Population Dynamics of Rhizoctonia Species in Tall Fescue and Creeping Bentgrass in Response to Disease Management Programs

Abstract

Rhizoctonia species are important pathogens of all turfgrasses, yet little is known about the structure of populations of these pathogens or the effect of disease control programs on population dynamics. Isolates of Rhizoctonia were obtained from a single creeping bentgrass putting green in 2002 and 2003 and from multiple tall fescue landscapes in 2003. Creeping bentgrass isolates were obtained by placing 5 mm soil cores on alkaline water agar, incubated at ~24 C for 1 to 2 days and the actively growing hyphae were transferred to potato dextrose agar (PDA). Tall fescue isolates were obtained from foliar lesions using the same procedures. Isolates were characterized using morphological characteristics, anastomosis reactions, nuclear staining, and ribosomal DNA (rDNA) sequences. From creeping bentgrass, 21 distinct clones of R. solani AG 2-2 IIIB were present in the sample population in 2002. Seven of these clones were also recovered on multiple sampling dates. Fifteen of the 21 clones from 2002 were recovered in 2003. All seven clones that were isolated in multiple sampling dates in 2002 were also recovered in 2003. There were 29 and 21 isolates of R. zeae collected from the sample population in 2002 and 2003, respectively. All 21 clones exhibited similar EC₅₀ values to mancozeb, chlorothanlonil and azoxystrobin. Of 224 isolates collected from tall fescue during 2003, 88 were R. solani AG 1, 105 were binucleate Rhizoctonia CAG 1, 14 were R. zeae, two were R. cerealis, and 15 were not identified to species or AG. One hundred and seventy four of the isolates were assayed for sensitivity to the fungicides flutalonil, iprodione, tebuconazole, and trifloxystrobin. Mean EC₅₀ values varied across fungicides and species, but generally were below 3.3 mg L⁻¹. The diversity of Rhizoctonia populations associated with tall fescue, and the range in fungicide sensitivities among species indicates that response to management programs may vary with location and environment. The effects of phosphorous acid on fungal growth, disease incidence and turfgrass quality was evaluated in vitro and in vivo. Mean EC₅₀ values of azoxystrobin (+SHAM) for R. solani 2-2 IIIB, and AG 1, P. aphanidermatum, and R. zeae were 1.27, 0.04, 0.012, and 0.145 mg L⁻¹, respectively. Applications of phosphorous acid to creeping bentgrass and tall fescue did not significantly reduce brown patch incidence or increase turfgrass quality in field or greenhouse experiments. The benefit of phosphorous acid as a fungicide or plant nutrient is questionable.