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High levels of fludioxonil resistance in Botrytis fragariae and investigation of potential resistance mechanisms
Mengjun Hu: Clemson University; Guido Schnabel: Clemson University
<div>Fludioxonil, a phenylpyrrole, is a fungicide commonly used for control of gray mold of strawberry caused by <i>Botrytis spp</i>., including <i>B. cinerea</i> and <i>B. fragariae</i>. In contrast to other site-specific fungicides, resistance frequencies have been comparably low in strawberry fields in the eastern United States. Therefore, the fungicide has been used extensively in rotational programs in recent years. In 2016, <i>B. fragariae</i> isolates were collected from strawberry fields in the east coast. They displayed MIC values greater than 100 µg/ml. In contrast, previously reported MIC values of <i>B. fragariae </i>from Germany ranged from 0.5 to 3.3 µg/ml. Resistance to fludioxonil in <i>B. cinerea</i> from eastern strawberry fields is conferred by mutations in the <i>mrr1</i> gene, leading to enhanced expression of the ATP-binding cassette (ABC) transporter gene <i>atrB</i>. Analyses of <i>mrr1</i> sequences indicated no variation in nucleotide sequence compared to sensitive isolates, but real-time gene expression analysis revealed the <i>atrB</i> gene was constitutional higher expressed in resistant <i>B. fragariae </i>isolates compared to sensitive isolates. Relative <i>atrB</i> copy numbers were identical in resistant and sensitive isolates indicating a lack of gene duplication. Nucleotide sequence analysis of <i>Bos1</i>, the osmosensing histidine kinase, did not reveal variations between sensitive and resistant isolates as well. In lab mutants and more recently in field isolates from China, mutations in this gene have been reported to be associated with resistance to dicarboxamides and phenylpyrroles. The high levels of fludioxonil resistance in <i>B. fragariae</i> isolates may pose a selective advantage over <i>B. cinerea </i>in commercial strawberry fields. <p> </div>

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