\u3ci\u3eClavibacter nebraskensis\u3c/i\u3e causing Goss\u27s wilt of maize: Five decades of detaining the enemy in the New World

Goss\u27s bacterial wilt and leaf blight of maize (Zea mays) caused by the gram-positive coryneform bacterium Clavibacter nebraskensis is an economically important disease in North America. C. nebraskensis is included within the high-risk list of quarantine pathogens by several plant protection organizations (EPPO code: CORBMI), hence it is under strict quarantine control around the world. The causal agent was reported for the first time on maize in Nebraska (USA) in 1969. After an outbreak during the 1970s, prevalence of the disease decreased in the 1980s to the early 2000s, before the disease resurged causing a serious threat to maize production in North America. The re-emergence of Goss\u27s wilt in the corn belt of the United States led to several novel achievements in understanding the pathogen biology and disease control. In this review, we provide an updated overview of the pathogen taxonomy, biology, and epidemiology as well as management strategies of Goss\u27s wilt disease. First, a taxonomic history of the pathogen is provided followed by symptomology and host range, genetic diversity, and pathogenicity mechanisms of the bacterium. Then, utility of high-throughput molecular approaches in the precise detection and identification of the pathogen and the management strategies of the disease are explained. Finally, we highlight the role of integrated pest management strategies to combat the risk of Goss\u27s wilt in the 21st century maize industry. Disease symptoms: Large (2–15 cm) tan to grey elongated oval lesions with wavy, irregular water-soaked margins on the leaves. The lesions often start at the leaf tip or are associated with wounding caused by hail or wind damage. Small (1 mm in diameter), dark, discontinuous water-soaked spots, known as “freckles”, can be observed in the periphery of lesions. When backlit, the freckles appear translucent. Early infection (prior to growth stage V6) may become systemic and cause seedlings to wilt, wither, and die. Coalescence of lesions results in leaf blighting. Host range: Maize (Zea mays) is the only economic host of the pathogen. A number of Poaceae species are reported to act as secondary hosts for C. nebraskensis. Taxonomic status of the pathogen: Class: Actinobacteria; Order: Micrococcales; Family: Microbacteriaceae; Genus: Clavibacter; Species: Clavibacter nebraskensis. Synonyms: Corynebacterium nebraskense (Schuster, 1970) Vidaver & Mandel 1974; Corynebacterium michiganense pv. nebraskense (Vidaver & Mandel 1974) Dye & Kemp 1977; Corynebacterium michiganense subsp. nebraskense (Vidaver & Mandel 1974) Carlson & Vidaver 1982; Clavibacter michiganense subsp. nebraskense (Vidaver & Mandel 1974) Davis et al. 1984; Clavibacter michiganensis subsp. nebraskensis (Vidaver & Mandel 1974) Davis et al. 1984. Type materials: ATCC 27794T; CFBP 2405T; ICMP 3298T; LMG 3700T; NCPPB 2581T. Microbiological properties: Cells are gram-positive, orange-pigmented, pleomorphic club-or rod-shaped, nonspore-forming, nonmotile, and without flagella, approximately 0.5 × 1–2.0 μm. Distribution: The pathogen is restricted to Canada and the United States. Phytosanitary categorization: EPPO code CORBNE

Evaluation of common bean lines for their reaction to the common bacterial blight pathogen

Common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli (Xap) is an economically important disease of common beans (Phaseolus vulgaris L.). Since there is no satisfactory chemical control for CBB, the use of resistant cultivars is an important management strategy. In the present study, twenty nine lines and one cultivar of common bean were evaluated for their reaction to Xap under greenhouse and field conditions. The experiments were conducted in randomized complete blocks with three replications. Reaction to Xap was assessed as diseased leaf area (DLA) and the number of spots on the leaves. Data analysis indicated that cultivar Khomein and the Ks21479 and Ks31169 lines were the most susceptible, while Ks51103, BF13607 and BF13608 lines were the most resistant. The data obtained from greenhouse and field experiments were in agreement. None of the evaluated lines and/or cultivars was immune for CBB; however, the three resistant lines were identified for use in cultivation or as sources of resistance to CBB in plant breeding programs

A powerful lamp weapon against the threat of the quarantine plant pathogen curtobacterium flaccumfaciens pv. Flaccumfaciens

Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff) is a Gram-positive phytopathogenic bacterium attacking leguminous crops and causing systemic diseases such as the bacterial wilt of beans and bacterial spot of soybeans. Since the early 20th century, Cff is reported to be present in North America, where it still causes high economic losses. Currently, Cff is an emerging plant pathogen, rapidly spreading worldwide and occurring in many bean-producing countries. Infected seeds are the main dissemination pathway for Cff, both over short and long distances. Cff remains viable in the seeds for long times, even in field conditions. According to the most recent EU legislation, Cff is included among the quarantine pests not known to occur in the Union territory, and for which the phytosanitary inspection consists mainly of the visual examination of imported bean seeds. The seedborne nature of Cff combined with the globalization of trades urgently call for the implementation of a highly specific diagnostic test for Cff, to be routinely and easily used at the official ports of entry and into the fields. This paper reports the development of a LAMP (Loop-Mediated Isothermal Amplification) specific for Cff, that allows the detection of Cff in infected seeds, both by fluorescence and visual monitoring, after 30 min of reaction and with a detection limit at around 4 fg/µL of pure Cff genomic DNA. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Multilocus sequence analysis reveals a novel phylogroup of Xanthomonas euvesicatoria pv. perforans causing bacterial spot of tomato in Iran

