This research evaluated the efficacy of essential oils in the management of cucumber
powdery mildew. Essential oils of lemongrass, lemon, thyme, peppermint, abundance blend, purification
blend, and thieves blend were tested in vitro and under greenhouse conditions in two
separate experiments. The effects of essential oils were tested against powdery mildew disease at
concentrations of 1.0–2.5 mL/L, and the consequent impact of the oils on plant growth was evaluated.
Powdery mildew fungus, Podosphaera xanthii, was identified using sequencing of the ITS region.
The essential oils significantly reduced disease incidence up to 77.3% compared with the positive
control (p < 0.5). Moreover, the essential oils increased the plant length (up to 187 cm), leaf area (up
to 27.5 cm2), fresh weight (up to 123 g), dry weight (up to 22.5 g), number of flowers (16.3), and
metabolite content compared with the positive control (p < 0.5). Cell membrane injury decreased
significantly in the oil-treated pants (p < 0.5), indicating the protective effect of essential oils. This
study recommends the application of essential oils in an appropriate dose (2.5 mL/L) to protect
cucumber plants against powdery mildew. Overdose of the oils (more than 2.5 mL/L) should be
avoided due to adverse effects.