Physiologicaland Biochemical Mechanism Of Prevention For Cucumber Downy Mildew In Seedlings Stage By High Temperature Control

Downy mildew, caused by the fungus Pseudoperonospora cubensis, is most destructive to cucumber, though all cucurbits are susceptible. Downy mildew can reduce yield, fruit quality, and harvesting time. Control of downy mildew on cucurbits is achieved primarily by the use of resistant varieties and/or fungicide spray programs. It is well known that chemical control of plant diseases mostly causes environmental pollution and increase the accumulated toxic substances in human food chain. Also, because drug-resistant fungus increased, to further control the disease difficult. Biological control method is a very effective method to control this disease. High temperature control for cucumber downy mildew is the most representative ecological control method. However, the physiological and biochemical mechanism of prevention for cucumber downy mildew by high temperature control was rarely reported, especially cucumber inoculated with P.cubensis after high temperature stress was never reported.The physiological and biochemical change of prevention for downy mildew by high temperature control was studied using cucumber susceptible variety"Changchun mici". Cucumber two-leaf stage seedlings inoculated with P.cubensis were treated with high temperature. The main results were as follows:1.The best effect of prevention was reached after seedlings inoculated with P.cubensis 8h and then treated for 90 min at 45℃. The earlier the time of high temperature control was, the better effect of prevention was.2. The change of chlorophyll, soluble sugar, malondialdehyde(MDA), peroxidase(POD), catalase(CAT), superoxide dismutase(SOD) and chitinase of cucumber seedlings inoculated with P.cubensis by high temperature control were as follows: Compared with seedlings inoculated with P.cubensis, chlorophyll and soluble sugar contents can significantly increase and malondialdehyde(MDA) content significantly decrease by high temperature control after inoculation. Compared with control , the activity of peroxidase(POD),catalase(CAT),superoxide dismutase(SOD) and chitinase can increase by high temperature control after inoculation. 3. Sodium dodecylsulfonate-polyacrylamide gel electrophoresis (SDS-PAGE) result showed that a 28kD protein was induced by pathogens, and this 28kD protein was one kind of chitinase. High temperature control can weaken its expression. The expression of chitinase protein in intercellular was higher than it was in intracellular. Western blot result indicated that the expression of chitinase reduced after high temperature control.The overall studies showed that high temperature control can prevent downy mildew from two aspects. On the one hand, high temperature control can reduce the stress of cucumber seedlings through killing pathogens. On the other hand, high temperature control can enhance soluble sugar content and strengthen protective enzymes activity, chitinase activity and the express of pathogenesis-related protein, and these can improve resistance of cucumber to downy mildew. These results provide theoretical basis for the study of prevention mechanism of high temperature control and for better application of high temperature control in the filed.