Plant Disease

Huanglongbing (HLB) is a destructive citrus disease that affects citrus production worldwide. ‘Candidatus Liberibacter asiaticus’ (CLas), a phloem-limited bacterium, is the associated causal agent of HLB. The current standard for detection of CLas is real-time quantitative polymerase chain reaction (qPCR) using either the CLas 16S rRNA gene or the ribonucleotide reductase (RNR) gene-specific primers/probe. qPCR requires well-equipped laboratories and trained personnel, which is not convenient for rapid field detection of CLas-infected trees. Recombinase polymerase amplification (RPA) assay is a fast, portable alternative to PCR-based diagnostic methods. In this study, an RPA assay was developed to detect CLas in crude citrus extracts utilizing isothermal amplification, without the need for DNA purification. Primers were designed to amplify a region of the CLas RNR gene, and a fluorescent labeled probe allowed for detection of the amplicon in real-time within 8 mins at 39°C. The assay was specific to CLas, and the sensitivity was comparable to qPCR, with a detection limit cycle threshold of 34. Additionally, the RPA assay was combined with a lateral flow device for a point-of-use assay that is field deployable. Both assays were 100% accurate in detecting CLas in fresh citrus crude extracts from leaf midribs and roots from five California strains of CLas tested in the Contained Research Facility in Davis, California. This assay will be important for distinguishing CLas-infected trees in California from those infected by other pathogens that cause similar disease symptoms and can help control HLB spread.

Grapevine yellows is one of the most damaging phytoplasma-associated diseases worldwide. It is linked to several phytoplasma species, which can vary regionally due to phytoplasma and insect-vector diversity. Specific, rapid, and reliable detection of the grapevine yellows pathogen has an important role in phytoplasma control. The purpose of this study was to develop and validate a specific loop-mediated isothermal amplification (LAMP) assay for detection of a distinct strain of grapevine ‘Candidatus Phytoplasma asteris’ that is present in South Africa, through implementation of a genome-informed test design approach. Several freely available, user-friendly, web-based tools were coupled to design the specific LAMP assays. The criteria for selection of the assays were set for each step of the process, which resulted in four experimentally operative LAMP assays that targeted the ftsH/hflB gene region, specific to the aster yellows phytoplasma strain from South Africa. A real-time PCR was developed, targeting the same genetic region, to provide extensive validation of the LAMP assay. The validated molecular assays are highly specific to the targeted aster yellows phytoplasma strain from South Africa, with good sensitivity and reproducibility. We show a genome-informed molecular test design and an efficient validation approach for molecular tests if reference and sample materials are sparse and hard to obtain. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

‘Candidatus Phytoplasma trifolii’ is a cell wall-less phytopathogenic bacterium that infects many agriculturally important plant species such as alfalfa, clover, eggplant, pepper, potato, and tomato. The phytoplasma is responsible for repeated outbreaks of potato purple top (PPT) and potato witches’ broom (PWB) that occurred along the Pacific Coast of the United States since 2002, inflicting significant economic losses. To effectively manage these phytoplasmal diseases, it is important to develop diagnostic tools for specific, sensitive, and rapid detection of the pathogens. Here we report the development of a DNA endonuclease targeted CRISPR trans reporter (DETECTR) assay that couples isothermal amplification and Cas12a transcleavage of fluorescent oligonucleotide reporter for highly sensitive and specific detection of ‘Candidatus Phytoplasma trifolii’-related strains responsible for PPT and PWB. The DETECTR assay was capable of specifically detecting the 16S-23S ribosomal DNA intergenic transcribed spacer sequences from PPT- and PWB-diseased samples at the attomolar sensitivity level. Furthermore, the DETECTR strategy allows flexibility to capture assay outputs with fluorescent microplate readers or lateral flow assays for potentially high-throughput and/or field-deployable disease diagnostics.

Citrus huanglongbing (HLB) is present in 10 provinces in China and is associated with ‘Candidatus Liberibacter asiaticus’ (CLas), which is transmitted by the Asian citrus psyllid (Diaphorina citri, ACP). To date, HLB and ACP have expanded to Yibin city of Sichuan Province, posing an imminent threat to the citrus belt of the upper and middle reaches of the Yangtze River, an important late-maturing citrus-producing area in China. To understand the epidemiological route of CLas and ACP in newly invaded regions of Sichuan and thereby better establish an HLB interception zone ranging from Leibo to Yibin, we evaluated the molecular variability of 19 CLas draft genomes from citrus or dodder (Cuscuta campestris). They include three type-specific prophage loci, three variable number tandem repeat loci, a miniature inverted-repeat transposable element, and population diversity of 44 ACP mitochondrial genomes. The results indicated that CLas isolates in the newly invaded area (Pingshan) were more diverse than those in the HLB endemic areas (Leibo and Ningnan). Phylogenetic analysis based on mitochondrial genomes demonstrated that ACPs in Leibo, Pingshan, and Xuzhou (rural areas) represent a new mitochondrial group (MG4), distinguished by the three unique single-nucleotide polymorphisms in cox1, nad4, and cytb. However, the ACPs sampled from the urban areas of Cuiping and Xuzhou belonged to the southeastern China group (MG2-1). Altogether, our study revealed multiple sources of ACP and CLas in the HLB interception zone and proposed their transmission route. This study contributes to the formulation of precise HLB prevention and control strategies in the HLB interception zone in Sichuan and could be useful for HLB management efforts in other regions.