Background: The interaction of plant nutrition and disease development has been well documented. Potassium (K) application has been widely recognized to decrease disease incidence in many host plants. However, contradictory reports about its impact on plant diseases have been ascertained therefore further research is needed for each host-pathogen system. The objective of the present study was to examine the effect of K levels on downy mildew severity and assess their impact on soil and nutritional status of cucumber plants. Methods: K (200-700 mg L-1) was applied to plants grown in pots under greenhouse conditions through fertigation, according to a randomized block design with four replicates. Nitrogen (N) and phosphorus (P) were given to plants at stable concentrations. The cucumber leaves were inoculated with a zoospore suspension of Pseudoperonospora cubensis, the causal agent of downy mildew. Leaf and lesion area as well as nutrients in plant tissues and soil were determined. Results: Plants treated with 300 and 400 mg L-1 of K demonstrated the lowest downy mildew severity. Above these rates, K supply led to downy mildew increase in cucumber leaves. The cubic regression curve can be fitted to the disease progress for downy mildew, irrespective of K doses. Leaf and soil nutrient content appeared to affect infection and leaf size. Reduced concentration of soil NH 4-N was noticed in fertigation levels of 300 and 400 mg L-1 K and the lowest Mn content appeared in leaves of cucumber plants grown in 400 mg L-1 K. Conclusion: The suppression effect of K on downy mildew had an optimum limit and beyond this, disease spread was observed. K enrichment of soil via potassium nitrate fertilizer could be considered as an efficient method for the control of downy mildew of cucumber plants.