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When pathogen populations diverge: why understanding species boundaries is critical for managing fungicide resistance

Marin Brewer: University of Georgia

<div>An increasing number of plant diseases originally described as being caused by a single species are now known to be caused by multiple, morphologically similar, but genetically distinct species. Members of these species complexes may vary in geographic range, host specificity, fungicide sensitivity, or other factors. We recently discovered that gummy stem blight (GSB) of cucurbits is caused by three closely related, but genetically distinct species. We studied how resistance profiles are associated with two of the GSB fungi found in the southeastern U.S. Nineteen <em>Stagonosporopsis caricae </em>isolates and 120 <em>S. citrulli</em> isolates collected from watermelon fields were genotyped with a <em>cytb</em> marker for azoxystrobin (QoI) resistance, and phenotyped for sensitivity to tebuconazole (DMI), boscalid (SDHI), and fluopyram (SDHI). Select isolates with varying sensitivity to DMI and SDHI fungicides were sequenced at <em>Cyp51 </em>and <em>SdhB, </em>respectively. All <em>S. caricae</em> isolates were resistant to tebuconazole and azoxystrobin, and sensitive to boscalid and fluopyram. All <em>S. citrulli</em> 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 <em>S. citrulli </em>isolates. We are continuing to monitor population dynamics in watermelon fields and differences in fungicide sensitivity within the two species to assess how populations may be changing in response to resistance profiles and fungicide use. Knowledge of population divergence and underlying biological differences, including fungicide resistance profiles, are critical for effectively managing plant-pathogenic fungi.</div>