What bacteria cause crown rot in cannabis, and how do they differ from fungal crown pathogens?
Bacterial crown rot in cannabis is associated with Rhizobium radiobacter (formerly Agrobacterium tumefaciens), Erwinia species, and related gram-negative bacteria. These are soilborne and waterborne organisms that enter plant tissue through wounds, the cuts made during propagation are the primary entry points.
The distinction from fungal crown pathogens (Rhizoctonia, Fusarium) matters because the diagnostic signs and the sanitation chemistry requirements differ:
Bacterial crown rot produces soft, wet, water-soaked lesions. The infected tissue smells foul, a byproduct of bacterial metabolism, and may feel slimy. Internal tissue is discolored brown to tan. The lesion progresses from the wound upward.
Fusarium crown rot produces dry, reddish-brown vascular discoloration that extends into the stem. The lesion is not slimy and does not have a foul odor.
Rhizoctonia produces reddish-brown, dry lesions at the stem-media interface, often with visible mycelium on the surface.
Distinguishing these in the field, without lab confirmation, relies primarily on lesion character: wet and malodorous for bacterial, dry and firm for fungal. Lab confirmation via plating onto selective media or PCR is the definitive diagnostic when the presentation is unclear.
How does bacterial crown rot spread through a propagation bench?
Cutting tools are the highest-risk transmission pathway. A single cutting scissors or scalpel that contacts a bacterially infected cutting base carries viable bacterial cells to every subsequent cutting made without re-sterilizing. In a propagation session where hundreds of cuttings are taken, a single contaminated tool can inoculate the entire batch.
Wet bench surfaces are the secondary pathway. Bacterial cells shed from infected cutting bases accumulate in the moisture on propagation bench surfaces, rooting cube trays, and dome interiors. The next cutting placed on that surface contacts the contaminated moisture at its base wound.
Water misting systems in propagation rooms can distribute bacterial contamination if the reservoir or lines are contaminated, the same water pathways that transmit Pythium apply to bacterial pathogens. Propagation systems using shared recirculating mist reservoirs face higher transmission risk than single-pass misting.
The pace of spread is faster than with fungal pathogens because bacterial reproduction rates are higher, a surface or tool contamination event doesn't have to wait for spore germination and hyphal extension.
What conditions make propagation rooms vulnerable to bacterial crown rot?
Three environmental conditions predominate in propagation rooms and increase bacterial crown rot risk:
Warm, high-humidity conditions. Propagation rooms maintain 75–80°F and 80–90% RH to support cutting establishment. These are also optimal conditions for rapid bacterial growth on warm, wet surfaces.
Dense bench packing. Cuttings placed close together on propagation benches create contact between healthy and potentially infected material. Condensation from domes drips between cuttings.
Continuous production with short batch intervals. Commercial propagation rooms cycle cuttings continuously, often with less than 24 hours between the removal of one batch and the placement of the next. This turnaround window is rarely long enough for a thorough decontamination unless a specific protocol exists for it.
Mother plant age also matters. Older mother plants that have been pruned repeatedly accumulate wound tissue at pruning sites, potential entry points that have been exposed to the cutting environment repeatedly. Monitoring mother plants for crown tissue health is an often-overlooked early indicator.
How do you distinguish bacterial crown rot from normal propagation attrition?
The diagnostic challenge is that cuttings fail to root or collapse at the base for several reasons: inadequate humidity, incorrect cutting depth, temperature stress, overwatering, and root pathogen infection all produce wilting and failure at roughly the same developmental stage as bacterial crown rot.
The indicators that point specifically to bacterial disease:
- Soft, wet lesion at the base. Normal propagation attrition produces dry, collapsed tissue; bacterial infection produces water-soaked, soft lesions that may feel slimy.
- Pattern on the bench. If failures cluster in adjacent positions on the bench rather than distributing randomly, a localized contamination source, a tool used in that section, a drip point, a contact surface, is likely.
- Foul odor. Bacterial decomposition of plant tissue produces a distinctly unpleasant smell that healthy tissue and even fungal infections don't produce at the same stage.
- Rate of progression. Bacterial crown rot can collapse a cutting within 3–5 days of placement; normal attrition from environmental stress typically presents over a longer timeline.
When the presentation matches these indicators, prompt removal of affected material and immediate tool sanitization limits the scope.
What does a bacterial crown rot prevention program require?
Prevention is built on three specific practices:
Cutting tool sanitization between plants. A 70% isopropyl alcohol wipe or a brief ClO₂ solution dip between each cutting is the operational control that prevents tool-to-plant transmission. The protocol has to be per-plant, not per-batch, a contaminated plant in the middle of a batch seeds every subsequent plant made with the same unsterilized tool.
Bench and tray decontamination between batches. Rooting trays, dome interiors, and bench surfaces require decontamination with an oxidizing chemistry before each new batch. The warm, wet conditions that optimize root development also optimize bacterial survival on surfaces.
Mother plant monitoring. Regular inspection of mother plant crown tissue for early lesion development, and isolation or removal of plants showing crown symptoms, reduces the inoculum available to cuttings.