Cannabis and hemp are attacked by a wide range of fungi, oomycetes, bacteria, viruses, nematodes, and viroids. Collectively, more than 100 pathogens/diseases have been documented in the scientific literature, with fungi/oomycetes the most common, and viroids/viruses emerging as major threats in intensive production. FrontiersMDPI

How do pathogens cut yield?

Bud/inflorescence diseases (e.g., gray mold/bud rot from Botrytis cinerea) destroy marketable flowers and degrade quality. PMCMDPI

Root and crown rots (Pythium spp., Fusarium spp.) stunt plants, cause wilting, and can kill seedlings or mature plants, reducing stand and final biomass. PMCFrontiers

Powdery mildew (e.g., Golovinomyces) reduces photosynthesis and vigor, indirectly lowering yields and complicating harvest quality. PMC

Viroids/viruses (especially Hop latent viroid, HLVd) can silently spread through propagation, leading to stunting, brittle stems, poor flower set, and reduced cannabinoid production; outbreaks can be facility-wide. OSU Extension ServicePMC+1

Mycotoxin-producing molds (Aspergillus, Penicillium, Fusarium) threaten product safety testing and saleability, turning a harvest into a loss even when biomass looks acceptable. Frontiers

Real World Numbers

1. U.S.–Wide Cannabis Disease Survey (APS, 39 States)

A comprehensive survey received 148 valid responses from growers and extension specialists across 39 states, revealing regional disease patterns:

Fungal diseases like botrytis bud rot, powdery mildew, Cercospora leaf spot, Fusarium damping-off, and Fusarium canker were most frequently reported. These diseases were especially prevalent in the Northeast and South—areas with higher rainfall. There, these pathogens had significantly higher incidence than others like sooty mold or Stemphylium leaf spot (P < 0.0001).

In contrast, the Great Plains saw fewer than 50% of respondents reporting any fungal disease, and the West reported only about 15 of 30 fungal diseases assessed in the survey.

Viral diseases, including Hop latent viroid (HLVd) and Beet curly top virus, were more commonly reported in the Great Plains and West. HLVd appeared across all regions, but was most prominent in the regions historically associated with hops production.

Oomycete pathogens (e.g., Pythium) were consistently reported across regions, while nematode reports were rare overall.
APS Journals

2. California HLVd Prevalence (Dark Heart Nursery, 2021)

A large-scale field survey in California revealed that over 90% of cannabis facilities tested positive for Hop latent viroid (HLVd). Among those facilities, roughly 30% of sampled plants showed disease symptoms.

Infected plants suffered yield penalties: ~30% biomass loss and up to 50% reduction in THC.
Technology NetworksGanjapreneur

3. HLVd in Canada’s Licensed Facilities (2020–2023)

In licensed Canadian cannabis operations, 15,947 samples were tested with RT-PCR. HLVd was detected in 5.3% to 92% of samples depending on province and year — averaging 25.6% national incidence.

Infected plants displayed symptoms ranging from stunting and leaf mottling to reduced stem length (12–42%) and significantly lower THC/terpene content.
Green Jungle

4. First California Production Practices Survey (UC/CA Agriculture Journal)

A pre-legalization California grower survey (n = ~60) provided early insight into pathogen perceptions:

43% of growers reported unspecified disease issues, followed by 30% noting molds (including bud rot, gray mold), and 12% referencing rots (root/stem/fusarium).

While self-reported, this reveals early concern around fungal threats even before large-scale commercial operations.
ResearchGate

These findings underscore the critical importance of early, accurate pathogen detection. HLVd alone is pervasive and devastating, and fungal and oomycete threats vary regionally but remain consistently problematic. Diagnostic speed and preventive screening, especially for HLVd, become essential tools for preserving yield, consistency, and product quality.

