Purple City Labs

Purple City Labs

Robust Testing for
Clean Plants

Since 2019, Purple City Labs has focused on developing strategies to identify and eradicate Hop Latent Viroid, the greatest pathogenic threat to cannabis cultivators today. After two years of in-house testing and a successful beta program we’re pleased to offer:

HLVd OnSight: RT-LAMP diagnostic test for Hop Latent Viroid

Purple City Labs

Hop Latent Viroid (HLVd)* is a circular RNA molecule dependent on its host plant’s metabolism for replication.

In 2019, researchers identified it as the primary cause of economically devastating symptoms observed in cannabis plants.

Developed in our Lab,
proven in our Nursery

HLVd OnSight is the test Purple City Labs developed for our own nursery to eliminate HLVd. It was such a success for us that we decided to commercialize it and make it available to others.

This high-throughput method with rapid turnaround time allows every plant in a nursery to be screened, affordably and efficiently, multiple times through its lifecycle. One person can test 100s of plants with same-day results. OnSight is the most cost-effective and validated test available for in-house HLVd testing.


  1. Why switch from PCR testing to HLVd OnSight?

    Having in-house testing allows you to integrate testing in your routine nursery practices — on your own schedule, with immediate results and the ability to perform repeat tests for confirmation. PCR-based tests that entail samples sent to outside labs require longer turnaround times and are more costly, creating a barrier for quick and effective mitigation of Hop Latent Viroid. Additionally, RNA, which makes up HLVd, is a fickle molecule that rapidly degrades unless preserved well during transport. Reducing the time between sampling and testing minimizes this risk.

  2. How accurate is HLVd OnSight?

    HLVd OnSight is as accurate as many PCR-based tests on the market. The reaction is able to detect approximately 375 copies of the viroid per microliter. Even if it is possible to design molecular reactions with higher levels of sensitivity, the OnSight system as a whole is the most effective at population level screening as OnSight. See below for some comparison studies with other labs.

  3. How long will it take to clear my facility of HLVd?

    Depending on the level of infection, six to nine months is often sufficient for most operations. Purple City Genetics brought down HLVd levels to 0-1% (depending on cultivar) after using OnSight intensively for 6 months.

  4. Do I need lab experience to run the HLVd OnSight test?

    No experience is needed! Our step-by-step SOP and instructional videos are easy to follow and our lab team can offer additional support on how to effectively run the tests.

  5. Do I need equipment for OnSight? Why is lab equipment sold separately?

    You will need some basic lab equipment to run OnSight. This is not expensive and costs around $1,000. We have sourced and validated the equipment from different vendors and can direct you to them. We also offer loaner equipment if you would like to give OnSight a try before buying your own.

  6. Why are you selling a proprietary test developed for your nursery?

    Purple City has been a deep partner to experienced cultivators across the marketplace since 2008. We're selling HLVd OnSight after experiencing how effective the test is within our nursery. We recognize HLVd is a collective problem for everyone who cultivates cannabis, and are excited to share our solution with the cannabis community.

  7. What tissue types can I test?

    You can test all tissue types including leaves, stem, petiole and roots, depending on the particular application of OnSight in your facility. Our team will gladly help you identify a sampling plan for effective screening of your plants.

Comparison of Purple City Labs
OnSight for HLVd (LAMP method) with
other testing Labs (PCR method)*

OnSight V1

May ‘21

We trained a San Francisco nursery to run a beta version of OnSight on stem tissue. They also collected leaf samples from the same plant being tested and sent them to a PCR-based testing lab.

Total Plants tested: 80
OnSight Positives: 29
Lab Positives: 10

All 10 positives detected by the lab were also detected by OnSight.

The 19 plants with a discrepancy between the two testing methods were re-analyzed by PCL and 16 were confirmed to be positive, while 3 were actually negative. OnSight v1 was more sensitive than the PCR-based lab but had a slight false negative rate.

OnSight V2

November ‘21

A nursery in Oregon wanted to compare OnSight to two labs using PCR. They sampled 28 plants and sent them to PCL and the two other labs.

