Downy mildew of onion (Allium cepa), caused by Peronospora destructor, is one of the most important onion diseases that
mainly controlled by fungicides. This investigation aimed to evaluate the effect of foliar application of two forms of humic substances (HS) individually or in combinations with two fungicides against downy mildew, growth, photosynthetic capacity and metabolism changes of onion. The trials were carried out during the growing seasons 2016 and 2017. Final disease severity (FDS%) andarea under disease progress curve (AUDPC) were used to assess the potential of twoforms of (HS), and two fungicides to control downy mildew of onion. All treatments resulted in minimizing the FDS (%) and AUDPC, compared to control treatment, during the two seasons. The most effective treatment was (HS)-leonardite combined with fungicide Ridomil Gold 68% (WP), which exhibited a significant decrease in FDS (%) and AUDPC in relation to other treatments and control. All treatments improved onion growth parameters i.e., bulb diameter, plant height, dry eight/plant, dry matter, average yield compared to untreated control plants. The chemical changes in treated plants i.e., enzymes activities, total carbohydrates, total nitrogen (%), total soluble solids (TSS %) were also assessed. Principal component analysis gave evidence to the importance of all parameters in this study, especially FDS (%), as a reliable indicator for evaluation of these materials against downy mildew disease.