The pathogen landscape (what shows up most often)

Fungi & Oomycetes

Botrytis cinerea (gray mold, bud rot): Most damaging during flowering in humid, dense canopies; can rapidly collapse colas near harvest. MDPI

Fusarium spp.: Wilt, crown/root rot, cankers; widespread in media and wounds; multiple pathogenic forms reported on cannabis. Frontiers

Pythium spp.: Damping-off and root rot in hydroponics and soilless media; favors high moisture and warm solution temps. Frontiers

Powdery mildew (Golovinomyces): Common foliar disease indoors and out; quality and yield impacts from reduced plant vigor. PMC

Viroids & Viruses

Hop latent viroid (HLVd): The most prominent viroid threat in modern cannabis; linked to yield and quality loss, with documented rapid spread via tools, workers, and clones. (Extension guidance and peer-reviewed reviews available.) OSU Extension ServicePMCMDPI

Other viruses (e.g., beet curly top virus, lettuce chlorosis virus) are reported in surveys and reviews; prevalence varies by region and production system. PMC

Bacteria & Nematodes

Bacterial leaf spots/wilts (Xanthomonas, Pseudomonas) appear sporadically but can cause significant local losses. (Survey/review coverage.) APS Journals

Root-knot nematodes (Meloidogyne spp.) impair nutrient uptake, causing chronic stunting and low yields in infested beds/fields. (State diagnostic manuals and surveys.) Nevada Department of Agriculture

Symptom photo guides (trusted, non-commercial)

OSU Extension (HLVd in hemp) – leaf/stem symptom progression photos and scouting notes. OSU Extension Service

UF/IFAS “Field Guide to Hemp Diseases” (Florida) – image-rich disease ID with concise management notes (useful beyond FL). Ask IFAS - Powered by EDIS

Frontiers reviews – figures showing common molds in grows/medium and representative symptoms. Frontiers

Practical prevention & management (evidence-based)

1) Start clean (propagation hygiene)

Source mother stock from tested material; quarantine and test incoming clones (PCR/RT-PCR for HLVd/viruses; plating/qPCR for common fungi). MDPIPMC

Sterilize cutting tools between plants/benches; viroids spread mechanically. OSU Extension Service

2) Environment & cultural controls

Keep VPD/airflow dialed in during late flower; dense, wet canopies invite Botrytis and powdery mildew. MDPI

Disinfect and manage water systems (reservoir temps, biofilm control) to deter Pythium in hydroponics/soilless media. Frontiers

Rotate/refresh or heat-treat media known to carry molds; unpasteurized coco can harbor problematic fungi. Frontiers

3) Scouting & thresholds

Use a standardized scouting sheet keyed to the APS survey list; sample early when you see off-types (stunting, chlorosis, brittle stems, cola collapse). APS Journals

4) Diagnostics

Confirm suspect issues before treating:

Fungi/oomycetes – plating/qPCR;

Viroids/viruses – RT-qPCR;

Bacteria – culture and gene assays.
Integrated reviews outline appropriate tests across the crop cycle. MDPI

5) Sanitation & traffic flow

Worker/tool/bench sanitation and one-way plant movement lower spread of both molds and HLVd. Extension protocols provide practical steps fit for busy facilities. OSU Extension Service

6) Harvest & post-harvest

Gentle handling and tight humidity control reduce flower wounding and post-harvest mold risk; mycotoxin reviews explain testing and regulatory screens. Frontiers

Want to go deeper?

Fungal pathogens in medical cannabis (Israel, multi-year) – Koch’s postulates confirmed for key bud/root pathogens; practical seasonality insights. PMC

Integrated disease management in greenhouse cannabis (review) – stage-by-stage IDM tactics, diagnostics, and decision points. MDPI

Cannabis virology (review) – current virome, HLVd spread/impact, and survey findings on facility prevalence. PMCInvasive Species Information

Sources (academically credible)

APS Plant Health Progress — Occurrence and Distribution of Common Diseases and Pests of U.S. Cannabis: A Survey (2023). APS Journals+1ResearchGate

Frontiers in Plant Science — Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. (2019). Frontiers

Frontiers in Agronomy — Diseases of Cannabis sativa Caused by Diverse Fusarium Species (2021). Frontiers

MDPI – Plants — Integrated Management of Pathogens and Microbes in Cannabis Cultivation (2024). MDPI

MDPI – Viruses — A Guide to Cannabis Virology (2023). PMCInvasive Species Information

Oregon State University Extension — Hop latent viroid in hemp (EM 9570, 2024). OSU Extension Service

Frontiers in Microbiology — Fungal and Mycotoxin Contaminants in Cannabis and Hemp Flowers (2023). Frontiers

NIH/PMC (peer-reviewed case study) — Fungal Pathogens Affecting Production and Quality of Medical Cannabis (Punja et al., 2020). PMC