Effects of Phosphorus Acid Compounds on the Incidence of Rhizoctonia solani in Tall Fescue and Creeping Bentgrass

Phosphorous acid-containing compounds have been shown to be effective in controlling certain fungal pathogens. The objective of this study was to evaluate the effectiveness of the active ingredient of phosphorous acid (phosphite) in vitro and in vivo. One isolate each of Rhizoctonia solani AG 1, R. solani AG 2–2IIIB, Rhizoctonia zeae, and Pythium aphanidermatum were obtained from turfgrasses and assayed for sensitivity to phosphorous acid and the fungicides fosetyl-Al, azoxystrobin, and azoxystrobin amended with salicylhydroxamic acid (SHAM). Six concentrations of each compound (0.01–10 mg L⁻¹) were established in 9-cm diameter Petri dishes. Radial growth of the colonies on the amended agar was measured to determine the relative growth of colonies compared with the growth of the colonies on unamended medium. Parallel greenhouse and field studies were initiated to determine the effects of phosphorous acid on disease incidence and overall turf quality. Mean 50% effective concentrations were observed for all four pathogens with the azoxystrobin amended with SHAM treatment. Effective concentrations for the 2–2IIIB, AG1, P. aphanidermatum, and R. zeae were 1.27, 0.04, 0.012, and 0.145 mg L⁻¹, respectively. Azoxystrobin alone did not reduce colony growth by 50% in the 2–2IIIB and R. zeae isolate. Greenhouse and field studies showed differences (p < 0.05) between two phosphorous acid rates, turf quality, and disease incidence of R. solani in tall fescue (Festuca arundinacea Schreb. ) and creeping bentgrass (Agrostis stolonifera L. ) turf and the control. However, disease incidence was at an unacceptable level in the phosphorous acid treatments. Phosphorous acid applications did increase the elemental phosphorus composition of shoot tissue.