A multilocus sequence analysis (MLSA) was performed on five housekeeping genes (fusA, gapA, gltA, lacF and lepA) of 22 Xanthomonas euvesicatoria strains recently isolated from alfalfa, pepper and tomato plants in Iran. In addition, 161 strains isolated worldwide from pepper, poinsettia, rose and tomato plants were included in the analysis. All X.euvesicatoria pv. perforans isolates from tomato plants in Iran clustered in a monophyletic group, although five MLSA haplotypes were detected among them. The Iranian tomato strains presented 10 nucleotide differences in the lepA gene sequences compared to the known worldwide population of X.euvesicatoria pv. perforans. Statistical analyses revealed a recombination event that had occurred in the lepA gene of the strains isolated from tomato in Iran. BOX-PCR analysis confirmed the inclusion of Iranian tomato strains within X.euvesicatoria pv. perforans. Furthermore, X.euvesicatoria pv. euvesicatoria strains isolated from pepper in Iran differed in one nucleotide in the lepA gene sequence from the known worldwide population of the pathovar, and clustered in a group containing strains isolated in Nigeria. The strains isolated from alfalfa in Iran clustered with the type strain of X.euvesicatoria pv. alfalfae. Altogether, the results reveal the existence of a phylogenetically novel population of X.euvesicatoria pv. perforans in Iran which needs further in-depth analysis to pinpoint the epidemiological impact of these strains

Pathogenicity and phylogenetic analysis of Clavibacter michiganensis strains associated with tomato plants in Iran

During 2013-2016, 277 tomato fields were surveyed across Iran to monitor the status of bacterial canker of tomato, caused by Clavibacter michiganensis subsp. michiganensis. Altogether, 450 plant samples were collected, both with and without symptoms, from which 35 bacterial strains were recovered. These were positive for the PCR test performed using the Clavibacter-specific primer pair CMR16F1/CMR16R1. Based on the phylogeny of the gyrB gene sequences, 31, three and one of the 35 strains were identified as C. michiganensis, Microbacterium sp. and Agrococcus sp., respectively. The 31 strains of C. michiganensis were further identified as C. michiganensis subsp. michiganensis (23 strains), C. michiganensis subsp. tessellarius (six strains) and Clavibacter spp. (two strains). This was subsequently confirmed by multilocus sequence analysis (MLSA) of five housekeeping genes (atpD, gyrB, ppk, recA and rpoB). In pathogenicity tests, all 23 strains induced wilting symptoms on tomato plants in greenhouse conditions, while no symptoms were observed on eggplant, bell pepper and chili pepper plants. All evaluated pathogenicity determinant genes (celA, pat-1, tomA, ppaA, chpC and chpG) were detected in 18 out of 31 C. michiganensis strains, using eight specific primer pairs. Estimation of the number of nucleotide differences, sequence similarity matrix and MLSA clustered two peach-coloured strains (Tom495 and Tom532) separately from all nine previously described subspecies, thereby suggesting these two strains are a new subspecies of C. michiganensis. However, a detailed taxonomic study using multiphased molecular approaches is needed to delineate a formal taxonomic name for these atypical strains

Two novel genomospecies in the Agrobacterium tumefaciens species complex associated with rose crown gall

In this study, we explored the pathogenicity and phylogenetic position of Agrobacterium spp. strains isolated from crown gall tissues on annual, perennial, and ornamental plants in Iran. Of the 43 strains studied, 10 strains were identified as Allorhizobium vitis (formerly Agrobacterium vitis) using the species-specific primer pair PGF/PGR. Thirty-three remaining strains were studied using multilocus sequence analysis of four housekeeping genes (i.e., atpD, gyrB, recA, and rpoB), from which seven strains were identified as A. larrymoorei and one strain was identified as A. rubi (Rer); the remaining 25 strains were scattered within the A. tumefaciens species complex. Two strains were identified as genomospecies 1 (G1), seven strains were identified as A. radiobacter (G4), seven strains were identified as A. deltaense (G7), two strains were identified as A. nepotum (G14), and one strain was identified as "A. viscosum" (G15). The strains Rnr, Rnw, and Rew as well as the two strains OT33 and R13 all isolated from rose and the strain Ap1 isolated from apple were clustered in three atypical clades within the A. tumefaciens species complex. All but eight strains (i.e., Nec10, Ph38, Ph49, fic9, Fic72, R13, OT33, and Ap1) were pathogenic on tomato and sunflower seedlings in greenhouse conditions, whereas all but three strains (i.e., fic9, Fic72, and OT33) showed tumorigenicity on carrot root discs. The phylogenetic analysis and nucleotide diversity statistics suggested the existence of two novel genomospecies within the A. tumefaciens species complex, which we named "G19" and "G20." Hence, we propose the strains Rew, Rnw, and Rnr as the members of "G19" and the strains R13 and OT33 as the members of G20, whereas the phylogenetic status of the atypical strain Ap1 remains undetermined