First Report of Resistance to Pyraclostrobin, Boscalid, and Thiophanate-methyl in Colletotrichum gloeosporioides from Blueberry in Georgia

Colletotrichum gloeosporioides causes anthracnose fruit rot and leaf spot on blueberries. For controlling anthracnose, blueberry growers mostly rely on pre- and postharvest fungicide applications in addition to orchard sanitation. Single-site fungicides including quinone outside inhibitors (QoIs), such as pyraclostrobin and azoxystrobin as well as fungicides containing the succinate dehydrogenase inhibitor (SDHI) boscalid are used frequently to control anthracnose rots and other diseases on blueberry; however, development of fungicide resistance is a real risk because a limited number of fungicides are now available for blueberry disease management. In 2019, three isolates of C. gloeosporioides were cultured from blueberry fruit collected from southern highbush blueberry cultivar ‘Farthing’ in two commercial blueberry fields in Pierce County, Georgia, where ripe rot had been a problem. Fungicide sensitivity tests were conducted using a mycelial growth inhibition assay as described previously. A total of nine fungicides were evaluated to determine the sensitivity of these C. gloeosporioides isolates. All three isolates were resistant to thiophanate-methyl, the QoI fungicide pyraclostrobin, and the SDHI fungicide boscalid. These findings suggest that continuous monitoring of fungicide resistance is necessary to avoid the unwarranted application of single-site fungicides.