DAYTIME SOLAR HEAT TREATMENT FOR THE SUPPRESSION OF FOLIAR PLANT PATHOGENS IN POLYETHYLENE-COVERED GREENHOUSES

A series of experiments were carried out to identify potential climate-management techniques for the suppression of humidity-promoted and powdery mildew diseases in greenhouse-grown sweet pepper, tomato and sweet basil. Elevated daytime temperatures were obtained by closing the side walls of the greenhouses. When sweet-pepper greenhouses were closed, the temperatures inside these structures reached 35°C and we observed significant suppression of Leveillula taurica (anamorph: Oidiopsis sicula). When commercial-like greenhouses were kept closed during autumn and winter, the temperatures inside these greenhouses reached 27-34°C and we observed season-long suppression of tomato powdery mildew (Oidium neolycopersici). Severe epidemics developed in aerated greenhouses maintained according to the common practice (control). The combination of day-heating with applications of disease control agents, such as plant extracts, biocontrol agents and sulfur, improved the control of powdery mildew diseases in both sweet pepper and tomato. Temperatures in polyethylene-covered sweet basil greenhouses that were kept closed for 6 h each day reached 42°C during the winter period. The incidence of gray mold (Botrytis cinerea) in these greenhouses was significantly reduced and basil yields were increased. A significant negative correlation was observed between the duration of temperatures above 30°C and the incidence of gray mold. The suppressive effects of high daytime temperatures on the examined pathosystems can be attributed to a direct effect of temperature on the pathogen inocula, as well as an indirect effect involving the induction of resistance in the host plants. It should be noted that partial aeration to release humidity is essential in daytime-heated greenhouses, in order to prevent outbreaks of diseases that thrive under humid conditions. In tomato greenhouses, additional measures were taken to ensure pollination and fruit set, processes that are negatively affected by high temperatures.