Sensitivity of <i>Rhizoctonia solani</i> to fertilizers and fungicides and their synergistic effects

Abstract Rhizoctonia solani reduces yield and quality of potato worldwide and some studies report the appearance of resistant strains due to the extensive use of fungicides. To overcome this problem, we studied the effect of commercial fertilizers alone and in combination with fungicides on the in‐vitro growth of R. solani in order to manage the use of fungicides and reduce the risk of resistance development in this pathogen. First, the study of the effect of three fungicides, azoxystrobin, pencycuron and fludioxonil, on the in‐vitro growth of R. solani showed that fludioxonil (EC 50 = 0.06 × 10 −3 μg a.i./mL) and pencycuron (EC 50 = 0.1 × 10 −3 μg a.i./mL) were very effective in inhibiting the growth of this pathogen at very low concentrations; however, azoxystrobin showed less antifungal activity (EC 50 = 74.85 × 10 −3 μg a.i./mL). The effect of five commercial fertilizers, namely di‐ammonium phosphate, superphosphate, phosphoric acid, potassium sulphate and ammonium nitrate, at different concentrations (0, 0.01, 0.1, 1 and 10 mg of fertilizer element [P 2 O 5 , K 2 O or N]/mL) showed that all fertilizers reduced the in‐vitro growth of R. solani . Furthermore, the studied fertilizers, particularly phosphorus‐based ones, improved the antifungal activity of the three fungicides. For instance, superphosphate at 0.1 mg of P 2 O 5 /mL in combination with pencycuron reduced the diameter of the fungal colony by 76.7% in comparison to the fungicide alone. In addition, the antifungal activity of azoxystrobin was improved by the NPK fertilizers, indicating their effect in reducing the risk of resistance development in R. solani to this fungicide. This study demonstrates the usefulness of fertilizers in an integrated management programme to enhance fungicide efficacy against R. solani , reducing the risk of resistance development in this pathogen.