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Baseline sensitivity of tangerine-infecting Alternaria alternata to boscalid and molecular characterization of the SDH complex
B. VEGA (1), M. M. Dewdney (1). (1) University of Florida, Lake Alfred, FL, U.S.A.

Altenaria brown spot (ABS) affects leaves, twigs, and young fruit, reducing fruit quality of fresh fruit tangerines in Florida. Disease control has depended on fungicide applications, but with the emergence of quinone outside inhibitor (QoI) resistance in <i>A.</i> <i>alternata</i> populations, the use of new chemical options is paramount. Boscalid, a succinate dehydrogenase inhibitor (SDHI), was registered in 2011 to control ABS. The objective of this study was to determine baseline sensitivity to boscalid in <i>A. alternata </i>isolates collected over multiple years from 14 Florida counties. When assessed, conidia germination and mycelial growth were not completely suppressed even at high concentrations. The most consistent effective concentration to inhibit 50% growth (EC50) was found with the resazurin-based microtiter test. The mean and median EC50 values of 419 isolates were 0.60 and 0.44 µg/ml, respectively. The mean EC50 values of isolates from different cultivars were not significantly different (<i>P </i>= 0.1350). Similarly, the mean EC50 values of isolates from different virulence classes were not significantly different (<i>P </i>= 0.1888). The SDH complex (<i>SdhB</i>, <i>SdhC</i>, and <i>SdhD </i>genes) was partially sequenced. Polymorphism in all SDH subunits was found, but especially in SDHD where 12 mutations were found in 13 of the 15 isolates analyzed but no mutations conferring resistance were found. It is concluded that <i>A. alternata </i>populations of Florida are currently sensitive to boscalid

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