Pathovar-Specific PCR Method for Detection and Identification of Xanthomonas translucens pv. undulosa

TaqMan Real-Time PCR Assay for Specific Detection and Differentiation of Xanthomonas translucens pv. undulosa from Other Pathovars Targeting a Recombination Mediator Gene, recF

Bacterial leaf streak and black chaff diseases of wheat caused by Xanthomonas translucens pv. undulosa is becoming a major constraint to growers and trade since it is seedborne. Molecular tools for specific detection/differentiation of pv. undulosa are lacking. We report the development of a TaqMan real-time PCR for specific detection/identification of pv. undulosa targeting the recombination mediator gene (recF). Analysis of the complete recF (1,117 bp) sequences identified the gene as a reliable phylogenetic marker for identification of pv. undulosa, differentiating it from the other pathovars; recF-based sequence homology values among the 11 pathovars correlated well with genome-based DNA-DNA hybridization values. The discriminatory power of recF to differentiate pv. undulosa from the other pathovars is due to nucleotide polymorphic positions. We used these nucleotide polymorphisms to develop a TaqMan PCR for specific detection of pv. undulosa. The specificity of the assay was validated using 67 bacterial and fungal/oomycete strains. The selected primers and the double-quenched FAM-labeled TaqMan probe were specific for the detection of 11 pv. undulosa/secalis strains. The 56 strains of other X. translucens pathovars (n = 39) and non-Xanthomonas spp. (n = 17) did not exhibit any detectable fluorescence. Also, greenhouse-inoculated and naturally infected wheat leaf samples showed positive reactions for the presence of pv. undulosa DNA but not healthy control plants. The TaqMan assay reliably detected as low as 1-pg DNA amount and 10 colony forming units of the target pathogen per reaction. This TaqMan assay could be useful to regulatory agencies with economic benefits to wheat growers.

Detection and Differentiation of Xanthomonas translucens Pathovars translucens and undulosa from Wheat and Barley by Duplex Quantitative PCR

Two probe-based quantitative PCR (qPCR) systems, namely P-Xtt and P-Xtu, were developed to diagnose cereal bacterial leaf streak pathogens Xanthomonas translucens pv. translucens and pv. undulosa, respectively. P-Xtt is specific to pv. translucens, and P-Xtu is specific to pv. undulosa, pv. cerealis, pv. secalis, and pv. pistaciae. P-Xtt and P-Xtu worked on all accessible strains of pv. translucens and pv. undulosa, respectively. Both systems could detect 100 copies of the target gBlock DNA. The two systems could be used in both singleplex qPCR and duplex qPCR with similar efficiencies. On genomic DNA from strains of various X. translucens pathovars, both singleplex and duplex qPCR could specifically detect and differentiate pv. translucens and pv. undulosa. The duplex qPCR could detect pv. translucens and pv. undulosa from genomic DNA of 1,000 bacterial cells. On infected barley and wheat grain samples and on one infected wheat leaf sample, the duplex qPCR showed similar efficiency compared to a previously published qPCR system but with the additional capability of pathovar differentiation. The duplex qPCR system developed in this study will be useful in studies on bacterial leaf streak and detection/differentiation of the pathogens.

Epidemiology, Host Resistance, and Genomics of the Small Grain Cereals Pathogen Xanthomonas translucens: New Advances and Future Prospects

Bacterial leaf streak (BLS) primarily affects barley and wheat and is mainly caused by the pathogens Xanthomonas translucens pv. translucens and X. translucens pv. undulosa, respectively. BLS is distributed globally and poses a risk to food security and the supply of malting barley. X. translucens pv. cerealis can infect both wheat and barley but is rarely isolated from these hosts in natural infections. These pathogens have undergone a confusing taxonomic history, and the biology has been poorly understood, making it difficult to develop effective control measures. Recent advancements in the ability and accessibility to sequence bacterial genomes have shed light on phylogenetic relationships between strains and identified genes that may play a role in virulence, such as those that encode Type III effectors. In addition, sources of resistance to BLS have been identified in barley and wheat lines, and ongoing efforts are being made to map these genes and evaluate germplasm. Although there are still gaps in BLS research, progress has been made in recent years to further understand epidemiology, diagnostics, pathogen virulence, and host resistance.

Novel plant disease detection techniques-a brief review

Plant pathogens cause severe losses to agricultural yield worldwide. Tracking plant health and early disease detection is important to reduce the disease spread and thus economic loss. Though visual scouting has been practiced from former times, detection of asymptomatic disease conditions is difficult. So, DNA-based and serological methods gained importance in plant disease detection. The progress in advanced technologies challenges the development of rapid, non-invasive, and on-field detection techniques such as spectroscopy. This review highlights various direct and indirect ways of detecting plant diseases like Enzyme-linked immunosorbent assay, Lateral flow assays, Polymerase chain reaction, spectroscopic techniques and biosensors. Although these techniques are sensitive and pathogen-specific, they are more laborious and time-intensive. As a consequence, a lot of interest is gained in in-field adaptable point-of-care devices with artificial intelligence-assisted pathogen detection at an early stage. More recently computer-aided techniques like neural networks are gaining significance in plant disease detection by image processing. In addition, a concise report on the latest progress achieved in plant disease detection techniques is provided.