Effect of nitrogen fertilization on cucumber downy mildew

Abstract: The objective of this study was to examine the effect of different concentrations of nitrogen on downy mildew severity on cucumber plants. Nitrogen (100-600 mg L^-1) was applied to plants grown in pots under greenhouse conditions through irrigation water. The cucumber leaves were inoculated with a zoospore suspension of Pseudoperonospora cubensis, the causal agent of powdery mildew. Disease symptoms in plants treated with 300 mg L^-1 nitrogen were significantly lower and 24% severity reduction was recorded. A positive effect on the leaf area was also noticed in cucumber plants with the application of 300 mg L^-1 nitrogen. The results indicated that a cubic regression curve can be fitted to the disease progress for downy mildew, regardless of N treatments. Furthermore, the N enrichment affected the NH₄-N and NO₃-N content in leaves and soil as well as P content in the leaves. The application of 100 mg L^-1 N significantly reduced Mn content in leaves. Therefore, nitrogen supply in the form of ammonium nitrate fertilizer could be considered as an efficient method for the control of downy mildew of cucumber.