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Virulence of multi-fungicide resistant Zymoseptoria tritici isolates under greenhouse conditions

Cruz Avila-Adame: Dow AgroSciences LLC

<div><em>Zymoseptoria tritici</em> (causing Septoria leaf blotch of wheat) is managed with fungicides differing in mode of action (MoA), including beta-tubulin, cytochrome b-Qo, and 14-alpha demethylase inhibitors. Concurrently, the European fungal population developed resistance to these chemistries. To determine whether accumulation of mutations conferring resistance to multiple MoAs impairs virulence/aggressiveness, 10 isolates from 5 countries were compared through 5 cycles of infection on wheat seedlings. Five isolates carried the E198A and G143A mutations conferring resistance to beta-tubulin and Qo inhibitors, respectively, as well as multiple mutations in CYP51 decreasing sensitivity to 14-alpha demethylase inhibitors. Three other isolates had mutations associated with resistance to 1 or 2 fungicide classes but not to all 3. Two isolates were fully sensitive to all 3 fungicide classes. The resistant and sensitive isolates were all virulent, infecting wheat with typical disease symptoms; however, only 8 isolates caused more than 95% disease severity 20 days after inoculation at the end of the 5 cycle. Two isolates resistant to 3 fungicide classes were less aggressive than fully sensitive isolates but overall there was no consistent effect of resistance phenotype on disease development.</div>