Clavibacter nebraskensis causing Goss's wilt of maize: Five decades of detaining the enemy in the New World

Clavibacter michiganensis Reframed: The Story of How the Genomics Era Made a New Face for an Old Enemy

OBJECTIVE

Bacterial wilt and canker of tomato caused by the gram-positive corynebacterial species Clavibacter michiganensis is an economically important disease threatening the tomato industry in both open-air and greenhouse productions around the world. The disease occurs in many countries, with a particular importance in regions characterised by high temperature and water scarcity. Management of bacterial canker has been a major problem since its original description in 1909. This is due in part to the seedborne nature of the pathogen, allowing the bacterium to be transmitted over long distances via infected seeds, as well as a lack of effective treatment to clean seeds. Detection of the pathogen from seeds is difficult due to high competition on culture media with diverse members of the seed-associated microbiota. Identification of the pathogen can also be difficult owing to the presence of different colony variants on culture media. In this review, we provide a historical perspective and an updated overview on the aetiology, epidemiology and management strategies of the bacterial canker disease. We also gathered recent molecular findings in the pathogenicity mechanisms and bioecology of C. michiganensis to boost management of the bacterial canker disease in the 21st century tomato industry.

TAXONOMY

Class: Actinobacteria; Order: Micrococcales; Family: Microbacteriaceae; Genus: Clavibacter; Species: Clavibacter michiganensis.

DISEASE SYMPTOMS

Interveinal leaf chlorosis leading to necrotic areas. Canker on stems and lateral branches of the plant. Discolouration of vascular and pith tissues to dark yellow or brown. Small and early ripened fruits or discolouration of the placenta from white to yellow in the interior part of the ripening fruits.

HOST RANGE

Tomato (Solanum lycopersicum) is the main host of the pathogen while natural infection has also been reported on eggplant, pepper and wild nightshade plants.

SYNONYMS (HISTORICAL/NON-PREFERRED SCIENTIFIC NAMES)

Aplanobacter michiganensis; Pseudomonas michiganense; Pseudomonas michiganensis; Bacterium michiganense; Phytomonas michiganensis; Mycobacterium michiganense; Erwinia michiganensis (=michiganense); Corynebacterium michiganense; Corynebacterium michiganense pv. michiganense; Corynebacterium michiganense subsp. michiganense; Clavibacter michiganensis subsp. michiganensis.

MICROBIOLOGICAL PROPERTIES

The bacterium produces domed, round and shiny mucoid colonies on general culture media. Colonies are usually yellow-pigmented, while pink-pigmented strains are occasionally observed. Cells are gram-positive, aerobic, non-motile, non-spore-producing curved rods (coryneform).

DISTRIBUTION

Present in all continents.

PHYTOSANITARY CATEGORIZATION

EPPO A2 List no. 50, EU 2019/2072 RNQP Annex IV. See EPPO (https://gd.eppo.int/taxon/CORBMI/categorization) and CABI (https://www.cabidigitallibrary.org/doi/10.1079/cabicompendium.15338) databases for further country-specific categorisations. EPPO code: CORBMI.

Identification of RppSLN from an Elite Landrace: A Major Locus Conferring Resistance to Southern Corn Rust in Maize (Zea mays L.)

Southern corn rust (SCR) is one of the most destructive foliar diseases in maize (Zea mays L.), resulting in significant yield losses. Therefore, the continuous identification of disease-resistant germplasm and the deployment of resistant hybrids is essential for durably controlling SCR. The objective of this research was to identify and characterize resistance loci against SCR in maize to expand disease management strategies. Here, we identified a maize landrace with high resistance to SCR 'Silunuo' (SLN) approaching complete immunity. We backcrossed it with a susceptible inbred line, N531, to generate a stable SCR-resistant introgression line N531_R. By crossing it with F35 (a susceptible inbred line), we created a large F2 segregating population and mapped a major SCR-resistant locus on chromosome 10, known as RppSLN. Based on the genome assembly and annotation, we found that RppSLN harbors two NBS-LRR (nucleotide binding site-leucine-rich repeat) genes, namely Zmays10G000430 and Zmays10G000440. These NBS-LRR genes were significantly induced during artificial inoculation with Puccinia polysora, suggesting that they might be candidate genes collectively contributing to the resistance level at this locus. In conclusion, this study identified a major SCR resistance locus directly isolated from a landrace, providing valuable support and information for expanding new disease-resistant germplasms and promoting the utilization of landraces.

