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, Department of Plant Sciences, University of Tennessee, Knoxville, TN 37996

Corresponding author: Erik H. Ervin. ervin@vt.edu

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, resulting in the need for affordable and efficacious products for the prevention of summer diseases.

Phosphite (H₂PO₃^-) is distinct from phosphate (HPO₄^-) in that it is not fully oxidized, with a H occupying the space of an O in the structure. This slight difference renders the phosphite form of P unavailable for plant 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 objective of this study was to compare the efficacy of various commercially-available phosphites for controlling Pythium blight on a perennial ryegrass fairway in Virginia.

The study was conducted on an irrigated perennial ryegrass fairway (0.5 inch mowing height) at Waterfront Country Club in Moneta, VA. Non pythium-active fungicides were applied on a preventive basis to control other summer diseases. Six treatments were applied to foliage with a CO₂-pressurized sprayer calibrated to deliver 40 gal H₂O per acre at 35 psi four times every 14 days during periods favorable for disease development: 9 June, 22 June, 13 July, and 1 August 2005, and 30 June, 13 July, 26 July, and 1 August 2006. These treatments were applied at label rates: an untreated control; Chipco Signature [Aluminum tris (O-ethyl phosphanate, 80%) + a blue green pigment and other ingredients (20%)] at 4 oz/1000 ft²; Aliette [Aluminum tris (O-ethyl phosphanate, 80%)] at 4 oz/1000 ft²; Vital (potassium phosphite, 54.5%, Phoenix Environmental Care) at 6 fl oz/1000 ft²; Ele-Max Foliar Phosphite (0-28-26, derived from potassium hydroxide and phosphorous acid; Helena Chemical) at 5 fl oz/1000 ft²; and as a non-phosphite fungicidal control, Subdue Maxx (mefenoxam, 22%; Syngenta) at 1 fl oz/1000 ft². Pythium blight occurred naturally in the first or second week of July each year. Percentage of pythium blight was visually rated every 7 to 14 days.

Disease was first noted on 13 July in both years, but damage was less than 4% in all plots (Table 1). By 12 Aug 2005, the untreated had 46% damage (Fig. 1a), while all other treatments were statistically similar at less than 4% disease (Figs. 1b and 1c). Similar results were seen in 2006, although the magnitude of control was not as great.

Based on these data, phosphite-based fungicides and fertilizers can be used for preventive control of Pythium blight on perennial ryegrass, although complete control should not be expected each year.

Table 1. Pythium blight disease incidence as influenced by phosphite formulations under field conditions.

Year	Treatment^x	Rate per 1000 ft²	Date
			13 Jul	20 Jul	1 Aug	12 Aug
			Pythium blight (%)
2005	Control	0	1.5 ab	8.8 a	20.0 a	46.3 a
	Signature	4 oz	0 b	0.3 b	0.5 b	2.8 b
	Aliette	4 oz	0 b	0.8 b	0.8 b	3.9 b
	Vital	6 fl oz	0 b	0.3 b	0.5 b	1.3 b
	Subdue	1 fl oz	3.3 a	1.8 b	1.8 b	2.5 b
	Elemax	5 fl oz	0 b	0 b	0.5 b	2.4 b
2006	Control	0	0.5 a	12.3 a	14.3 a	27.5 a
	Signature	4 oz	0 a	3.0 a	0.5 b	5.8 c
	Aliette	4 oz	0 a	4.0 a	5.0 b	16.0 bc
	Vital	6 fl oz	0.8 a	3.8 a	3.8 b	11.8 bc
	Subdue	1 fl oz	0 a	1.8 a	5.3 b	20.5 ab
	Elemax	5 fl oz	0 a	2.0 a	1.8 b	13.8 bc

^x Means followed with same letters within each column for each are not significantly different at P ≤ 0.05.

Fig. 1. Photos showing field symptoms of Pythium leaf blight on a perennial ryegrass fairway on 12 August 2005, due to the treatments: Untreated (a), Chipco Signature (b), and Ele-Max (c).

Literature Cited

1. Smillie, R. H., Grant, B. R., and Guest, D. 1989. The mode of action of phosphite: Evidence for both direct and indirect modes of action on three Phytophthora spp. in plants. Phytopathol. 79:921-926.

2. Rickard, D. A. 2000. Review of phosphorus acid and its salts as fertilizer materials. J. Plant Nutr. 23:161-180.

3. Landschoot, P., and Cook, J. 2005. Sorting out the phosphonate products. Golf Course Manage. 73(11):73-77.

4. Uddin, W., Soika, M. D., Soika, E. L., and Francl, A. 2004. Pythium foliar blight control with the use of fungicides, 2003. Fung. Nemat. Tests 59:T037.