agronomy Article Effects of Calcium, Magnesium and Potassium on Sweet Basil Downy Mildew (Peronospora belbahrii) Yigal Elad 1, * , Ziv Kleinman 2 , Ziv Nisan 1,2,3 , Dalia Rav-David 1 and Uri Yermiyahu 4   Citation: Elad, Y.; Kleinman, Z.; Nisan, Z.; Rav-David, D.; Yermiyahu, U. Effects of Calcium, Magnesium and Potassium on Sweet Basil Downy Mildew (Peronospora belbahrii). Agronomy 2021, 11, 688. https:// doi.org/10.3390/agronomy11040688 Academic Editor: Diego Rubiales Received: 21 March 2021 Accepted: 1 April 2021 Published: 4 April 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 Department Plant Pathology and Weed Research, The Volcani Center, Agricultural Research Organization, Rishon LeZion 7534509, Israel; zivnisan@gmail.com (Z.N.); dalia@volcani.agri.gov.il (D.R.-D.) 2 Bikat HaYarden Research and Development, Tzevi Research Station, Bikat HaYarden 91906, Israel; ziv.kleinman@mail.huji.ac.il 3 The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot 7610001, Israel 4 Gilat Research Center, Agricultural Research Organization, Negev 85280, Israel; uri4@volcani.agri.gov.il * Correspondence: elady@volcani.agri.gov.il Abstract: Downy mildew (Peronospora belbahrii) is a major disease of sweet basil (Ocimum basilicum). We examined the effects of potassium, calcium and magnesium, individually and in combination, on sweet basil downy mildew (SBDM) in potted plants and under commercial-greenhouse conditions over six growing seasons. An increased K concentration in the fertigation solution increased SBDM severity, whereas foliar-applied KCl and K 2 SO 4 suppressed SBDM. The application of higher con- centrations of those salts increased the K concentrations in the shoots and significantly alleviated SBDM. Increased concentrations of Ca or Mg in the fertigation solution decreased SBDM severity, as did foliar-applied CaCl 2 . However, the combination of Ca and Mg did not have any synergistic effect. Foliar-applied K 2 SO 4 provided better disease suppression than some of these treatments. The 3.3 mM Mg + fungicide treatment and the 5.0 mM Mg + fungicide treatment each provided synergistic disease control in one of two experiments. SBDM severity was significantly reduced by MgCl 2 and MgSO 4 (both 3.3 mM Mg), as compared with the basic Mg fertigation (1.6 mM), with MgCl 2 providing better control. The combined Mg salts + fungicide treatments reduced SBDM better than any of those treatments alone. These results demonstrate that macro-elements can contribute to SBDM control. Keywords: calcium; downy mildew; fertigation; irrigation; macro-elements; magnesium; Ocimum basilicum; potassium; yield 1. Introduction Macro-elements play an important role in plant nutrition and plant health. Among these, the cations K + , Ca 2+ and Mg 2+ are particularly important [1,2]. K + is the most abundant cation in plant tissue; its concentration in the cytoplasm reaches 200 mM, and it accounts for up to 6% of dry plant weight [3]. K + is readily translocated in the phloem and xylem and moves through tissues and into cells via K channels [4]. K + helps to regulate the electrical charge in cells and control the acidity of the cytosol and chloroplasts and plays a role in enzymatic reactions [5]. K + also affects the process of photosynthesis, including the turgor pressure that opens and closes stomata, other osmoregulation, cell elongation, upregulation of enzyme expression and protein synthesis [3,6]. In sweet basil, K deficiency inhibits growth and leads to the development of thin stems, leaf-edge necrosis and slow-developing roots (Yigal Elad, unpublished data). When present at optimal concentrations, K decreases plants’ susceptibility to dis- ease [1]. It increases the thickness of the walls of epidermal cells [4] and decreases the availability of sugar, amino acids and organic acids that are essential to the parasites’ nutrition [3]. K regulates the plant’s reaction to stress, including reactive oxygen species, as Agronomy 2021, 11, 688. https://doi.org/10.3390/agronomy11040688 https://www.mdpi.com/journal/agronomy