The Role of the Relative Humidity on the Development of Downy Mildew Infection of Cucumber in the Greenhouse in Baghdad

Abstract

This study conducted at the Dujail, north of Baghdad, aims to control the downy mildew disease of cucumber based on the ventilation of greenhouses by making three ventilation holes in the plastic cover, in comparison to cultivation into a regular greenhouse (without ventilation holes). Three approved cucumber hybrid were included in this study; Mustaqbal, Jamilah, and Faris. The infection percentage and disease severity rates were calculated since the onset of the disease on March 21st, 2022 until May 18th, 2022. The results showed that the average temperatures (minimum and maximum) and relative humidity were 5-25 ºC and 97% inside the regular greenhouse were suitable for the Pseudoperonospora cubensis natural infection. The highest percentage of infection and disease severity were 30.81 and 23.33%, respectively, reported into the regular greenhouse (without ventilation), compared with the infection percentage and disease severity in the ventilated greenhouses, which were 1.28 and 6.99%, respectively. The percentages of infection of Mustaqbal, Faris and Jamila in the regular greenhouse (without ventilation) were 31.69, 31.40 and 29.35%, respectively, compared to 1.47, 1.17 and 1.21% noticed in the greenhouses with three ventilation holes for the same cucumber varieties, respectively. This study found that three ventilation holes in plastic cover of the greenhouse reduced the percentage infection and disease severity in cucumber plants, by 95.8 and 70% respectively.