First Report of Resistance to Pyraclostrobin in Pseudocercospora spp. from Blackberry (Rubus fruticosus) in Georgia, U.S.A.

Pseudocercospora leaf spot is frequently observed in blackberry production in the southeastern U.S. To manage the impacts of leaf spot diseases, blackberry growers in the southeastern U.S. typically rely on the application single-site fungicides including the quinone outside inhibitors (QoI) (FRAC 11) pyraclostrobin and azoxystrobin. In recent years, despite the regular application of chemical fungicides to manage leaf spots, blackberry growers and extension personnel in Georgia have noticed relatively poor control of these diseases. To investigate potential control failures due to fungicide resistance development, Pseudocercospora were isolated from symptomatic leaves collected from blackberry (Rubus fruticosus) in five commercial fields in southeastern Georgia. Using a mycelial growth inhibition assay, isolates were screened for resistance to pyraclostrobin. Among the 13 isolates tested, 12 were determined to be resistant to the fungicide. Subsequent sequencing of the cytochrome b gene indicated the presence of the G143A mutation in all fungicide-resistant isolates. The identification of fungicide resistance in 12 of 13 isolates and 4 of 5 examined commercial blackberry sites in Georgia suggests that resistance to this fungicide may be prevalent among Pseudocercospora on blackberry in Georgia and that future fungicide resistance monitoring efforts in Georgia blackberry plantings may be warranted. This represents the first report of QoI fungicide resistance in Pseudocercospora spp. from blackberry.