| Summary: | Species of the fungus <i>Fusarium</i> cause Fusarium head blight (FHB) of cereal crops and contaminate grain with sesquiterpenoid mycotoxins, including culmorin (CUL) and trichothecenes. While the phytotoxicity of trichothecenes, such as deoxynivalenol (DON), and their role in virulence are well characterized, less is known about the phytotoxicity of CUL and its role in the development of FHB. Herein, we evaluated the phytotoxic effects of purified CUL and CUL-trichothecene mixtures using <i>Chlamydomonas reinhardtii</i> growth and <i>Triticum aestivum</i> (wheat) root elongation assays. By itself, CUL did not affect growth in either system. However, mixtures of CUL with DON, 3-acetyldeoxynivalenol, 15-acetyldeoxynivalenol, or NX-3, but not with nivalenol, inhibited growth in a synergistic manner. Synergistic phytotoxic effects of CUL and DON were also observed on multiple plant varieties and species. The severity of wheat FHB caused by 15 isolates of <i>Fusarium graminearum</i> was negatively correlated with the CUL/DON ratio, but positively correlated with the sum of both CUL and DON. Additionally, during the first week of infection, CUL biosynthetic genes were more highly expressed than the <i>TRI5</i> trichothecene biosynthetic gene. Furthermore, genomic analysis of <i>Fusarium</i> species revealed that CUL and trichothecene biosynthetic genes consistently co-occur among species closely related to <i>F. graminearum</i>.
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