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Poster: Biology & Disease Mgmt: Chemical Control


Sensitivity of Sclerotinia sclerotiorum to thiophanate-methyl, fluazinam, and procymidone.
M. Lehner (1), R. Silva (1), T. Paula Jr. (2), E. Mizubuti (1) (1) Universidade Federal de Vicosa, Brazil; (2) EPAMIG, Brazil

The sensitivity of 282 Brazilian isolates of Sclerotinia sclerotiorum to thiophanate-methyl (TM), fluazinam, and procymidone was assessed using discriminatory doses and concentration that results in 50% mycelial growth inhibition (EC50). No isolate was resistant to fluazinam or procymidone. The EC50 values varied from 0.003 to 0.007 and from 0.11 to 0.72 mg/ml for fluazinam and procymidone, respectively. One isolate collected in 2010 was resistant to TM (EC50 > 100 mg/mL). The 2010 TM-resistant isolate had a L240F mutation in the b-tubulin gene. This is the first report of mutation at codon 240 conferring resistance to benzimidazole in S. sclerotiorum. The high-resolution melting analysis allowed the distinction of TM-sensitive and TM-resistant isolates by specific melting peaks and curves. The 2010 TM-resistant isolate had mycelial growth, sclerotia production, and aggressiveness comparable with that of the sensitive isolates. In 2015, 8 isolates were collected from the same region where the 2010 TM-resistant isolate was obtained. All 2015 isolates were sensitive to fluazinam and procymidone, but resistant to TM. The 2015 TM-resistant isolates exhibited considerable instability in mycelial growth rates after consecutive transferrings on PDA. Resistance to fluazinam and procymidone was nonexistent or rare. Resistance to TM may occur, but at this point no robust inference can be made about the persistence of TM-resistant isolates in the field.