Characterization, Pathogenicity, and In Vitro Sensitivity of Rhizoctonia spp. Associated with Leaf and Sheath Spot of Bermudagrass Putting Greens in North Carolina and Alabama

Abstract

Twenty isolates of a fungus resembling Rhizoctonia zeae were obtained from three golf courses with bermudagrass [Cynodon dactylon (L.) Pers.] greens that were exhibiting leaf and sheath spot symptoms in 2013 and 2014. Morphological and molecular identification showed that 17 isolates were identified as R. zeae and three were identified as Waitea circinata var. zeae. Pathogenicity of the 20 isolates was determined by inoculating hybrid bermudagrass (‘Champion’) plants in a growth chamber. After inoculation, plants were incubated in a growth chamber at 34/30°C (day/night) and sealed in a closed container to ensure 100% humidity. After 7 d, disease was visually estimated using a modified Horsfall‐Barrett scale (0–10). All isolates were pathogenic to the hybrid bermudagrass cultivar Champion and aggressiveness among isolates varied. Sensitivity of seven isolates collected in 2013 to 16 fungicides (flutolanil, pyraclostrobin, fluoxastrobin, azoxystrobin, propiconazole, triticonazole, triadimefon, penthiopyrad, fluxapyroxad, azoxystrobin + difenoconazole, tebuconazole, difenoconazole, iprodione, polyoxin‐D zinc salt, fluxapyroxad + pyraclostrobin, and chlorothalonil) was tested. The sensitivity of 13 isolates collected in 2014 to five fungicides (chlorothalonil, azoxystrobin, pyraclostrobin, difenoconazole, and flutolanil) was tested. In general, all the isolates were extremely sensitive to demtheylation and succinate dehydrogenase inhibitor fungicides (50% effective concentration [EC50] < 1 mg a.i. L−1) and all isolates showed insensitivity to quinone outside inhibitor fungicides (EC50 > 10 mg a.i. L−1). These results demonstrated that R. zeae is a pathogen of ultradwarf bermudagrass under the imposed environmental conditions. Rhizoctonia zeae is sensitive to many fungicides, but based on in vitro sensitivity data presented here, some may not perform well under field conditions.

Nitrogen source impacts Rhizoctonia leaf and sheath spot severity in ultradwarf bermudagrass

“…Until the 1990s, most host‐pathogen research with R. zeae had been conducted with cool‐season turfgrasses as a host (Martin & Lucas, 1983; Martin et al., 1983; Martin & Lucas, 1984). Isolation of R. zeae from bermudagrass putting green turf became frequent in the late 1990s and early 2000s with the adoption of ultradwarf bermudagrass (UDBG) as a putting surface (Kerns et al., 2017; Martin, 2009). Rhizoctonia zeae was most often isolated from bermudagrass exhibiting frog‐eye patch symptoms ranging from 10 to 40 cm in diameter, possibly larger, in which an outer ring was bleached and tan in color, with the turfgrass in the center of the patch generally unaffected or green (Martin, 2009).…”

“…Although the disease is formally referred to as RLSS, many in the turfgrass industry have adopted the name “mini‐ring” based on symptom appearance in bermudagrass putting greens. In the US Southeast, symptoms can develop as early as mid‐July, but more frequently appear in September or October and persist until dormancy (Kerns et al., 2017; Inguagiato & Martin, 2015).…”

“…When intensively managed, UDBG can tolerate low mowing heights, at or below 4 mm, and has similar texture, density, and playability as creeping bentgrass (Hanna, 1996; Hannah & Elsner, 1999). This has driven the conversion of creeping bentgrass playing surfaces to UDBG in the transition zone of the United States due to the superior heat and summer stress tolerance of UDBG (Kerns et al., 2017; McCarty, 2018). The practice of “no‐till” conversions from creeping bentgrass to UDBG have reduced transitional costs and speed‐of‐establishment, making it possible for low‐ and high‐budget golf courses to transition to UDBG (O'Brien & Hartwiger, 2011).…”