Three chemical inducers salicylic acid (SA), bion and indol butyric acid (IBA) at different concentrations were applied under field trials to control onion downy mildew and purple blotch and study their effectiveness on onion bulb yield during two successive seasons (2014/2015 and 2015/2016). Chemical inducers were used as a foliar treatment only as well compiled with Amistar Top SC 32.5% (Azoxstrobin + difenoconazole) as a dipping for onion transplants in permanent field. Recommended fungicide Amistar Top was the best treatment recorded the highest percentage efficacy in reducing downy mildew and purple blotch diseases severity compared to other treatments. Treatment with Chemical inducers showed a significant reduction of downy mildew and purple blotch severity and increased onion bulb yield. In this respect, the highest reduction in disease severity and increase in onion bulb yield were detected with IBA at 200 ppm treatments followed by bion treatment during the two growing seasons. In general, disease severity of downy mildew and purple blotch showed more reduction when onion transplants were dipped in the fungicide before planting. Spraying with IBA at 200 ppm on onion plants treated their transplants by dipping in fungicide tested was the best treatment in reducing downy mildew and purple blotch diseases severity followed by bion treatment at 8 mM. Increase the concentration of the chemical inducers caused more reduction in disease severity of both downy mildew and purple blotch diseases. Data clearly indicated that phenolic contents (free, conjugated, and total phenols) and activity of the oxidative enzymes (peroxidase, polyphenol oxidase and catalase) were affected by the three tested concentrations of any chemical inducer. But, applying fungicide as a dipping treatment with different concentrations of chemical inducers had no significant effect on increasing the phenolic contents or the activity of oxidative enzymes in leaves of onion plants during the two growing seasons, 2014-2015 and 2015-2016.