A host shift as the origin of tomato bacterial canker caused by Clavibacter michiganensis

The Actinomycetota (formerly Actinobacteria) genus Clavibacter includes phytopathogens with devasting effects in several crops. Clavibacter michiganensis, the causal agent of tomato bacterial canker, is the most notorious species of the genus. Yet, its origin and natural reservoirs remain elusive, and its populations show pathogenicity profiles with unpredictable plant disease outcomes. Here, we generate and analyse a decade-long genomic dataset of Clavibacter from wild and commercial tomato cultivars, providing evolutionary insights that directed phenotypic characterization. Our phylogeny situates the last common ancestor of C. michiganensis next to Clavibacter isolates from grasses rather than to the sole strain we could isolate from wild tomatoes. Pathogenicity profiling of C. michiganensis isolates, together with C. phaseoli and C. californiensis as sister taxa and the wild tomato strain, was found to be congruent with the proposed phylogenetic relationships. We then identified gene enrichment after the evolutionary event, leading to the appearance of the C. michiganesis clade, including known pathogenicity factors but also hitherto unnoticed genes with the ability to encode adaptive traits for a pathogenic lifestyle. The holistic perspective provided by our evolutionary analyses hints towards a host shift event as the origin of C. michiganensis as a tomato pathogen and the existence of pathogenic genes that remain to be characterized.

Biologically inspired nanoformulations for the control of bacterial canker pathogens Clavibacter michiganensis subsp. michiganensis and subsp. capsici

Clavibacter michiganensis subsp. michiganensis (Cmm) and C. michiganensis subsp. capsici (Cmc) are phytopathogenic bacteria that cause bacterial canker disease in tomatoes and peppers, respectively. Bacterial canker disease poses serious challenges to solanaceous crops, causing significant yield losses and economic costs. Effective management necessitates the development of sustainable control strategies employing nanobiotechnology. In this study, the antibacterial effects of four Aspergillus sojae-mediated nanoformulations, including cobalt oxide nanoparticles (Co3O4 NPs), zinc oxide nanoparticles (ZnO NPs), cobalt ferrite nanoparticles (CoFe2O4 NPs), and CoFe2O4/functionalized multi-walled carbon nanotube (fMWCNT) bionanocomposite, were evaluated against Cmm and Cmc. The diameters of the zone of inhibition of A. sojae-mediated Co3O4 NPs, ZnO NPs, CoFe2O4 NPs, and CoFe2O4/fMWCNT bionanocomposite against Cmm and Cmc were 23.60 mm, 22.09 mm, 27.65 mm, 22.51 mm, and 19.33 mm, 17.66 mm, 21.64 mm, 18.77 mm, respectively. The broth microdilution assay was conducted to determine the minimal inhibitory and bactericidal concentrations. The MICs of Co3O4 NPs, ZnO NPs, CoFe2O4 NPs, and CoFe2O4/fMWCNT bionanocomposite against Cmm were 2.50 mg/mL, 1.25 mg/mL, 2.50 mg/mL, and 2.50 mg/mL, respectively. While, their respective MBCs against Cmm were 5.00 mg/mL, 2.50 mg/mL, 5.00 mg/mL, and 5.00 mg/mL. The respective MICs of Co3O4 NPs, ZnO NPs, CoFe2O4 NPs, and CoFe2O4/fMWCNT bionanocomposite against Cmc were 2.50 mg/mL, 1.25 mg/mL, 5.00 mg/mL, and 5.00 mg/mL. While, their respective MBCs against Cmc were 5.00 mg/mL, 2.50 mg/mL, 10.00 mg/mL, and 10.00 mg/mL. The morphological and ultrastructural changes of Cmm and Cmc cells were observed using field-emission scanning and transmission electron microscopy before and after treatment with sub-minimal inhibitory concentrations of the nanoformulations. Nanoformulation-treated bacterial cells became deformed and disrupted, displaying pits, deep cavities, and groove-like structures. The cell membrane detached from the bacterial cell wall, electron-dense particles accumulated in the cytoplasm, cellular components disintegrated, and the cells were lysed. Direct physical interactions between the prepared nanoformulations with Cmm and Cmc cells might be the major mechanism for their antibacterial potency. Further research is required for the in vivo application of the mycosynthesized nanoformulations as countermeasures to combat bacterial phytopathogens.

