P-622 - Dual Resistance to SDHI and DMI fungicides in Monilinia fructicola from South Carolina Peach Orchards

Abstract Text: Peach growers in South Carolina (SC), USA, rely on FRAC 3, 7, and 11 fungicides for brown rot control. In 2024, outbreaks raised concerns about Monilinia fructicola insensitivity to fungicides. Thirty-two single-spored isolates were collected from three orchards. Five 2024 isolates from two orchards and three historic isolates were tested for in vitro sensitivity to boscalid, isofetamid, fluopyram, fluxapyroxad, and pyraclostrobin based on EC₅₀ values. SdhA, SdhB, SdhC, and SdhD genes were sequenced, and the presence of the DMI resistance marker 'Mona' was evaluated. All isolates were sensitive to pyraclostrobin (EC₅₀ < 0.01 µg/ml). Compared to historic isolates, 2024 ones showed reduced sensitivity to boscalid (EC₅₀ = 2.23–6.84 µg/ml), isofetamid (1.15–36.9 µg/ml), fluopyram (1.91– >10 µg/ml), and fluxapyroxad (1.24–15.07 µg/ml). No mutations were found in SdhA or SdhD. All five 2024 isolates had an A-to-C substitution at codon 230 in SdhB, resulting in N230H, and one also had a C-to-A substitution at codon 80 in SdhC (P80H). All five 2024 isolates carried the 'Mona' element, associated with DMI reduced sensitivity in the southeastern USA. Further screening of all 32 isolates showed that 27 had N230H, two had P80H, and three had both. 'Mona' was present in all. The emergence of SDHI resistance in M. fructicola populations already carrying DMI resistance threatens sustainable peach production in SC.