Alternative use of systemic and contact fungicides with resistant cultivars to manage potato late blight (Phytophthora infestans) disease at Holetta, Ethiopia

Late blight caused by Phytophthora infestans, is a major potato disease worldwide, including in Ethiopia. The pathogen has high genetic variability, intensity, and prevalence. Therefore, it is very important to control this disease using resistant cultivars, cultural practices, and fungicides. Thus, the current research aimed to evaluate the integrated effect of alternative use of systemic and contact fungicides with resistant cultivars on leaf blight during the 2023 and 2024 production years. Twenty-six treatment combinations, 13 fungicide alternatives including control and two cultivars, replicated 3 times were used in a randomized complete block design in a factorial arrangement. Logistic and Gompertz models were used to linearize the disease data. The association between tuber yield and final disease and area under disease progress curve was analyzed using Pearson correlation in the ellipses predictor. ANOVA tests indicate that there was significant interaction between alternative use of systemic and contact fungicides on resistant cultivars for both final disease and AUDPC parameters. The control treatment of Shenkolla cultivar was significantly different from the application of fungicide contact (C) + C + systemic (S) + S in the cultivar Belete for final disease and AUDPC. The application of fungicide C + S + C + S in the same cultivar was significantly different from C + C + C + C and S + S + S + S in the cultivar Shenkolla, and S + S + S + S in the cultivar Belete. Again, application of fungicide C + S + C + S in the cultivar Belete was significantly different from C + C + C + C and S + S + S + S in the cultivar Shenkolla, and S + S + S + S in the cultivar Belete. Three sequences resulted in significantly decreased disease severity, while some alternate (or rotational) sequences provided statistically similar responses to the disease severity as of C + C + C + C. Their severities were S + C + S + C (17%), C + C + S + C (27%), and C + C + S + S (22%). The maximum final disease (88%) and AUDPC (2207) were scored on the control Shenkolla cultivar. The cultivar Belete treated in the C + C + S + S alternative way scored the minimum 34 and 663 for final disease and AUDPC, respectively, during the 2024 production year. According to the sigmoidal graph, the disease pressure was much higher on the control treatment but reduced on the cultivar Shenkolla treated in the C + C + S + S alternative way at 62–70 days of planting. In the control cultivar, Shenkolla, the disease rate increased by 13 as days increased by 8. Final disease severity and AUDPC show (r = − 0.76) and (r = − 0.83) strong correlation with yield, respectively. The cultivar Belete treated by the C + C + S + S alternative way, scored the maximum yield of 18.1 t/ha, whereas the cultivar Shenkolla, sprayed in the S + S + S + C alternative way, produced 4.1t/ha; both treatments were significantly different. It is concluded that using systemic and contact fungicides in alternative ways is much more effective than using the same fungicide at different times along with a resistant cultivar.