Phytotoxic Effect of Phosphites on Phosphorus Deficient Creeping Bentgrass Putting Green Turf.

Phosphite-based materials are commonly used in turfgrass management both as a fungicide and as a phosphorus fertilizer. Phosphite must first be transformed to phosphate in order to become available to the plant as a source of P, a bacteria-facilitated process that can take many months.  In phosphate-deficient soils, application of phosphites has been shown to cause phytotoxicity to plants as they falsely sense that levels of phosphate within the plant are sufficient. This effect is not present in plants that are also fertilized with phosphate. A field experiment and a greenhouse experiment were initiated in the spring of 2016. The field experiment consisted of four phosphite-based fungicide materials applied both with and without an equivalent rate of phosphate (applied as a titration of Phosphoric Acid and Potassium Hydroxide) to a creeping bentgrass (Agrostis stolonifera) putting green turf. Treatments were initiated when soil temperatures reached 55 and 65°F and were reapplied every 14-days. Plots were assessed visually for phytotoxicity and turf quality. The greenhouse experiment consisted of 5 rates of a phosphite-material applied with and without equivalent rates of phosphate (Phosphoric Acid & Potassium Hydroxide) to newly seeded pots of creeping bentgrass with a silica sand-based rootzone. Pots were assessed visually for phytotoxicity and Dark Green Color Index (DGCI) measurements were made using the Spectrum FieldScout GreenIndex+ app (Spectrum Technologies, 3600 Thayer Court, Aurora, IL 60504). The objective of this study was to determine whether the phytotoxic effect caused by the phosphite application could be mitigated with the application of an equivalent rate of phosphate.