الملخص
Cucumber (Cucumis sativus L.) is one of the most important crop of vegetables that is produced in protected houses and open fields. However, this crop is subjected for different infectious diseases that reduce the productivity and increase the production cost. Powdery mildew, caused by Podosphaera xanthii, is the most serious disease that infect the cucumber plants and represents a big challenge facing the growers, especially in the protected houses. In Ahba, Saudia Arabia, this disease comprises a great challenge, whereas the climate is most appropriate for the disease progress. Recently, Saudi Arabia encourages the production of crops, especially vegetables and fruits by green agriculture (organic farming), where any application of chemicals or pesticides are prohibited. Using of safe, effective and economic biological or natural method to protect the crops free of disease is greatly encouraged. Therefore, this study introduces the essential oils of some medicinal plants as alternative biological method to the chemical fungicides on the control of powdery mildews of cucumber. The main objective of this study is the application of essential oilsindividually and in mixture in the control of powdery mildew disease, as well as the exploring of their mode of action. Two-seasons experiment was conducted to test the efficacy of different essential oils in the control of the powdery mildew of cucumber, after section of the most effective oils based on the in vitro test. Lemongrass, lemon, thyme, peppermint, thieves blend or abundance blend oils, in addition to a mixture of the first four oils, were tested against Podosphaera xanthii in vitro and in vivo. The effect of oil application of the morphology and physiology of the cucumber plants was evaluated. In addition, the chemical analysis of the essential oils was carried out using the GC-MS equipment. Results approved identity of the causal pathogen of cucumber powdery mildew as Podosphaera xanthii. In vitro test approved the fungitoxic effect of the essential oils against the pathogenic fungus in dose of 2000-2500 ppm. Under greenhouse conditions, the essential oils succeeded to reduce the disease severity and incidence percentage and the most effective oils were lemongrass the abundance blend in vivo. Application of the essential oils enhanced increased plant length, leave area, fresh and dry weight and number of flowers compared with the controls. The content of chlorophyll a and b, carbohydrates and proteins were significantly increased compared with the controls because of application of the essential oils. Cell membrane stability percentage and electrolytes leakage from the cells were enhanced by the application of the essential oils. GC-MS analysis of the applied essential oils approved the presence fungitoxic compounds such as cymene, terpinene, phenol, linalool, thymol, pinene, caryophyllene oxide, phellandrene, careen and others. The study recommends the use of such essential oils in definite dose (2500 ppm) to protect the cucumber free of infection and production of this important vegetable without application of any fungicides to conserve the human health and reduce the input of the harmful and hazed materials into the environment. However, these oils should be applied in definite doses (2000 -2500 ppm) to avoid the adverse effect that could be produced by the overdosing.
