agriculture Article Effective Management of Cucumber Powdery Mildew with Essential Oils Yasser S. Mostafa 1 , Mohamed Hashem 1,2, * , Ali M. Alshehri 1 , Saad Alamri 1 , Ebrahem M. Eid 1,3 , El-Sayed H.E. Ziedan 4 and Sulaiman A. Alrumman 1   Citation: Mostafa, Y.S.; Hashem, M.; Alshehri, A.M.; Alamri, S.; Eid, E.M.; Ziedan, E.-S.H.; Alrumman, S.A. Effective Management of Cucumber Powdery Mildew with Essential Oils. Agriculture 2021, 11, 1177. https:// doi.org/10.3390/agriculture11111177 Academic Editors: Renata Bažok and Maja ˇ Caˇ cija Received: 5 November 2021 Accepted: 20 November 2021 Published: 22 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Biological Science Department, Faculty of Science, King Khalid University, Abha 61321, Saudi Arabia; Ysmosutafa@kku.edu.sa (Y.S.M.); aahshehry@kku.edu.sa (A.M.A.); saralomari@kku.edu.sa (S.A.); ebrahem.eid@sci.kfs.edu.eg (E.M.E.); Salrumman@kku.edu.sa (S.A.A.) 2 Botany and Microbiology Department, Faculty of Science, Assiut University, Assiut 71516, Egypt 3 Botany Department, Faculty of Science, Kafrelsheikh University, Kafr El-Sheikh 33516, Egypt 4 Plant Pathology, National Research, Dokki, Cairo 12622, Egypt; eh.ziedan@nrc.sci.eg * Correspondence: mhashem@kku.edu.sa; Tel.: +966-172-417-625 Abstract: This research evaluated the efficacy of essential oils in the management of cucumber powdery mildew. Essential oils of lemongrass, lemon, thyme, peppermint, abundance blend, pu- rification 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 cm 2 ), 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. Keywords: cucumber; essential oils; crop management; disease control; Podosphaera xanthii; powdery mildew 1. Introduction Cucumber (Cucumis sativus L.) is one of the most important and widely distributed vegetable crops due to its rapid growth and early maturity. Cucumber is produced either in protected houses or in open fields [1,2]. The global cultivation of this crop covers 2,231,402 hectares, with production of about 87.805 tons [3]. However, cultivation in the Arab countries is about 67,000 hectares. Among these countries, Saudi Arabia cultivates 11,764 hectares and produces 149,074 tons, with a production rate of 1.27 tones/ha [4]. Cucumber fruit is a good source of minerals and vitamins and delivers Na, Mg, K, Ca, S, Si, F, and Fe. Cucumber contains carbohydrates, protein, thiamin, riboflavin, vitamin C, and niacin. Moreover, the presence of minerals is important to maintain human blood alkalinity. In addition, the vitamins in cucumber fruit facilitate strength and the growth of connecting tissue, as well as relieve joint pain. Cucumber also contains secoisolariciresinol, lignans, lariciresinol, and pinoresinol, which help to reduce the risks of different cancers [5]. In many countries of the Middle East, such as Egypt, open fields are the common culti- vation areas for cucumbers [6]. However, in other countries suffering from water shortages, such as Gulf countries, greenhouses are the main production sources of cucumber [7]. In Saudi Arabia, the total greenhouse area for cucumber production in 2013 was 2605 hectares, which produced 236,087 tons [8]. Agriculture 2021, 11, 1177. https://doi.org/10.3390/agriculture11111177 https://www.mdpi.com/journal/agriculture