What causes Rhizoctonia outbreaks in cannabis propagation?
Rhizoctonia solani is a contact pathogen, its mycelium grows across surfaces and enters plant tissue through physical contact, wounds, and the natural openings at the base of cuttings. Unlike airborne pathogens, it doesn't need a vector to travel; it grows directly from infected substrate to healthy plant material.
In propagation environments, the transmission pathway is straightforward: contaminated rooting media or growing trays, wet bench surfaces colonized with mycelium, and reused containers that weren't fully decontaminated between clone batches. Rooting cubes and rockwool blocks placed on a contaminated surface provide direct contact between the pathogen and the cutting's base wound.
The pathogen is encouraged by warm, moist conditions (72–82°F), which is precisely the propagation room environment. High humidity, limited airflow, and closely packed cuttings on benches create exactly the conditions Rhizoctonia prefers.
What do Rhizoctonia symptoms look like?
Rhizoctonia produces several distinct symptom patterns depending on when infection occurs:
- Damping-off: Seedlings or cuttings collapse at the soil line, with a brown, water-soaked lesion that girdles the stem. The plant falls over as if the stem has been cut, the most visible and immediate symptom.
- Crown lesions: In established plants, Rhizoctonia causes reddish-brown to dark brown lesions at the crown, the junction between the stem and the root system. The lesion may girdle the crown, restricting water and nutrient flow and causing progressive wilting.
- Stem girdling: Where the mycelium has grown up the lower stem, a distinct browning and collapse of stem tissue appears at or just above the growing media surface.
Distinguishing Rhizoctonia from Fusarium crown rot and Pythium root rot requires attention to the lesion character: Rhizoctonia lesions tend to be dry and reddish-brown with a clear margin, while Pythium-infected tissue is typically water-soaked and slimy, and Fusarium shows internal vascular discoloration that extends up the stem. Lab confirmation via plating or PCR is the most reliable diagnostic when multiple pathogens may be present.
Why are clone rooms and propagation benches the highest-risk sites?
Several structural features of propagation environments amplify Rhizoctonia risk:
- Dense plant spacing: Cuttings are placed closely together on propagation benches. Mycelial growth can move from one cutting to the next through direct surface contact across wet bench material.
- Wet bench surfaces: Propagation domes and misting systems maintain high humidity and produce regular condensation on bench surfaces. Wet bench surfaces are colonized by Rhizoctonia mycelium before visible plant symptoms appear.
- Reused containers and trays: Rooting trays, net pots, and propagation domes that are not fully decontaminated between batches carry sclerotia and mycelium from infected batches to clean ones.
- Wounds at the cutting base: Every clone cutting has a fresh wound at its base, the sterilization cut that facilitates rooting. This wound is the primary entry point for Rhizoctonia.
- Continuous propagation cycles: An infected batch that isn't identified before the bench is reloaded seeds the next batch immediately.
How does Rhizoctonia persist between crop cycles?
Rhizoctonia forms sclerotia, compact aggregations of mycelium with a protective outer layer that resist desiccation, temperature extremes, and standard disinfectants. These structures can persist on bench surfaces, in growing media particles, in container walls, and in facility substrate for months to years without a living host.
This persistence is why recurring Rhizoctonia in a propagation room is not random, it reflects an inoculum reservoir that was never eliminated. Facilities that decontaminate bench surfaces but don't address growing media disposal, container reuse, or bench frames create conditions where sclerotia survive the sanitation step and reinfect the next batch.
The most reliable indicator that sclerotia are persisting in the environment is pattern: if Rhizoctonia damping-off recurs in the same area of the bench with each new batch, the inoculum source is local and structural, not being re-introduced from outside the facility.
What does an effective Rhizoctonia prevention program require?
Prevention requires eliminating the pathogen's persistence sites between batches.
Growing media: Never reuse growing media that has been in contact with infected plants. Rockwool, coco, and perlite carry sclerotia that don't disappear with drying or storage.
Container and tray decontamination: Rooting trays, net pots, and propagation domes require decontamination with an oxidizing chemistry between every batch, not just between full crop cycles. Rhizoctonia cycles with your clone batches if the interval is longer.
Bench surfaces: All surfaces that cutting bases contact require full decontamination between batches. Wet bench surfaces are a direct transmission route.
Cutting tool sanitization: Each cut should be made with a sterile blade. The base wound is an entry point, and tools carry mycelium between plants in an actively infected batch.