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.

Molecular, Phenotypical, and Host-Range Characterization of Robbsia andropogonis Strains Isolated from Bougainvillea spp. in Mexico.

Characteristic leaf spot and blight symptoms caused by Robbsia andropogonis on bougainvillea plants were found in three locations in different provinces of Mexico from 2019 to 2020. Eleven bacterial isolates with morphology similar to R. andropogonis were obtained from the diseased bougainvillea leaves. The isolates were confirmed as R. andropogonis by phenotypic tests and 16S rRNA, rpoD, and gyrB gene sequencing. In addition to bougainvillea, the strains were pathogenic to 10 agriculturally significant crops, including maize (Zea mays), sorghum (Sorghum bicolor), barley (Hordeum vulgare), coffee (Coffea arabiga), carnation (Dianthus caryophilus), Mexican lime (Citrus × aurantifolia), common bean (Phaseolus vulgaris), broadbeans (Vicia faba), and pea (Pisum sativum), but not runner bean (Phaseolus coccineus). The haplotypes network reveals the genetic variability among Mexican strains and its phylogeographic relationship with Japan, the U.S.A., and China. The presence of this pathogen represents a challenge for plant protection strategies in Mexico.

Localized expression of antimicrobial proteins mitigates huanglongbing symptoms in Mexican lime.

Citrus huanglongbing (HLB) is a devastating disease associated with Candidatus Liberibacter asiaticus spp. (CLas), a bacterium restricted to the sieve tube system of the phloem that is transmitted by the psyllid vector, Diaphorina citri. In this study, the human antimicrobial peptides, lysozyme and ß-defensin 2, were targeted to the vascular tissue of Mexican lime (Citrus x aurantifolia [Christm.] Swingle) by fusion to a phloem-restricted protein. Localized expression was achieved, via Agrobacterium tumefaciens-mediated transformation of the stem, which led to protein expression and mobilization within the vascular tissue of heterotrophic tissues. HLB-infected plants were monitored for 360 days. Lower bacteria titers were observed in plants expressing either ß-defensin 2, lysozyme, or the combination thereof, and these plants had increased photosynthesis, compared to untreated control trees. Thus, targeting of antimicrobial proteins to the vascular tissue was effective in decreasing CLas titer, and alleviating citrus greening symptoms. Based on these findings, this strategy could be used to effectively treat plants that are already infected with bacterial pathogens that reside in the phloem translocation stream.

Characterization of a proposed dichorhavirus associated with the citrus leprosis disease and analysis of the host response.

The causal agents of Citrus leprosis are viruses; however, extant diagnostic methods to identify them have failed to detect known viruses in orange, mandarin, lime and bitter orange trees with severe leprosis symptoms in Mexico, an important citrus producer. Using high throughput sequencing, a virus associated with citrus leprosis was identified, belonging to the proposed Dichorhavirus genus. The virus was termed Citrus Necrotic Spot Virus (CNSV) and contains two negative-strand RNA components; virions accumulate in the cytoplasm and are associated with plasmodesmata-channels interconnecting neighboring cells-suggesting a mode of spread within the plant. The present study provides insights into the nature of this pathogen and the corresponding plant response, which is likely similar to other pathogens that do not spread systemically in plants.