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Oral: Fungicide Resistance


Fungicide resistance profiles of unique and clonal genotypes of gummy stem blight fungi from the southeastern United States
H. LI (1), K. Stevenson (2), M. Brewer (1) (1) University of Georgia, U.S.A.; (2) University of Georgia, U.S.A.

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Gummy stem blight (GSB) is a devastating disease of cucurbits that is usually managed with fungicide applications. However, GSB fungi have rapidly evolved resistance to multiple classes of fungicides. To better understand the evolution and persistence of fungicide resistance in field populations, we studied resistance profiles associated with unique and clonal genotypes of GSB fungi. Nineteen Stagonosporopsis caricae isolates and 120 S. citrulli isolates collected from watermelon fields in the southeastern U.S. were genotyped with 16 SSR markers and a cytb marker for azoxystrobin (QoI) resistance, and phenotyped for sensitivity to tebuconazole (DMI), boscalid (SDHI), and fluopyram (SDHI). Cyp51 and sdhB genes of select isolates with varying sensitivity to DMI and SDHI fungicides, respectively, were sequenced. All S. caricae isolates were resistant to tebuconazole and azoxystrobin, and sensitive to boscalid and fluopyram. All S. citrulli isolates were sensitive to tebuconazole, sensitive to fluopyram with one exception, and resistant to azoxystrobin with two exceptions. Phenotypic differences in response to boscalid were detected among S. citrulli isolates, but the phenotype was not associated with clonal genotype, suggesting multiple evolution events. This study provides critical information for effectively managing both species of GSB fungi present in the southeastern U.S., while providing novel insights on the evolution of resistance in plant-pathogenic fungi.