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Subjects:
Agronomy
Tomato is one of the world’s most commonly grown and consumed vegetables. However, it can be attacked by the Gram-positive bacterium Clavibacter michiganensis subsp. michiganensis (Cmm), which causes bacterial canker on tomato plants, resulting in significant financial losses in field production and greenhouses worldwide.
- bacterial canker
- C michiganensis subsp. michiganensis
- biocontrol
1. Introduction
The world’s population is steadily increasing and may exceed over 9 billion people by 2050 [1]. However, approximately 2 billion people worldwide are moderately to severely food insecure and face an increased risk of hunger, malnutrition, and health disorders. Therefore, agricultural systems are challenged with finding appropriate solutions for more sustainable food production, with the requirement to increase total food production by 70–100% to satisfy the global population’s needs and meet the growing consumer demand for healthy food that is free of synthetic agrochemicals [2]. Plant pathogens constitute a severe challenge to agricultural productivity and food production worldwide, with spillover effects on natural resources [3]. Every year, crops are damaged by plant diseases caused by phytopathogens, which affect 10 to 20% of the world’s production, resulting in significant yield losses estimated at billions of dollars and preventing 800 million people from being adequately fed [4]. Among these pathogens, there are over 200 species of phytobacteria which could cause serious diseases in agricultural ecosystems worldwide. Indeed, most research on the interaction of bacterial pathogens with their target plants has focused primarily on the Gram-negative group, as they are the major soil pathogens and are readily available for molecular analysis. In contrast, the Gram-positive group of phytobacteria has unfortunately not attracted the same interest from molecular phytopathologists, even though some of them have caused significant crop losses in agriculture [5].
The Gram-positive Clavibacter michiganensis subsp. michiganensis (Cmm) is the causal agent of bacterial wilt and canker of tomato plants [6], which is considered one of the most potentially contagious and destructive diseases of this crop [5,7]. This bacterium is a seed-borne pathogen that is widespread in different areas of tomato production around the world, and has caused devastating epidemics, thus resulting in significant financial losses in greenhouse and field production [8,9]. It has become an economically serious threat to tomato producers worldwide [10]. For this reason, the European and Mediterranean Plant Protection Organization (EPPO) has declared C. michiganensis subsp. michiganensis as a quarantine organism at the international level [11]. Forty-six complete genomes of Clavibacter strains were extracted at the Bacterial and Viral Bioinformatics Resource Center (BV-BRC) [24]. To evaluate the relevancy of the sequenced strains among the Clavibacter genus, a genome-based phylogenetic tree based on 46 complete genome sequences was constructed. The tree was built with the Bacterial and Viral Bioinformatics Resource Center (BV-BRC) [24]. Only Clavibacter strains that have chromosome-level genome assembly were selected for analysis. The tree was visualized by using ITOL [25].
Despite the seriousness of tomato bacterial canker, no control strategy has proven to be completely effective to date [12]. Since no Cmm-resistant seeds are commercially available, and genetic progress in selecting resistant lines remains modest, the management of bacterial canker remains difficult [13]. Currently, Cmm has been controlled primarily by strict prevention, which is aimed at reducing the risk of Cmm-spread and new epidemics [14]. The most frequent methods used to control Cmm infection include the use of various synthetic pesticides and antibiotics [15]. However, the widespread application of these agrochemicals has led to growing concerns about environmental pollution and health risks [16]. In addition, chemical control is costly and ineffective in managing bacterial canker [14]. Therefore, researchers have focused on developing more efficient and safer alternatives to manage the disease, while improving tomato crop quality and production. This has become a major priority in modern agriculture [17].
The use of PGPRs (plant growth-promoting rhizobacteria) as biocontrol agents (BCAs) has emerged as a promising alternative to synthetic chemicals, providing a cost-efficient and ecological approach to plant preservation [18,19]. PGPRs are free-living bacteria in the soil that have the potential to promote plant growth, improve crop yield, and limit pathogen infection through complex direct or indirect mechanisms, including growth promotion, antibiosis, and induced resistance in host plants, thereby contributing to effective disease control [20,21]. Therefore, PGPRs represent powerful sustainable agriculture tools which are now a common practice worldwide and a key trend for the future [22].
2. Importance of Bacterial Canker Disease
Tomato (Lycopersicon esculentum Mill.) is widely consumed and recognized as one of the world’s most widespread vegetable crops [26]. It accounts for 72% of the global value of fresh vegetables produced [27], achieving a global production of 182 million tons in 2018 [28]. In Morocco, tomato cultivation holds a prominent place in the export-oriented agricultural economy [29]. The crops are mainly grown on 15,239 hectares in the Souss-Massa region, with a total production of 1,231,250 tons and productivity of 81 tons/hectare in 2018 [28]. Furthermore, tomato is an indispensable part of the human diet, and its production provides income to many smallholder farmers in the poorest regions of the globe [30,31]. However, it is vulnerable to a broad variety of diseases, which significantly impact plant growth and even survival, thereby affecting crop quality and production [18]. Bacterial canker caused by the actinobacterium, C. michiganensis subsp. michiganensis is among the most devastating and contagious diseases that severely affect tomato plants [5,6,12]. It was first identified by Smith in Michigan, USA in 1909 [32]. The Cmm is a seed-borne pathogen [33]. Its long-distance movement is mediated by infected seeds, which may explain its current dispersion in most tomato-producing areas around the world [34,35,36,37]. C. michiganensis has caused devastating epidemics, resulting in severe economic damage in greenhouses and open-field production [12,38]. In addition, this destructive disease has severely affected the production and performance of tomato crops, with substantial yield losses ranging from 20 to 84%, thereby posing a real economic threat for tomato growers worldwide [10]. For this reason, the EPPO has declared C. michiganensis subsp. michiganensis as a quarantine organism at the international level [11]. Indeed, the severity of bacterial canker depends on several factors including the year, cultivar, cultural practices, phenological stage of host infection, weather conditions, and inoculum concentration [8,12].
The bacterial canker has been identified in Morocco since 1942 and has drastically damaged all tomato cultivation areas, with varying severity in different regions. In particular, the bacterial pathogen Cmm has been the principal reason for the premature death of tomatoes in the Souss-Massa-Draa valley in the Agadir region, affecting the yield and causing a severe decrease in fruit weight under field conditions, which represents a serious threat to tomato cultivars [29,39,40].
This entry is adapted from the peer-reviewed paper 10.3390/microorganisms11030726
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