CRISPR-Cas13 Test Spots Three Quarantine Sugarcane Viruses Without Amplification

Sugarcane is the most cultivated crop in the world by volume, underpinning an industry that produces roughly 179 million metric tons each year and supplying far more than sweeteners, from biofuels to building materials. Yet the crop remains acutely vulnerable to viral pathogens that can quietly strip away yield, with some fields reporting losses of up to 80 percent even in elite, supposedly resistant cultivars. Three viruses classified as quarantine-significant—Sugarcane mosaic virus (ScMV), Sugarcane streak mosaic virus (ScSMV), and Sugarcane yellow leaf virus (ScYLV)—are among the worst offenders, spreading rapidly through aphid vectors and the trade of infected planting material, often without showing symptoms until damage is already done. A research team led by Joseph Lagner and Yiping Qi at the University of Maryland, working with the USDA Animal and Plant Health Inspection Service, now reports a CRISPR-based diagnostic that can detect and distinguish all three viruses directly from plant RNA in a single assay, without any pre-amplification step.

The new assay, described in Plant Cell Reports, relies on LwaCas13a, a type VI CRISPR effector protein originally harnessed for human pathogen detection in the SHERLOCK platform. Unlike Cas9 or Cas12, Cas13 targets RNA rather than DNA and does not require a protospacer adjacent motif, which simplifies guide design considerably. Once the protein’s CRISPR RNA guide finds a complementary viral sequence, the enzyme undergoes a conformational change that unleashes indiscriminate collateral cleavage of nearby single-stranded RNA. The researchers exploit this behavior with a fluorescent reporter oligonucleotide carrying a FAM fluorophore on one end and a quencher on the other; cleavage separates the two molecules, producing a measurable rise in fluorescence that signals a positive detection.

To build the test, the team obtained sequence data for all three viruses from the USDA-APHIS Plant Germplasm Quarantine Program in Maryland and aligned the genomes using the MUSCLE algorithm in Geneious Prime software. They designed five candidate 28-nucleotide crRNA target sites for each virus and screened every candidate against the NCBI BLAST database to rule out off-target matches to related viruses, host plant genetic material, or other organisms associated with sugarcane production. The reverse complements of the selected sites were appended to the LwaCas13a direct repeat sequence to generate functional guide RNAs, which were then synthesized commercially.

Because working with quarantine pathogens requires specialized containment, the team first validated the assay using synthetic pseudo-virus targets. They embedded the five 28-nucleotide target sequences for each virus into 700-base-pair synthetic DNA constructs separated by scrambled sequences, placed under a T7 promoter, and produced single-stranded RNA by in vitro transcription. Serial log dilutions of these RNA targets established the limit of detection, which varied among target sites but reached approximately 1.5 picomolar at its best. The top-performing guides were site 5 on ScMV, site 3 on ScSMV, and site 1 on ScYLV. The authors note that this sensitivity represents roughly a tenfold improvement over a recent report of amplification-free Cas13 detection of plant viruses, although they caution that the intrinsic enzyme kinetics of Cas13 still limit how low the detection floor can go without pre-amplification.

A key feature of the platform is its multiplexing capacity. In a cross-reactivity matrix, the researchers combined every pseudo-virus target with every crRNA and found that fluorescence rose only when a viral target matched its corresponding guide, with matched wells producing relative fluorescence units between roughly 39,000 and 60,000 compared with 4,500 to 8,100 for mismatched combinations. Strikingly, after just 15 to 20 minutes the fluorescent signal was strong enough to be visible with a standard gel documentation reader, an instrument not designed for such measurements. This strict specificity means a single microplate can screen a sample for any of the three viruses in one multiplexed well while simultaneously running simplex wells for each virus individually, yielding both detection and genotyping in one run.

