Influence of Light Management on the Sporulation of Downy Mildew on Sweet Basil

Downy mildew of basil (Ocimum basilicum), caused by the pathogen Peronospora belbahrii, seriously affects greenhouse production of basil in Europe. Generally, infection of new plant tissue occurs by zoospores released from sporangia. Sporulation occurs under high humidity. Under optimal conditions, dense mats of black sporangia are produced on the abaxial side of the leaves, creating a high epidemic potential. For some oomycetes, it is known that they show a direct reaction to light. For Plasmopara viticola, it was shown that under continuous light there was no formation of sporangia; a dark period is needed to induce sporulation. To assess the influence of different light intervals on the sporulation of downy mildew on basil, a series of experiments were conducted under controlled conditions. Plants were spray inoculated with sporangia and incubated in a humidity chamber. Subsequently, plants were moved to the growth chamber with different light regimes (same temperature and humidity but different intervals of artificial light and darkness). Disease assessment was 10 to 16 days after inoculation. There was a clear influence of light intervals on incidence and severity of sporulation of basil downy mildew. While the negative control with 24 h of light showed no sporulation at all, there was full sporulation in the positive control (14 h light, 10 h darkness). Treatments in which the 10 h darkness period was interrupted with light (either 3 h darkness (D), 4 h light (L), 3 h darkness or 2 h D/2 h L/2 h D/2 h L/2 h D) showed either reduced or no sporulation. It seems that downy mildew of basil needs a period of darkness for the induction of sporulation. We therefore conclude that it is possible to influence the epidemiology of basil downy mildew with light. The use of "light breaks" during the darkness period could be one piece of a strategy against downy mildew epidemics on basil in greenhouses.