Seasonal dynamics and fungicide sensitivity of organisms causing brown patch of tall fescue in North Carolina

ABSTRACT

Brown patch, caused by multiple species of Rhizoctonia and Rhizoctonia-like fungi, is the most severe summer disease of tall fescue in home lawns across the southeastern United States. Home lawns were surveyed in central North Carolina from 2013 to 2015 to determine the organisms present during typical epidemics of brown patch in tall fescue. Isolates of Rhizoctonia and Rhizoctonia-like fungi were obtained by sampling 147 locations in July 2013 and May and July 2014. In addition, 11 sites were sampled once a week for 12 consecutive weeks from late May to the end of July 2015. All isolates were identified to species and anastomosis group with nuc rDNA internal transcribed spacer (ITS) sequence analysis. Isolations from brown patch lesions in May 2014 predominately yielded Ceratobasidium cereale (77% of the organisms recovered), whereas the organisms recovered in July 2013 and 2014 were R. solani AG 2-2-IIIB (44%), R. solani AG 1-IB (37%), and R. zeae (14%). In 2015, Ceratobasidium cereale was isolated from all 11 locations in May but was replaced by Rhizoctonia species in June and July. Sensitivity of the May 2014 isolates to multiple concentrations of the fungicides azoxystrobin, flutolanil, fluxapyroxad, and propiconazole was compared with sensitivity of isolates collected in 2003, to determine whether multiple years of exposure to fungicides applied for brown patch control had altered fungicide sensitivity. Historical isolates of R. solani, which had never been exposed to fungicide applications for brown patch control, were also included for comparison. Mean EC₅₀ values (concentration of fungicide needed to inhibit mycelial growth by 50%) varied across fungicides and species, but no resistance was observed, and there was no apparent shift in sensitivity over the years. An additional 94 isolates from 2015 were screened against azoxystrobin, flutolanil, fluxapyroxad, and propiconazole, and fungicide insensitivity was not observed.

KEYWORDS:

• Binucleate Rhizoctonia
• Thanatephorus cucumeris
• turfgrass
• Waitea circinata var. zeae

Funding

This research was supported by the North Carolina State University Center for Turfgrass Environmental Research and Education and the North Carolina Agricultural Research Service.