The goal of this study was to evaluate the effectiveness of aqueous extracts from five traditional Egyptian medicinal plants in preventing Sphaerotheca fuliginea’s powdery mildew disease, which affects cucumber plants. Aqueous extracts from each of the examined plants suppressed the pathogen’s conidia germination in vitro. In trials using detached leaves and greenhouses, these extracts lessened the severity of the disease. Compared to other plant extracts, Curcuma longa rhizome extract showed the greatest potency against the pathogen. The aqueous extract of Curcuma longa showed the largest improvement in disease suppression compared to the control in the greenhouse experiment. The results showed that total phenol and associated defense enzyme levels (POD and PPO) were elevated by plant extracts from all studied plants. These findings might suggest that total phenol and associated defense enzymes strengthen the cucumber’s resistance to the disease. The C. longa extract had more total phenol than the extracts from the other plants. The phenolic components in the C. longa rhizome extract were varied, and these variations were detected and quantified using high-performance liquid chromatography (HPLC). The content of curcumin (3220.8 μg · g ^–1 dry weight) was the highest. In comparison to the control, the foliar application of the C. longa extract considerably increased the cucumber fruit yield and its constituent parts. This is the first time, to my knowledge, that the C. longa rhizome extract has been utilized to improve cucumber plants’ production and its constituent parts. The pathogen appeared as small colonies with fewer mycelia and immature conidia in the treated cucumber leaves with 20% of C. longa rhizome extract according to an examination by SEM. Overall, the results indicated that the extract of C. longa rhizome, was a promising, effective, and environmentally friendly management measure against powdery mildew disease of cucumbers, and thus could be used in the production of organically grown vegetables.