The effect of nitrogen and NH 4 + fertilization on Peronospora belbahrii downy mildew of sweet basil

Downy mildew ( Peronospora belbahrii ) is a severe disease in sweet basil. The aim of the present research was to study the effect of total nitrogen (N) concentration and NH 4 + partial concentration in sweet basil fertigation on sweet basil downy mildew (SBDM). The study was conducted under controlled conditions, semi-commercial conditions, and commercial-like (field) conditions. Increasing the total N concentration in the irrigation water from 1.1 to 10.7 mM resulted in a gradual increase in N concentration in the shoots, and SBDM severity in potted sweet basil plants. Similarly, under commercial-like greenhouse conditions, higher N in the irrigation water resulted in higher SBDM severity. A high rate of 30–40% of the total N of NH 4 + in the irrigation water, was associated with lower SBDM severity. Mineral concentration in the shoots of sweet basil was measured in all greenhouse plots. Shoot concentrations of N, P, K, and Ca were positively correlated with SBDM severity, while shoot concentrations of Mg and Cl negatively correlated with disease severity. In conclusion, N fertigation affected SBDM severity. We suggest that the effect of N fertigation on disease may be due to the change in N concentration in the tissue, or the change in other nutritional element (s).