© 2009 Plant Management Network. Accepted for publication 23 August 2009. Published 19 October 2009. Efficacy of Phosphite Fungicides and Fertilizers for Control of Pythium Blight on a Perennial Ryegrass Fairway in Virginia Erik H. Ervin, Department of Crop and Soil Environmental Sciences, and David S. McCall, Department of Plant Pathology, Physiology, and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061; and Brandon J. Horvath, Dep artment of Plant Sciences, Universit y of Tennessee, Knoxville, TN 37996 Corresponding author: Erik H. Ervin. ervin@vt.edu Ervin, E. H., McCall, D. S., and Horvath, B. J. 2009. Efficacy of phosphite fungicides and fertilizers for control of pythium blight on a perennial ryegrass fairway in Virginia. Online. Applied Turfgrass Science doi:10.1094/ATS-2009-1019-01-BR. Perennial ryegrass ( Lolium perenne L.) is a cool-season turfgrass of temperate climates. Due to its high susceptibility to summer diseases such as Pythium blight [ Pythium aphanidermatum (Edson) Fitzp.] and gray leaf spot ( Magnaporthe oryzae Couch), it is rarely planted as a monostand in hot and humid summer climate areas such as the Mid-Atlantic. One exception is golf course tees and fairways in the Mid- Atlantic, resultin g in the need for affordable and efficacious products for the prevention of summer diseases. Phosphite (H PO ) is distinct from phosphate (HPO ) in that it is not fully oxidized, with a H occupying the space of an O in the structure. This slight difference renders the p hos p hite form of P unavailable for p lant nutrition and is thought to be the key to its action in plants and susceptible fungi (2). Current understanding of how phosphites control certain diseases involves both direct and indirect effects. First, phosphites have been shown to be directly fungitoxic to genera of oomycetes such as Phytophthora and Pythium . Phosphite perturbs P metabolism in these oomycetes by causing a massive accumulation of poly- and pyro-phosphate and inhibiting certain enzyme activities that are essential for fungal energy production and use (1). Perturbation of phosphorus metabolism in plants due to phosphite treatment appears to cause greater expression of defense-related genes (1). Phosphite fungicides and fertilizers are formulated in various ways. Potassium phosphite, formed by neutralizing the acidity of phosphonic acid with potassium hydroxide, is the active ingredient in some EPA-registered fungicides (e.g., Chipco Signature, Vital Sign, Magellan), plus a number of fertilizers (e.g., EleMax, K-Phite, PK Plus). Phosphonic acid can be reacted with ethanol to form ethyl phosphonate. Aluminum ions are added to neutralize the ethyl-phosphonate to form aluminum tris, the active ingredient in the industry standards: Chipco Signature and Aliette (Bayer Environmental Science). Phosphonates and phosphites can be considered as equivalent compounds as upon plant absorption all formulations become incorporated into plant cells as phosphite ions (2). Landschoot and Cook present an extensive review of this topic as it pertains to turfgrass systems (3). However, only one of eight recent Plant Disease Management Reports present phosphite efficacy data for control of Pythium blight on perennial ryegrass (4), but at this site in Pennsylvania disease had to be artificially induced. The ob jective of this stud y was to com p are the efficacy of various commercially-available phosphites for controlling Pythium blight on a perennial ryegrass fairway in Virginia. 2 3 - 4 - 19 October 2009 Applied Turfgrass Science