Total Plants tested: 28
OnSight Positives: 9
PCR Lab 1 Positives: 9
PCR Lab 2 Positives: 19

The 9 positive plants detected with OnSight were also detected by the two labs. We were not able to run secondary analysis on the conflicting samples.

OnSight V2

April ‘22

A lab in Canada wanted to update their ISO accreditation to include using OnSight for HLVd detection. In order to do this they needed to conduct a blind proficiency test with 15 positive and 15 negative plant samples. 

The lab using OnSight successfully diagnosed all 30 samples accurately.

OnSight V3

October ‘22

After substantial improvements to the sensitivity and specificity of OnSight, the same nursery in Oregon wanted to run another trial comparing OnSight and PCR to each other by sending split samples to Purple City Labs and three labs.  

Total Plants tested: 14
OnSight Positives: 2
PCR Lab 1 Positives: 2
PCR Lab 2 Positives: 2
PCR Lab 3 Positives: 0

The two positive samples were the same for OnSight and the two PCR labs.

  1. Research Made Real

    Purple City Labs harnesses the scientific tools developed for farming in order to contribute to the knowledge around cannabis while empowering the cannabis community at large.

    The lab was created to complement our breeding efforts and assist with challenges faced in the nursery. It provides Purple City Genetics with modern agronomic tools, designs analytically-driven field trials and discovers custom molecular markers that accelerate every part of our breeding program.
  2. * Cannabis infected with HLVd is often asymptomatic during the vegetative stage and can spread unnoticed like wildfire through a nursery via tools and wounds on the plants.

    Flowering plants with HLVd often have smaller, looser buds with drastically reduced trichomes and terpenes. One estimate puts cannabis plants infected with HLVd at half the cannabinoid content of healthy plants and an overall yield reduction of 30%.

    Unfortunately there is no treatment for HLVd and the only effective solution is integrating intensive, high-throughput testing into IPM programs.

  3. RT-LAMP diagnostic test for Hop Latent Viroid
    After Hop Latent Viroid was first identified in 2019, we began screening our entire nursery population using outside testing services. Results returned weeks later, and were often contradictory. We tried various tests and none delivered a solution to effectively rid our shop of this problematic viroid. Relying on outside parties was a frustrating experience, which prompted Purple City Labs to design a RT-LAMP diagnostic test for HLVd that fulfills the specific requirements of a cannabis nursery.

    Cost-effective, rapid and high-throughput testing
    Early on, it became clear that the nature of Hop Latent Viroid required repetitive testing at scale. Plants needed to be sampled comprehensively and continuously at various time points to ensure our facility remained free from the pathogen. This differs from the human diagnostic framework, in which many plant pathology labs are stuck, where costs are higher and sensitivity is paramount for one-off patient testing.
    Purple City Labs’ OnSight test is robust with crude samples of all types, and allows us to test hundreds of plants at our nursery, with same-day turnaround. Freedom to run our own tests provides flexibility, allows us to ask interesting scientific questions and repeat tests to confirm results.

    Validated with Experience
    Purple City Genetics has been using OnSight intensively for the past 2 years, providing continuous feedback to Purple City Labs’ developers regarding the test and its processes, including sample types, sampling frequencies, and a streamlined user experience. Other test developers point only to experimental data of their assays, while OnSight is validated via our daily experience, in addition to crucial experimental data.
    Implementing an intensive, on-site testing regimen into Purple City Genetics’ IPM brought HLVd under control, cleaned our nursery, and allowed us to continually squash offenders before they become full-blown outbreaks.

    Perfected for you
    Purple City Labs developed their assay further than Purple City Genetics’ testing needs, to make it as easy and accurate as possible for others. Room temperature stable formulations and a streamlined process for newcomers to molecular biology were prioritized. During this further refinement, Purple City Labs conducted pilots to observe User experience as well as numerous comparative studies with other labs, including blind proficiency studies. OnSight is the best integrated and cost-effective solution to clean and monitor HLVd.

  4. * HLVd has been shown to exist in one part of a plant and not another or be at higher (i.e. detectable) concentrations in certain tissue types rather than others. When comparing labs to each other it is best to think of detecting HLVd from a sample and not diagnosing a plant. While it is not always possible, it is preferable to split samples to be sent to different labs.