Application of Next Generation Sequencing for Diagnostic Testing of Tree Fruit Viruses and Viroids

Conventional detection of viruses and virus-like diseases of plants is accomplished using a combination of molecular, serological, and biological indexing. These are the primary tools used by plant virologists to monitor and ensure trees are free of known viral pathogens. The biological indexing assay, or bioassay, is considered to be the “gold standard” as it is the only method of the three that can detect new, uncharacterized, or poorly characterized viral disease agents. Unfortunately, this method is also the most labor intensive and can take up to three years to complete. Next generation sequencing (NGS) is a technology with rapidly expanding possibilities including potential applications for the detection of plant viruses. In this study, comparisons are made between tree fruit testing by conventional and NGS methods, to demonstrate the efficacy of NGS. A comparison of 178 infected trees, many infected with several viral pathogens, demonstrated that conventional and NGS were equally capable of detecting known viruses and viroids. Comparable results were obtained for 170 of 178 of the specimens. Of the remaining eight specimens, some discrepancies were observed between viruses detected by the two methods, representing less than 5% of the specimens. NGS was further demonstrated to be equal or superior for the detection of new or poorly characterized viruses when compared with a conventional bioassay. These results validated both the effectiveness of conventional virus testing methods and the use of NGS as an additional or alternative method for plant virus detection.

Molecular Detection and Prevalence of Citrus Viroids in Texas

Viroids are graft- or mechanically transmissible agents, disseminated through budding. Biological indexing of commercially important citrus cultivars grown in the Lower Rio Grande Valley of Texas showed that many are infected with citrus viroids. Most of these trees carried more than one viroid. In most cases, the infected trees are asymptomatic carriers because sour orange, the predominant rootstock used in Texas, does not show symptoms of viroid infection. Detection of viroids through biological indexing on sensitive indicator plants followed by sequential polyacrylamide gel electrophoresis (sPAGE) is the gold standard but is time-consuming and requires plants to be kept at optimum conditions. A conditional use of reverse transcriptase–polymerase chain reaction (RT-PCR) provides an efficient and alternative detection of viroids for use in the Texas virus-free citrus budwood certification program. RT-PCR could be useful in Texas to help expedite the evaluation for the presence of viroids before conducting the final biologic indexing. Using RT-PCR, we could detect, clone, and sequence full-length viroids of Citrus exocortis viroid (CEVd), Hop stunt viroid (HSVd) (both cachexia and noncachexia variants), Citrus viroid-III (Citrus dwarfing viroid), and Citrus viroid-IV (Citrus bark cracking viroid) from a collection of viroid-inoculated grapefruit plants. The source plants were previously shown to be viroid-infected by biological indexing on Etrog citron plants. Based on our results, RT-PCR can be a conditional substitute for biological indexing of mother trees in foundation blocks and shoot tip-grafted trees in the virus-free budwood program. A positive RT-PCR result has a serendipitous value because those trees can be discarded from the pool before expensive biological indexing.