HLVd May Open the Door to Fusarium and Powdery Mildew: What a New 2026 Study Found
Most growers think of Hop Latent Viroid (HLVd) as a slow drain: shorter plants, weaker roots, smaller flowers and lower yield. A new study suggests the cost may not stop there. In some cultivars, HLVd-infected plants were also easier targets for two of the most common fungal problems in cannabis: Fusarium and powdery mildew.
The study
In April 2026, Dr. Zamir Punja published a study in the Canadian Journal of Plant Pathology asking a simple question: if a cannabis plant is carrying HLVd, does it handle other diseases differently?
To find out, he took cuttings from stock (mother) plants of the same genotypes that tested either positive or negative for HLVd by RT-PCR. He then challenged them with:
Fusarium oxysporum, which causes damping-off and root and stem rot. Vegetative cuttings from six genotypes were tested, plus stems from flowering plants of 'Sugar Cookies' and 'Pink Kush' taken two and five weeks into flower.
Powdery mildew (Golovinomyces ambrosiae). Rooted cuttings of 'Pink Kush', 'Sugar Cookies' and 'Pennywise' were tested, along with flowering plants of 'Sugar Cookies' and 'Pink Kush'.
Disease was scored after four weeks, and the experiments were repeated.
What they found
Fusarium hit harder in half the genotypes. In three of the six genotypes ('Pink Kush', 'Sugar Cookies' and 'Berry Cream Puff'), HLVd-positive cuttings developed more Fusarium disease than HLVd-negative cuttings. By three weeks, the HLVd-positive 'Pink Kush' cuttings had collapsed, with fungus growing up the stems, while the HLVd-negative cuttings showed only some yellowing. The other three genotypes ('Blue Dream', 'Pennywise' and 'Mac1') got sick to the same degree whether or not HLVd was present.
Flowering stems were affected too. Stems from HLVd-positive 'Sugar Cookies' plants in flower were heavily colonized by Fusarium and collapsed, and the fungus also spread into developing flowers. HLVd-negative stems had much less fungal growth and stayed upright.
Powdery mildew was worse on every genotype tested. All three genotypes showed more powdery mildew on HLVd-positive plants than on HLVd-negative ones. On flowering 'Sugar Cookies', the difference was striking.
HLVd sensitivity did not predict the fungal response. 'Mac1', which earlier work found to be highly susceptible to HLVd, showed no added Fusarium effect. 'Pink Kush', considered more tolerant of HLVd, did. In other words, you can't assume a cultivar is safe just because HLVd doesn't seem to bother it much.
Why would a viroid make fungi worse?
The study didn't test the mechanism directly, but the author points to a likely explanation. A 2025 study found that HLVd-infected cannabis tissue had reduced expression of a defense gene called PR-1, which is part of the plant's salicylic acid defense pathway. If HLVd quietly turns down part of the plant's immune response, fungi may find it easier to get established. Weaker root development in infected cuttings could also play a role. Punja is clear that this needs more research.
What this means for growers
1. The cost of HLVd may be bigger than yield loss. If you're fighting more damping-off in the clone room or a tougher powdery mildew season, HLVd could be part of the picture for some cultivars. In the study's words, the findings "have implications for cannabis producers confronted with HLVd infection."
2. Don't diagnose by eye. In this study, some yellowing, stunted mother plants were HLVd-negative but Fusarium-positive, and HLVd-positive 'Pink Kush' mothers looked perfectly healthy. Plants carrying both showed the worst vascular browning in their stems. Symptoms alone can't tell you what you're dealing with. Test.
3. Test your mothers. Every cutting from an infected mother carries HLVd forward. RT-PCR testing of stock plants is the only way to know what you're propagating.
4. Start clean. Meristem tissue culture followed by RT-PCR confirmation is how infected genetics are cleaned up and kept clean. At Zennetix, every Gen Zero clone starts from tested, HLVd-free tissue culture stock.
A few caveats
This is one study, with small numbers of cuttings per genotype, run under lab and greenhouse conditions. The effect depended strongly on genotype. Not every HLVd-positive plant will get powdery mildew or Fusarium, and the proposed mechanism still needs testing. But it adds to a growing body of evidence that HLVd isn't just a cosmetic or yield problem. It may make the rest of your disease pressure harder to manage.
Source
Punja, Z. K. (2026). Hop latent viroid-infected cannabis (Cannabis sativa L.) plants show enhanced susceptibility to Fusarium root and stem rot and powdery mildew infection. Canadian Journal of Plant Pathology, 48(5), 469–483. https://doi.org/10.1080/07060661.2026.2649769
Want to know whether your mothers are carrying HLVd, or need a clean start? Email us at sales@zennetix.com.
Comments