Effects of air temperature, relative humidity and canopy wetness on gray mold of cucumbers in unheated greenhouses.

Microclimatic variables were monitored in cucumber crops grown in polyethylene-covered, unheated greenhouses in Israel during the winter of 1987/88. The winter was characterized by a relatively large number of rainy days. The relative humidity (RH) in the greenhouses was high (>97%) during most of the day, resulting in long periods of dew persistence. Dew point temperature and duration of dew deposition were calculated for the plant canopy. Disease incidence was monitored in 2-m-high plants, both on senescing female flowers ('fruits') and on stems. Multiple linear correlations were calculated for gray mold incidence and duration of air temperature and RH at specific ranges, and of leaf wetness (LW). Disease was characterized by two stages, according to the rate of its development and the microclimatic conditions influencing it. In the first phase of the epidemic a high correlation was found between infected fruits and air temperature in the range of 11-25°C, and RH in the range of 97-100% or LW. In the second phase, disease incidence was better correlated with air temperature in the range of 11-16°C and RH above 85% (R = 0.681); there was no correlation between disease and LW at this stage. Development of stem infections was correlated with air temperature in the range of 11-16°C during the first phase of the epidemic. By contrast, the second phase was characterized by a close correlation between stem infections and RH in the range of 80-100% but also with air temperature in the range of 11-16°C, or with air temperature in the range of 11-25°C and RH 80-100%, and LW. [ABSTRACT FROM AUTHOR]