Propolis for the control of powdery mildew and the induction of phytoalexins in cucumber
Due to its antimicrobial properties propolis has been utilised by man for millennia, however, only recently has its use as an agricultural antimicrobial agent been tested. The capacity of this product to activate plant defence mechanisms is highly promising, due to the large number of substances in its composition. Therefore, the objective of this study was to evaluate the effect of different concentrations and application times of ethanol extracts of propolis (EEP) on powdery mildew (Podosphaera fuliginea) in cucumber crops, to establish the most suitable method for control and whether or not there is potential to induce plant defence mechanisms. The experiment was conducted in pots in a greenhouse. A completely randomised experimental design was employed, using a 6x3 factorial consisting of six concentrations (0; 0.5; 1; 2; 4 and 8%) and three EEP application times (24 hours before pathogen inoculation, 24 hours after pathogen inoculation and at the onset of first symptoms of the disease), replicated four times. Evaluations for disease incidence and severity were made weekly. The experiment was repeated for a second crop, following the same methodology established in the first. We also evaluated the effect of different EEP concentrations on the activation of plant defence mechanisms, through the induction of phytoalexins in soybean (Glycine max) cotyledons. A significant interaction was observed between the factors (application time x concentration) for disease incidence. For the area under the disease progress curve (AUDPC) for severity there was no interaction observed between the factors, however, we observed significant differences for the two factors separately. The AUDPC for incidence was less for the application 24 hours after pathogen inoculation, principally at the highest concentration. The 8% concentration of EEP controlled disease severity by 31.33 and 43.68% for the first and second crops, respectively. The application of the concentrations 24 hours before and 24 hours after pathogen inoculation showed less AUDPC for severity, but this difference was not significant for either crop. The EEP induced increasing phytoalexin levels in soybean cotyledons as the applied EEP concentration increased. Ethanol extracts of propolis could have potential for the control of powdery mildew, principally in cucumbers, through the preventive application of an 8% concentration.