Fungicide sensitivity of Clarireedia spp. isolates from golf courses in China

Abstract

Dollar spot, caused by Clarireedia spp. (formerly Sclerotinia homoeocarpa), is a devastating turfgrass disease worldwide. Strong selection pressure imposed by continuous fungicide applications has caused the emergence of fungicide resistance in fungal populations. In this context, a total of 358 isolates of Clarireedia spp., collected from 25 locations in the north, east and south regions of China, were assayed for in vitro sensitivities to thiophanate-methyl, iprodione, propiconazole, and boscalid using discriminatory concentrations of 10 μg/mL, 1 μg/mL, 0.1 μg/mL and 10 μg/mL, respectively. Relative mycelial growth (RMG) was used to determine sensitivity or resistance based on comparison with baseline populations. A total of 18, 11 and 14 of the 25 locations exhibited significantly reduced sensitivity (P < 0.05) to propiconazole, boscalid and iprodione, respectively. Thirteen locations had isolates resistant to thiophanate-methyl while 81%, 67%, 63% and 62% of the isolates had reduced sensitivity to propiconazole, boscalid, iprodione and thiophanate-methy1, respectively. Of the 328 isolates with the history of fungicide use screened, 94% had reduced sensitivity to at least two fungicides. Isolates resistant to thiophanate-methyl had significantly higher EC50 values than sensitive isolates for propiconazole but not for boscalid. Low to highly significant correlations were observed between fungicide sensitivities for isolates from the 25 locations, the three regions, and for all isolates in total. Our findings suggest wide-spread cross-resistance to commonly employed fungicides to manage Clarireedia spp. throughout China and underscore the challenges for managing dollar spot.