Effects of Hybrids, Foliar Treatments, and Infection Conditions on Bacterial Leaf Streak of Sweet Corn

Bacterial leaf streak (BLS) of corn caused by Xanthomonas vasicola pv. vasculorum was first reported in the United States in 2017. The biology and management of BLS are poorly understood. The objective of this work was to determine the effects of hybrids, foliar treatments, and infection conditions (timing, temperature, and inoculation site) on BLS of sweet corn. Field studies were conducted to determine if hybrid or foliar disease management treatments influenced BLS development and yield. Corn leaves were inoculated in plots with X. vasicola pv. vasculorum, and noninoculated plots were used for comparison. The leaf incidence and severity of BLS differed significantly among sweet corn hybrids, suggesting different levels of susceptibility to BLS. Grain yield was significantly reduced (14.7%) by BLS for one hybrid. The corn growth stage at the time of infection influenced BLS, with incidence and severity significantly greater after inoculation at stage V6 than at V9. Foliar application of Kocide, LifeGard, and Kocide and LifeGard significantly reduced leaf severity compared with nontreated controls in field studies. Kocide significantly reduced leaf incidence, but no treatments significantly increased yield versus controls. In comparisons of inoculation methods in a growth chamber, lesion length on leaves was significantly greater on stalk-inoculated than leaf-inoculated plants. Lesions developed on leaf-inoculated plants only at inoculation sites, whereas lesions developed on stalk-inoculated plants on multiple leaves. In controlled environments, lesion length on leaves was significantly greater at 21°C than at 27 and 32°C. This study expands our understanding of factors that influence the development and management of BLS of sweet corn.

Herbicide safener isoxadifen-ethyl associated with increased Goss's wilt severity in corn (Zea mays)

BACKGROUND

Goss's bacterial wilt and leaf blight (Goss's wilt), caused by the bacterium Clavibacter nebraskensis, is a corn disease that has been a top ten yield-reducing disease in North America in the past 15 years. Isoxadifen-ethyl is an herbicide safener that effectively increases cytochrome P450 activity in corn which enhances a plant's metabolism of herbicide molecules. Recent research found a potential link between isoxadifen-ethyl and increased Goss's wilt severity.

RESULTS

The application of isoxadifen-ethyl increased (P = 0.014-0.046) area under disease progress curve (AUDPC) by 19%, 7%, and 9% at three environments, independent of accompanying herbicide or herbicide application timing. However, no significant differences in incidence of systemic wilt or corn grain yield occurred among treatments at any environment.

CONCLUSION

These data provide evidence for an association between isoxadifen-ethyl safener and Goss's wilt in corn. The reason for this association is unknown, but the safener may affect plant or pathogen physiological mechanisms. While the increased disease severity did not result in decreased grain yield in these experiments, an increase in pathogen inoculum due to higher disease severity could influence Goss' wilt epidemics in future years. © 2024 Society of Chemical Industry.

