Indication of the role of efflux pumps that may explain the reduced sensitivity to tebuconazole and reduced sensitivity to fengycin and surfactin in V. inaequalis

Apple scab can cause yield loss of up to 70%: this serious disease, caused by Venturia inaequalis, attacks leaves and fruits. Apple scab is managed with fungicides including triazoles which is the most widely used class. Cases of resistance to these fungicides are appearing increasingly. Several mechanisms may explain fungicide resistance. One of them is the overexpression of efflux pumps. Efflux pumps can release fungicides and modulate their intracellular accumulation.
To replace the current fungicides, more environmental-friendly alternatives are sought, such as biofungicides. In this study, we’ve tested the in vitro antifungal activity of three families of microbial metabolites produced by B. subtilis with a range of concentrations (fengycin, mycosubtilin and surfactin). Two Venturia inaequalis strains were used: a sensitive to tebuconazole (S755) and a reduced sensitivity to tebuconazole (Rs552). Through the determination of half inhibitory concentration (IC50 determined by non linear regression), results revealed that the strain with reduced sensitivity to tebuconazole also showed reduced sensitivity to fengycin and surfactin. On the other hand, Rs552 remains sensitive to mycosubtilin.
To determine the origin of the reduced sensitive of Rs552, tolnaftate, an indicator to the efflux pumps overexpression was used. Activity on S755 strain and no activity on Rs552 strain was observed, indicating a potential rule of efflux pump.