Resistance to Azoxystrobin and Thiophanate-Methyl Is Widespread in Colletotrichum spp. Isolates From the Mid-Atlantic Strawberry Fields.

Multiple Colletotrichum species have been found to be responsible for strawberry anthracnose, and prevalence of each species seems to vary by regions and/or host tissues. In this study, a total of 200 Colletotrichum isolates were obtained from different strawberry cultivars displaying anthracnose symptoms in the mid-Atlantic fields. Analysis of g3pdh, tub2, and/or internal transcribed spacer sequences revealed four Colletotrichum species, including C. nymphaeae, C. fioriniae, C. siamense, and C. lineola. C. nymphaeae was the predominant species, representing 90% of all isolates collected. This species was found from all strawberry organs/tissues examined, whereas C. siamense and C. fioriniae were limited to the crown and fruit, respectively. Further, all Colletotrichum isolates were screened for resistance to azoxystrobin in vitro, and all C. siamense isolates were additionally screened for resistance to thiophanate-methyl. The overall frequency of resistance to azoxystrobin and thiophanate-methyl was 48 and 67%, respectively. G143A in the cytochrome b gene was found in all C. nymphaeae and C. siamense isolates with high level of resistance, with EC50 >100 µg/ml, while F129L was found in two of the five C. nymphaeae isolates with moderate resistance, with EC50 values ranging from 2.6 to 7.8 µg/ml. All C. fioriniae isolates tested were found to be less sensitive to azoxystrobin, with EC50 values ranging from 9.7 to 14.4 µg/ml, despite no mutations detected in the cytochrome b gene. Moreover, E198A in tub2 was linked with C. siamense isolates resistant to thiophanate-methyl (EC50 >100 µg/ml). These results revealed that resistance in Colletotrichum spp. to primary fungicides is widespread in the mid-Atlantic strawberry fields.