Unlocking a Mystery: Characterizing the First Appearance of Clavibacter nebraskensis in Mexican Cornfields

The Goss's wilt and leaf blight is a disease of maize (Zea mays) caused by Clavibacter nebraskensis, which was widespread in the last several years throughout the Midwest in the United States, south in Texas, and north to Canada. The bacterium is included within the high-risk list of quarantine pathogens by many plant protection organizations and countries including Mexico. Severe blight symptoms on maize plants were found in different provinces from Coahuila and Tlaxcala, Mexico, in 2012 and 2021, respectively. Twenty bacterial isolates with morphology similar to C. nebraskensis were obtained from the diseased maize leaves. The isolates were confirmed by phenotypic tests and 16S rRNA and gyrB sequencing. Two strains were tested for pathogenicity tests on seven hybrid sweet corn cultivars available in Mexico, and the most sensitive cultivar was tested for all the strains to fulfill Koch's postulates. The phylogenetic reconstruction based on two single loci reveals a remarkable clustering of Mexican strains to American strains reported approximately 50 years ago. The presence of this pathogen represents a risk and a significant challenge for plant protection strategies in Mexico and maize diversity.

Identification of loci conferring resistance to 4 foliar diseases of maize

Foliar diseases of maize are among the most important diseases of maize worldwide. This study focused on 4 major foliar diseases of maize: Goss's wilt, gray leaf spot, northern corn leaf blight, and southern corn leaf blight. QTL mapping for resistance to Goss's wilt was conducted in 4 disease resistance introgression line populations with Oh7B as the common recurrent parent and Ki3, NC262, NC304, and NC344 as recurrent donor parents. Mapping results for Goss's wilt resistance were combined with previous studies for gray leaf spot, northern corn leaf blight, and southern corn leaf blight resistance in the same 4 populations. We conducted (1) individual linkage mapping analysis to identify QTL specific to each disease and population; (2) Mahalanobis distance analysis to identify putative multiple disease resistance regions for each population; and 3) joint linkage mapping to identify QTL across the 4 populations for each disease. We identified 3 lines that were resistant to all 4 diseases. We mapped 13 Goss's wilt QTLs in the individual populations and an additional 6 using joint linkage mapping. All Goss's wilt QTL had small effects, confirming that resistance to Goss's wilt is highly quantitative. We report several potentially important chromosomal bins associated with multiple disease resistance including 1.02, 1.03, 3.04, 4.06, 4.08, and 9.03. Together, these findings indicate that disease QTL distribution is not random and that there are locations in the genome that confer resistance to multiple diseases. Furthermore, resistance to bacterial and fungal diseases is not entirely distinct, and we identified lines resistant to both fungi and bacteria, as well as loci that confer resistance to both bacterial and fungal diseases.

Clavibacter lycopersici sp. nov.: a peach-colored actinobacterium isolated from symptomless tomato plant

In 2015, Gram-positive peach-coloured actinobacterial strains were isolated from symptomless tomato phyllosphere in Iran. Biochemical and physiological characteristics, as well as 16S rRNA phylogeny showed that the strains belong to Clavibacter sp., while they were non-pathogenic on the host of isolation, and morphologically distinct from the tomato pathogen C. michiganensis and other plant-associated bacteria. Multilocus sequence analysis of five housekeeping genes showed that the two peach-coloured strains CFBP 8615T (Tom532T) and CFBP 8616 (Tom495) were phylogenetically distinct from all validly described Clavibacter species. Whole genome sequence-based indices, i.e. average nucleotide identity (orthoANI) and digital DNA-DNA hybridization (dDDH), showed that the two peach-colored strains share nearly 100 % orthoANI value with one another, while they differ from all validly described Clavibacter species with the orthoANI/dDDH values <93 % and <50 %, respectively. Thus, based on both phenotypic features and orthoANI/dDDH indices the peach-coloured strains could belong to a new species within Clavibacter. In this study, we provide a formal species description for the peach-coloured tomato-associated Clavibacter strains. Clavibacter lycopersici sp. nov. is proposed for the new species with Tom532T = CFBP 8615T = ICMP 22100T as type strain.