The efficacies of tebuconazole plus azoxystrobin/Reynoutria spp. extract, sulfur (S)/Reynoutria spp. extract, Reynoutria spp. Extract/S, KH₂PO₄ plus S, Na₂SiO₃ (moduli m2 and m3) plus S, and NaHCO₃ against powdery mildew caused by Erysiphe corylacearum were investigated on the hazelnut cultivars Kara and Sarı. The study was carried out during the 2019 vegetation season in a hazelnut orchard in the Akçakoca district of Düzce, Turkey. The experiment was established in a randomized complete block design with three replications, and twelve plants in two plantings (ocaks) were used for each parcel. The data on different parameters were analyzed using Minitab® Statistical Software Release 19, and the mean values were compared by the Tukey multiple range test (P≤ 0.05). The alternation treatment program of tebuconazole plus azoxystrobin/Reynoutria spp. extract exhibited the highest efficacy (93.6%) against disease on the leaves of the Kara cultivar, followed by the other treatment programs with efficacies between 91.8% and 79.6%. On the leaves of the Sarı cultivar, the alternation treatment program tebuconazole plus azoxystrobin/Reynoutria spp. extract, with 96.2%, and the mixing treatment program KH₂PO₄ plus S, with 94.9%, showed the highest effectiveness, followed by the application programs Na₂SiO₃ (m3) plus S, Na₂SiO₃ (m2) plus S, and S/Reynoutria spp. extract, with efficacies of 92.5%, 92.2%, and 91.3%, respectively. Na₂SiO₃ (m3) plus S (100%), S/Reynoutria spp. extract (94.6%), and Na₂SiO₃ (m2) plus S (93.9%) were most effective against powdery mildew on the fruit clusters of the Kara cultivar, and Na₂SiO₃ (m2) plus S (98.7%), KH₂PO₄ plus S (97.8%), and Na₂SiO₃ (m3) plus S (94.9%) had the highest efficacies on the fruit clusters of the Sarı cultivar, followed by tebuconazole plus azoxystrobin/Reynoutria spp. extract and KH₂PO₄ plus S on the Kara cultivar, and tebuconazole plus azoxystrobin/Reynoutria spp. extract and S/Reynoutria spp. extract on the Sarı cultivar. Reynoutria spp. extract/S showed moderate efficacies between 65.0% and 79.6% against powdery mildew on the leaves and fruit clusters. While NaHCO₃ had moderate or high efficacies on the leaves of both cultivars (80.4% on the Kara cultivar and 57.9% on the Sarı cultivar), its effects were the lowest than those of other all treatment programs on the fruit clusters. The treatment programs had no phytotoxic effects on the leaves or fruit clusters of either hazelnut cultivar. The present study has demonstrated that all spraying programs except NaHCO₃ could be used effectively as eco-friendly spraying programs against powdery mildew in hazelnut.