Evidence Against Fusarium Poae and Siteroptes Graminum as Causal Agents of Silver Top of Grasses

Abstract

HARDISON, J. R. (Oregon Agric. Exp. Sta., Corvallis) Evidence against Fusarium poae and Siteroptes graminum as causal agents of silver top of grasses. Mycologia 51: 712-728. 1961.-Silver top, the premature death of inflorescences from damage to the lower part of the terminal or penultimate internode of grasses, was formerly considered a disease caused by Fusarium poac (Pk.) Wr. in association with a mite-vector Siteroptes gratninum (Reut.). This theory is challenged by a weak association of microorganisms determined by recovery from 2355 silver-top-affected culms of Festuca arundinacea Schreb., F. rubra var. commiutata Gaud. and Poa pratensis L. The exterior of the leaf sheath was surface-disinfected in 1957 but not in 1958. Nearly half of the plated stem segments remained sterile on PDA. More sterile stems were noted in collections early in the season than later, suggesting gradual invasion of dead stems by saprophytes. S. graminuml was not recovered from more than 95% of the grass stems studied. Fusarium poae is easily isolated but was recovered from only 2.1% of 236 damaged stems of F. arundinacea, 2.9% of 102 F. rubra var. commutata stems collected in a late-pastured field, and less than 20% of all grass stems studied. The incidence of F. poae increased as the season advanced, suggesting progressive invasion of stems killed by other agents. Newly killed stems generally had lower incidences of F. poae and S. graminum than older dead stems with white panicles. The general absence of microorganisms in most silver-top-affected culms of grasses in Oregon, a revaluation of the literature, and control of the disorder by DDT are interpreted as strong evidence against F. poae and S. graminuml as causal agents of a disease and suggest a return to insects as the primary agents causing silver top of grasses. Silver top, the premature death of the inflorescence resulting from an injury to the lower part of the terminal or pelultimate internode, affects many grasses throughout the world. Destruction of the entire inflores