plants Article Effects of Microelements on Downy Mildew (Peronospora belbahrii) of Sweet Basil Yigal Elad 1, * , Ziv Nisan 1,2,3 , Ziv Kleinman 2 , Dalia Rav-David 1 and Uri Yermiyahu 4   Citation: Elad, Y.; Nisan, Z.; Kleinman, Z.; Rav-David, D.; Yermiyahu, U. Effects of Microelements on Downy Mildew (Peronospora belbahrii) of Sweet Basil. Plants 2021, 10, 1793. https:// doi.org/10.3390/plants10091793 Academic Editor: Francesca Degola Received: 27 July 2021 Accepted: 26 August 2021 Published: 28 August 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, Agricultural Research Organization, The Volcani Institute, 68 Hamakabim Rd, 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 76100, Israel 4 Gilat Research Center, Agricultural Research Organization, Volcani Institute, M.P. Negev, Gilat 8528000, Israel; uri4@volcani.agri.gov.il * Correspondence: Correspondence: elady@volcani.agri.gov.il Abstract: We recently demonstrated that spraying or irrigating with Ca, Mg and K reduces the severity of sweet basil downy mildew (SBDM). Here, the effects of Mn, Zn, Cu and Fe on SBDM were tested in potted plants. The effects of Mn and Zn were also tested under semi-commercial and commercial-like field conditions. Spray applications of a mixture of EDTA-chelated microelements (i.e., Fe-EDTA, Mn-EDTA, Zn-EDTA, Cu-EDTA and Mo) reduces SBDM severity. The application of EDTA chelates of individual microelements (i.e., Fe-EDTA, Mn-EDTA and Zn-EDTA) significantly reduces SBDM in potted plants. Foliar applications of Mn-EDTA and Zn-EDTA are found to be effective under semi-commercial conditions and were, thus, further tested under commercial-like conditions. Under commercial-like conditions, foliar-applied Mn-EDTA and Zn-EDTA decreased SBDM severity by 46–71%. When applied through the irrigation solution, those two microelements reduce SBDM by more than 50%. Combining Mg with Mn-EDTA and Zn-EDTA in the irrigation solution does not provide any additional disease reduction. In the commercial-like field experiment, the microelement-mixture treatment, applied as a spray or via the irrigation solution, was combined with fungicides spray treatments. This combination provides synergistic disease control. The mode of action in this plant–pathogen system may involve features of altered host resistance. Keywords: agrotechnical control; cultural control; integrated management; downy mildew; magne- sium; manganese; microelements; plant disease; Ocimum basilicum; zinc 1. Introduction Microelements are essential for plant development and are needed in lower concen- trations than macro-elements. Microelements are active as co-factors of metalo-enzymes, and some of them play roles in the structural stability of membranes in vascular plants. Microelements are involved in metabolic processes, such as the synthesis of primary and secondary metabolites, energy metabolism, cell defense, the control of gene expression, hormone absorption and signal transduction [1]. Microelements affect the concentrations of phenols and lignin in plants [2] and also affect how plants respond to pathogens [3], including the induction of systemic acquired resistance. Substantial information on the effects of specific microelements on plant diseases was published in the second half of the 20th century. In the powdery mildew–cucumber patho-system, Mn and Cu were found to induce the release of Ca ions from membranes. Ca ions play a role in the defense system together with salicylic acid [4]. Interactions between microelements may affect both the pathogen and the host. For instance, Fe is essential for Fusarium pathogenicity, while Mn Plants 2021, 10, 1793. https://doi.org/10.3390/plants10091793 https://www.mdpi.com/journal/plants