A lower level of nitrogen will improve the resistance to powdery mildew in the cultivation of gerbera, sweet pepper, and cucumber. However, this effect is only achieved at the edge of nitrogen levels, which affects crop growth negatively. A lower level of nitrogen has no positive effect on resistance to botrytis. These are some of the results from a study by the Greenhouse Horticulture and Flower Bulbs Business Unit of Wageningen University & Research.
Lower nitrogen levels
WUR investigated the effect of a lower level of nitrogen on the growth and resilience of cucumber, bell pepper, chrysanthemum, and gerbera. It clearly showed that nitrogen levels can be reduced seriously in all crops. For cucumber, a nitrogen reduction of over 30% resulted in a negative effect on growth and production. The limiting value is considerably lower for chrysanthemum, gerbera, and bell pepper, where the effect on growth and production occurred only from a 70-80% reduction in nitrogen compared to normal. The room to maneuver for growers is therefore much higher in those crops.
Effect on mildew
The nitrogen uptake by the plant also has an effect on the susceptibility to powdery mildew, especially in gerbera and cucumber. However, a lower level also causes less growth. The study also examined whether the use of so-called elicitors, such as jasmonic acid or salicylic acid, substances that increase plant resistance, add anything. It turned out not to be the case, the effects were equal to the effect with just lowering the nitrogen level. The nitrogen affected the development of some pests, though the effects were not always straightforward. Aphid reproduced less and grew less rapidly at a low nitrogen level (tested in peppers). Caterpillars of the Turkish moth also grew less quickly (tested in chrysanthemum). Contradictory effects were found in the development of thrips and whitefly for the various crops.
The research, which WUR is conducting together with Vertify and the Control Flowers and Food Foundation, will run until next year. In the current phase, the effects in practical situations will be investigated. The research is a PPP and is financed by the Top Sector Horticulture & Propagation Materials, Stichting Kennis In Je Kas, and the crop cooperatives Gerbera, Chrysanthemum, Cucumber, and Paprika.