Mutations in the complex III of Zymoseptoria tritici and implications on fungicide sensitivity and fitness

Quinone outside inhibitors (QoI, FRAC Group 11) of the mitochondrial respiratory chain have played an important role in the control of Zymoseptoria tritici. Evolution of the G143A mutation in cytochrome b led to the development of resistance, resulting in a loss of effectiveness of FRAC Group 11 fungicides. Accordingly, two new complex III inhibitors, metyltetraprole and fenpicoxamid, were developed. Metyltetraprole, although classified as QoI, does not show cross-resistance to FRAC Group 11 fungicides on G143A mutants, due to its unique chemical structure. This is reflected by listing the compound in FRAC Group 11 A. Fenpicoxamid, as a quinone inside inhibitor (QiI, FRAC Group 21), is unaffected by the G143A mutation due to the different binding site at the same complex. Laboratory mutants conferring resistance to both compounds have been generated: Y132C mutants were generated by metyltetraprole and the G37V mutants by fenpicoxamid exposure either in wildtype strains or in combination with other cytochrome b mutations such as F129L and G143A. However, studies indicated an incompatibility between the G143A and Y132C mutations. Isolates with the Y132C mutation, particularly double mutants F129L + Y132C, showed reduced sensitivity to metyltetraprole in vitro and in vivo. Simultaneously, competition trials in microtiter plates and greenhouse tests indicated fitness penalties for all Y132C mutants. Since competitiveness and pathogenicity are crucial for isolate survival, metyltetraprole resistance in Z. tritici due to Y132C is unlikely to become dominant in the field, especially in populations with a high frequency of G143A. G37V mutants derived from both wildtype and G143A isolates exhibited reduced sensitivity to fenpicoxamid relative to their parent strains and demonstrated only minor fitness penalties in competition trials and greenhouse studies.