infected plants shows that infection, in preventing inflorescence production, causes a considerable increase in tiller density, without any other deleterious effects. These effects are of advantage to plants growing under conditions where vegetative growth is important. It is therefore concluded that infection of Agrostis sp. by Epichloe must be considered to be not an example of parasitism but of a curious form of symbiosis.