Response of Septoria (Zymoseptoria tritici) isolates with Qi and Qo mutations under multiple fungicide selection

Inatreq, active ingredient fenpicoxamid, is a new QiI fungicide product brought to the market by Corteva Agriscience for strong activity against Septoria (Z. tritici). This study examines the interaction between Qo and Qi mutant Septoria isolates to confirm negative cross-resistance between the two sites. And to observe the change in sensitivity of the isolates in a mixed population over several generations and under multiple rates of fungicide selection. Initial growth rate and fitness isolates were determined for four isolate strains; wildtype (IPO323), G37V single mutation (WF.2.44), G143A single mutation (37- 16), and G37V-G143A double mutation (716.3F.2.2). Initial IC50 values were determined for several fungicides in multiple fungicide groups, QiIs, QoIs, Azoles, SDHIs to confirm negative cross-resistance and determine concentrations needed for fungicide treatments. A mixed population trial was then conducted with a 6-rate dose response for three fungicide treatments; Inatreq (QiI), pyraclostrobin (QoI), and a 1:1 mix of Inatreq and pyraclostrobin. It was suspected that throughout 6 generations, sensitivity of single mutant isolates to the opposite fungicide site inhibitor would increase. The double mutation isolate would become more sensitive under fungicide selection than the single mutant isolates. The results of the study confirmed this hypothesis of negative cross-resistance between fungicide groups. Qo single mutant isolate was determined resistant to QoI fungicide but sensitive to QiI fungicides, and the opposite found true for the Qi single mutant. The double mutant isolate was sensitive to both fungicide groups, with increased sensitivity than the single mutants. Under double fungicide selection, all mutant strains were sensitive to the treatment, with increased sensitivity than under single fungicide selection.