Foliar applications of silicon fertilisers inhibit powdery mildew development in greenhouse cucumber

​Powdery mildews (Podosphaera xhantii) remain among the most important pathogenic fungi in greenhouse vegetable production, and are the cause of extensive fungicide use in European horticulture. Although the underlying mechanisms are not known, the ability of soluble silicon (Si) to reduce the impact of powdery mildew infection on several greenhouse crops has been amply described. Still, the use of Si in greenhouse production is very limited. One major reason for this is the challenge associated with the standard form of application, which is amendment to soil or nutrient solutions. Foliar applications could represent a more practical means of application, but have not been adequately tested. In the present study, foliar applications of two commercially available silicon (Si) based products were evaluated for their effect in reducing powdery mildew development in commercial greenhouse cucumber (Cucumis sativus) production. The products Carbon Silpower® and Carbon Defense® were supplied to the mildew susceptible cultivars ‘Euphoria’ and ‘Jessica’, one or two times per week in two different concentrations. All treatments resulted in a significantly lower mildew infection development than untreated control plants (water only). In general, more frequent applications improved the fungal inhibitive effect. The most effective treatment was a high concentration of Carbon Silpower solution (with 56 mM Si) applied twice per week, reducing the disease severity by as much as 87% compared to the control.