In vitro and in planta activity of some essential oils against Venturia inaequalis (Cooke) G. Winter.

The effect of the essential oils of thyme ( Thymus vulgaris ), cinnamon ( Cinnamomum verum ) and sweet orange ( Citrus sinensis ) on conidial germination of Venturia inaequalis was investigated in vitro . Moreover, the effect of cinnamon oil was further studied by analyzing its preventive and curative activity as well as its rainfastness in an apple seedling-Venturia pathosystem under growth chamber and greenhouse conditions. The essential oils suppressed the germination of conidia at concentrations of 0.01% and 0.1% effectively. The highest inhibition level was achieved with thyme oil. Cinnamon oil (0.2%) with Silwet Star adjuvant (0.02%) was protective and curative against Venturia inaequalis when applied 72 h, 24 h, 1 h before or 24 h, 72 h after inoculation of plants. Effective control was achieved when essential oil was applied by 24 h, 1 h or 24 h pre- or ost-inoculation, respectively. The 72 h application was less effective, however, it still showed decent control level. Although the simulated rainfall of light (1 mm), moderate (3 mm) and heavy (5 mm) rain decreased the control efficacy of cinnamon oil, the water amounts did not influence control level. The oil application resulted in a moderate and long lasting control (92% efficacy without rainfall, 65% efficacy with rainfall). The results of this study show that the tested essential oils are appropriate candidates for the development of biofungicides against apple scab.