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Geographic and cultivar distribution of QoI-resistant Alternaria alternata isolates, causal agent of Alternaria brown spot on Florida tangerine hybrids.
B. VEGA (1), M. M. Dewdney (2). (1) University of Florida, Gainesville, FL, U.S.A.; (2) University of Florida, CREC, Lake Alfred, FL, U.S.A.

Strobilurins are a commonly used fungicide class for A. alternata control in Florida, but there are increasing reports of control failure. To determine the sensitivity of the statewide A. alternata population, a survey was begun in 2010. More than 300 monoconidial isolates of A. alternata were tested from 28 citrus orchards with different QoI application histories in 7 counties. Sensitivity to azoxystrobin and pyraclostrobin was determined by RZ-based microtiter assay. A highly significant (P < 0.001) and strong correlation for fungicide cross-resistance was found between azoxystrobin and pyraclostrobin. Of the isolates tested, 63% were resistant to both fungicides. The EC<sub>50</sub>’s for resistant isolates were greater than 5 µg/ml for azoxystrobin and 1 µg/ml for pyraclostrobin. The EC<sub>50</sub>’s for sensitive isolates (37%) ranged from 0.016 to 0.696 µg/ml for azoxystrobin, and from 0.00345 to 0.0922 µg/ml for pyraclostrobin. The resistant phenotype was widespread among counties. Resistant isolates were recovered with higher frequency from the cultivar Minneola, followed by cultivars Murcott, Dancy and Orlando. Field disease severity was low in Sunburst and Orlando, moderate in Murcott and Minneola and high in Dancy and Lee. The presence of mixed sensitive and resistant populations also was observed in 13 of 28 citrus orchards. Molecular characterization of the cytochrome b gene will confirm if the resistant phenotype is associated with G143A substitution or an additional one. <p><p>Keywords: Fungus, Fruits-Nuts, Citrus

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