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Tall fescue (Festuca arundinacea Schreb.) is the predominant perennial cool-season grass grown in the USA. Typically, tall fescue is infected with the endophyte, Neotyphodium coenophialum Morgan- ones & Gams, which produces alkaloids that are toxic to grazing animals. Nontoxic endophyte-infected cultivars of tall fescue have been developed, but to maximize their utility for profitable livestock production a better understanding of conditions affecting seed and tiller transmission is needed to maintain endophytes in seed. Our understanding of mechanisms of endophyte transmission in planta is limited. Seasonal variations of endophyte in established tall fescue pastures in Watkinsville, GA, and seed fields near Salem, OR, were examined. Growth chamber experiments were conducted to examine temperature effects on plant and endophyte growth and to determine the cardinal minimum temperatures for each. Endophyte frequency varied over months in both Georgia and Oregon. Frequency averaged 93.4% when sampled April through December, but was 80.5% when sampled January through March in Georgia. Frequency averaged 64.5% when sampled February through April, but was 88.6% during other months in Oregon. Cardinal minimum temperature for plant growth was 5.2°C (± 0.5), but for endophyte was 10.3°C (± 0.7). Temperature appears to be a major variable affecting fluctuation of endophyte frequency in plant tissue. [ABSTRACT FROM AUTHOR]