Characterization of Monilinia fructicola Strains Resistant to Both Propiconazole and Boscalid

In 2011 and 2012, significant brown rot disease caused by Monilinia fructicola was observed in a peach orchard in Spartanburg County, SC, despite preharvest fungicide applications of demethylation inhibitor (DMI), quinone outside inhibitor (QoI), and succinate dehydrogenase inhibitor (SDHI) fungicides. All 22 isolates obtained in 2011 from this orchard were sensitive to the QoI fungicide, azoxystrobin, and the methyl benzimidazole carbamate (MBC) fungicide, thiophanate-methyl. Five were resistant to the DMI fungicide, propiconazole, and were selected, together with five propiconazole-sensitive isolates, for further investigations. One of the 10 isolates was resistant to propiconazole but sensitive to the SDHI fungicide, boscalid (EC₅₀ = 0.42 μg/ml), 3 were resistant to propiconazole with intermediate sensitivity to boscalid (EC₅₀ 0.72 to 2.1 μg/ml); 2 were sensitive to propiconazole with intermediate sensitivity to boscalid; 3 were sensitive to propiconazole but resistant to boscalid (EC₅₀ ≥ 2.1 μg/ml); and 1 (isolate MD22) was resistant to both propiconazole and boscalid. Disease incidence on detached fruit treated with formulated propiconazole or boscalid was significantly higher for MD22 compared to a sensitive control isolate. Continued monitoring of fungicide resistance in the same orchard in 2012 revealed an increase of isolates resistant to propiconazole from 22.7% in 2011 to 34.7%, and an increase of isolates resistant to both propiconazole and boscalid from 4.5% in 2011 to 18.4%. Propiconazole resistance was always associated with the presence of the 'Mona' mobile element located upstream of the sterol 14α-demethylase (MfCYP51) gene. To investigate whether mutations in the subunits of the succinate dehydrogenase enzyme were involved in boscalid resistance, significant portions of the M. fructicola SdhA, SdhB, SdhC, and SdhD genes were cloned and analyzed for 2 sensitive, 2 boscalid-resistant, and 6 dual-resistant isolates. Although sequence variation was found among the isolates, no single change correlated with resistance. Interestingly, analysis of isolates collected from orchards in 2001 and 2002, prior to the registration of boscalid, revealed a range of sensitivities to boscalid (EC₅₀ 0.03 to 3.46 μg/ml) including boscalid-resistant isolates. The presence of boscalid-resistant isolates in the baseline population was unexpected and requires further investigation.