Acibenzolar— S — methyl (commercial name: Bion®) is an effective compound for control of powdery mildew in some crops. It doesn’t show any fungicidal effect in vitro and acts via inducing natural defense mechanisms in plants. The aim of this research was to study the effectiveness of Bion® for the prevention of cucumber powdery mildew via in vivo as well as in situ tests. In greenhouse tests Bion® (0.1 g/l) was sprayed on the plants at either 6, 3 or 1 day(s), or 3 hours before the inoculation of plants with spore suspension of the pathogen, disease severity being evaluated based on the number of disease spots per leaf. The effectiveness of this compound was also compared with Bayleton (0.5 g/l), Anvil (0.125 g/l) and Topas (0.125 g/l) through the same method. Furthermore the effect of Bion® for systemic control of disease on other nontreated parts of plant was studied in comparison with other fungicides. In field tests, Bion (0.1 g/l), Anvil (0.25 g/l), Topas (0.25 g/l) and Sulfur WP (3 g/l) were sprayed on the plants immediately after the appearance of primary disease spots. The effectiveness of the compounds was evaluated 14 days after application by measuring the percentage of diseased leaves and severity of leaf infection. In greenhouse tests, application of Bion® at different time intervals before inoculation protected the plants from infection. Disease severity on Bion-treated plants wasn’t significantly different at different application times and was less than 1% of that of control. In comparison with other compounds, the efficacy of Bion® for disease control was comparable to that in Anvil, Bayleton and Topas with all the four compounds standing in a distinct upper class. Bion® systemically protected the other plant parts with its systemic action being better than that of the other compounds. In field tests, Bion® well controlled the disease and was in an upper comparison group together with Anvil and Topas.
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