Incidence of the endophyte fungus Neotyphodium coenophialum in pastures of tall fescue differing in age and soil characteristics

Neotyphodium coenophialum is an endophyte fungus living in a mutualistic symbiosis with tall fescue [Schedonorus arundinaceus (Schreb.) Dumont]. The endophyte infection affects both health and animal production given the toxicity of the infected plants (fescue toxicosis). Our objective was to determine the incidence of N. coenophialum in some fescue populations placed in both high and low relief, of sites with different aged pastures as well as to evaluate its relationship with soil and vegetation characteristics. Study sites were three and four fields having old and young pastures, respectively, located southeast of Buenos Aires Province, Argentina. In each field, two populations of tall fescue, one from a low relief and the other from a high one, were sampled. Low relief sites were characterized by poor saline-alkaline soil conditions with halophytic vegetation and relatively low ground cover, and high relief ones had better soil conditions dominated by mesophytic grasses and full ground cover.  The endophyte incidence across all the evaluated environmental conditions was higher than 95%. No differences were found attributable to soil characteristics and age of pastures, as predicted from the hypothesis of differential abiotic stress tolerance between infected (E+) and non infected (E-) plants. Therefore, it must be provisionally rejected. Alternative hypothesis must consider either effects of preferential herbivory on E- plants, high propagule pressure of infected fescue dispersing from the field margins, and/or  higher recruitment rate for E+ than E- seeds from the soil seed bank.   Key words: Endophyte-grass symbiosis, infection, fescue toxicosis, plant invasion, natural reseeding.