Evaluating the efficacy of humic acid in controlling powdery (Sphaerotheca fuliginea Schlecht., Pollacci) and downy (Pseudoperonospora cubensis Berk. and Curt.) mildews of cucumber and enhancing the cucumber yield components was explored through experiments of plastic houses under pathogens pressure during two growing seasons 2016/2017 and 2017/2018. Three concentrations 5, 10 and 15 ml/l of humic acid were sprayed five times with seven days interval against powdery and downy mildews compared to bio-agents (Bacillus subtilis and Trichoderma asperellum) and chemical registered fungicides Ridomil Gold MZ 68 WG with downy mildew and Topas 100 EC with powdery mildew. Maximum reduction percent in disease severity of both powdery (85.23 and 80.88) and downy (70.36 and 81.14) mildew diseases was recorded with the third concentration (15 ml/l) of humic acid compared to other used concentrations during both seasons respectively. The significant reduction effect on powdery and downy mildew diseases was increased with increasing of humic acid concentrations. All treatments increased peroxidase and polyphenol oxidase activity compared with control treatments at all periods of activity. Enzymes of peroxidase and polyphenol oxidase showed the highest activity when induced by T. asperellum as well ashumic acid at the third concentration. The highest activity was induced by T. asperellum and humic acid at concentrationof 15 ml/l during the growing season 2016/2017. Similar results were obtained during the second season 2017/2018. Foliar application of humic acid by different concentrations increased significantly the plant height, chlorophyll content, fresh and dry weights as well as yield components and fresh fruit shelf life. These results established that humic acid could play an essential role either for controlling powdery and downy mildews of cucumber through enzymes induction or advantage of enhancing the plant growth, yield and fresh fruit shelf life.