  Citation: Risoli, S.; Cotrozzi, L.; Sarrocco, S.; Nuzzaci, M.; Pellegrini, E.; Vitti, A. Trichoderma-Induced Resistance to Botrytis cinerea in Solanum Species: A Meta-Analysis. Plants 2022, 11, 180. https://doi.org/ 10.3390/plants11020180 Academic Editor: Paula Baptista Received: 16 December 2021 Accepted: 4 January 2022 Published: 11 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). plants Article Trichoderma-Induced Resistance to Botrytis cinerea in Solanum Species: A Meta-Analysis Samuele Risoli 1,2 , Lorenzo Cotrozzi 2,3, * , Sabrina Sarrocco 2, * , Maria Nuzzaci 4 , Elisa Pellegrini 2,3 and Antonella Vitti 4,5 1 University School for Advanced Studies IUSS Pavia, Piazza della Vittoria 15, 27100 Pavia, Italy; samuele.risoli@iusspavia.it 2 Department of Agriculture, Food and Environment, University of Pisa, Via del Borghetto 80, 56124 Pisa, Italy; elisa.pellegrini@unipi.it 3 Nutrafood Research Center, University of Pisa, Via del Borghetto 80, 56124 Pisa, Italy 4 School of Agricultural, Forestry, Food and Environmental Sciences, University of Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy; maria.nuzzaci@unibas.it (M.N.); avitti@unisa.it (A.V.) 5 Department of Pharmacy, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Italy * Correspondence: lorenzo.cotrozzi@unipi.it (L.C.); sabrina.sarrocco@unipi.it (S.S.); Tel.: +39-050-221-0563 (L.C.); +39-050-221-6103 (S.S.) Abstract: With the idea of summarizing the outcomes of studies focusing on the resistance induced by Trichoderma spp. against Botrytis cinerea in tomato, the present paper shows, for the first time, results of a meta-analysis performed on studies published from 2010 to 2021 concerning the cross-talk occurring in the tomato–Trichoderma-B. cinerea system. Starting from an initial set of 40 papers, the analysis was performed on 15 works and included nine parameters, as a result of a stringent selection mainly based on the availability of more than one article including the same indicator. The resulting work not only emphasizes the beneficial effects of Trichoderma in the control of grey mold in tomato leaves (reduction in disease intensity, severity and incidence and modulation of resistance genes in the host), but carefully drives the readers to reply to two questions: (i) What are the overall effects of Trichoderma on B. cinerea infection in tomato? (ii) Do the main effects of Trichoderma differ based on the tomato species, Trichoderma species, amount, type and duration of treatment? At the same time, this meta-analysis highlights some weak points of the available literature and should be seen as an invitation to improve future works to better the conceptualization and measure. Keywords: grey mold; systemic resistance; pathogenesis-related (PR) genes; defense-signal transduc- tion pathway; Trichoderma spp. 1. Introduction Tomato (Solanum lycopersicum) is one of the most cultivated horticultural crops world- wide, with more than 180 million tons produced over an area of about 5 million hectares in 2019 [1]. Tomato is also the most consumed vegetable worldwide and a major component of the Mediterranean diet, mainly because of its remarkable nutraceutical properties due to the considerable presence of antioxidant compounds, such as lycopene, ascorbic acid, phenols, flavonoids and tocopherols [2]. At the same time, tomato is one of the most economically important host plants of Botrytis cinerea (a pathogenic fungus belonging to the phylum Ascomycota causing grey mold [3]). Botrytis cinerea has been classified as the second most dangerous plant pathogen [4] because of its wide host range (including more than 200 species over both temperate and tropical areas) and its ability to attack leaves, stems, flowers and fruits, so determining severe pre- and post-harvest losses [5,6], as well as the low efficacy of chemical control due to its wide genetic variability and high capacity to acquire resistance against chemical fungicides [7]. For these reasons, B. cinerea has received increasing attention, also becoming Plants 2022, 11, 180. https://doi.org/10.3390/plants11020180 https://www.mdpi.com/journal/plants