The decisive test came with real plant material. Working inside the USDA-APHIS quarantine facility, the team extracted bulk RNA from infected sugarcane and sorghum plants—sorghum serving as an alternative host for ScMV—using the same commercial kit employed by the agency’s own diagnosticians. They first benchmarked the samples with five conventional RT-PCR protocols, including genus-level potyvirus assays and two separate primer sets for ScYLV that exist because of that virus’s considerable genetic diversity. The CRISPR assay then processed the same extracts directly, with no dilution and no cDNA synthesis step, reading fluorescence over a two-hour run at 37 degrees Celsius.

The two methods broadly agreed, but the discrepancies proved illuminating. A sorghum sample called Rio tested positive in the generic Potyvirus RT-PCR protocols, but only the CRISPR assay—and a follow-up sequencing step—could pin the infection specifically on ScMV, since the conventional protocols detect the genus rather than the species. Another sorghum sample, Z1536, produced signals above threshold for all three viruses in the CRISPR assay while remaining negative or ambiguous by RT-PCR, and sugarcane sample Z1502, which showed no RT-PCR bands at all, yielded a clear CRISPR positive for ScYLV. The authors attribute such gaps to several factors: low or seasonally fluctuating viral titers, mutations at primer binding sites that can silence PCR amplification, the added variability of the two-step cDNA-plus-amplification workflow, and possible weak cross-reactivity of the Poty2 primer set with ScSMV observed in sample Z1501.

Beyond speed, the platform offers practical advantages for diagnostic laboratories. The conventional workflow requires five separate RT-PCR protocols, each with cDNA preparation and post-amplification gel electrophoresis, and a skilled technician might need a day or two to fully characterize even one sample. The CRISPR assay completes detection and virus identification in four hours or less for high-titer infections, on a single plate, with fewer handling steps and correspondingly fewer opportunities for contamination. The fluorescence-based readout also provides a semi-quantitative measure of viral load, whereas conventional RT-PCR as performed here is essentially a presence-or-absence test. To guard against false positives from natural degradation of the RNA reporter during the long incubation, the team set a conservative threshold of three standard deviations above the mean of background-subtracted negative control values.

The researchers are candid about the method’s limits. At roughly 1.5 picomolar, the sensitivity may not suffice for very early or light infections, where RT-PCR or an added pre-amplification step would still be needed, and the study was constrained by a limited number of infected samples, leaving more rigorous statistical benchmarking on larger field collections as the obvious next step. Still, because the assay needs only a single conserved target region per virus—rather than the paired primer sites PCR demands—and Cas13 requires no PAM, redesigning guides as viruses evolve is straightforward. The team envisions extending the approach with kinetic barcoding for richer multiplexing, engineered guides for greater specificity, and enhanced collateral cleavage activity, ultimately expanding the panel to many more pathogens and molecular markers.

For an industry in which viruses can account for more than half of yield reduction and global commerce in planting material keeps introducing quarantine pathogens to new regions, a rapid, amplification-free test that names its target within hours could materially change surveillance practice. The authors suggest that, equipped with cold storage and a suitable plate reader, the workflow could even be adapted into field-deployable diagnostics, offering a way to catch outbreaks early enough to contain them before the damage reaches the mill—or the next season’s seed cane.

Subject of Research: Amplification-free CRISPR-Cas13 detection of quarantine-significant sugarcane RNA viruses

Article Title: CRISPR-Cas13-based amplification-free detection of three quarantine-significant sugarcane viruses

Article References: Lagner, J., Paulson, A., Schulden, T., Adhikari, B., Koob, J., Vakulskas, C., & Qi, Y. (2026). CRISPR-Cas13-based amplification-free detection of three quarantine-significant sugarcane viruses. Plant Cell Reports, 45(10), Article 307. https://doi.org/10.1007/s00299-026-03994-4

Image Credits: AI Generated

DOI: 10.1007/s00299-026-03994-4

Keywords: CRISPR-Cas13, LwaCas13a, sugarcane viruses, Sugarcane mosaic virus, Sugarcane streak mosaic virus, Sugarcane yellow leaf virus, plant pathology, molecular diagnostics, amplification-free detection, multiplexed assay, RNA viruses, agricultural biotechnology