Skip to Content

What causes mold on clones?

Mold growth is a common issue that can affect clones, just like any other plant. Clones are genetically identical copies of a mother plant created through cutting and rooting. While clones provide growers with desirable and consistent genetics, their identical DNA can also make them prone to shared vulnerabilities like mold.

What is clone mold?

Clone mold refers to any fungal growth that develops on cloned cannabis plants. Mold spores are microscopic fungi that float in the air and can land on clone surfaces when conditions are right for growth. The main molds that affect cannabis clones are:

  • Botrytis cinerea (gray mold)
  • Powdery mildew
  • Pythium
  • Fusarium
  • Rhizoctonia

These molds produce threadlike hyphae that penetrate clone leaves, stems, and roots. As mold colonies expand, they cause plant tissue to rot and die. Mold also releases spores that can spread infection to surrounding clones.

What causes mold on clones?

Mold spores are always present in the air, soil, and water. However, they will only germinate and grow when the right environmental conditions exist. The main factors that contribute to clone mold are:

1. Excess moisture

Moisture is the most critical requirement for mold growth. Clones with wet leaves, stems, or media provide an ideal environment. Excess humidity above 60% RH encourages condensation that wets plants. Overwatering and poor drainage also lead to prolonged moisture. Drip irrigation systems and crowded clone domes can raise moisture levels too high.

2. Insufficient air circulation

Standing water and poor gas exchange promote fungal and bacterial growth. Stagnant, still air prevents evaporation from clone leaves and media. Rooms with minimal air movement have higher relative humidity and reduced drying. Inside domes and propagation trays, tight spacing impedes air flow around each clone.

3. High temperatures

Warmer temperatures accelerate mold growth and reproduction. Optimal temperatures for most clone molds range from 65-85°F. Heat buildup in domes and mats can easily reach these levels and higher. High room temperatures and heating mats without thermostats overheat clones.

4. Low light

Low light slows clone transpiration, keeping leaves wetter for longer. Weak lighting reduces clone growth, leaving more unused nutrients in the media to feed mold. Compact, bushy clones under low light lack air movement within the canopy. Lighting under 70 watts per square meter enables mold.

5. High media pH

Media pH affects the availability of nutrients to plants. Mold growth is encouraged by high pH levels above 6.0-6.5 in soilless media. As pH rises, more phosphorus becomes locked out for the clones but available for mold use. High pH also interferes with beneficial root zone microorganisms that protect clones.

6. Unsterilized equipment

Clones are very susceptible to pathogens during propagation. Any unclean cloning tools, domes, trays, or work spaces can transfer mold spores to fresh cuttings. Propagation areas must be disinfected regularly to prevent disease transfer between batches of clones. Bleach or hydrogen peroxide effectively sterilize equipment.

7. Nutrient solution contamination

Dirty or unsterilized nutrient reservoirs allow mold and other microbes to grow rapidly. Contaminated cloning solution spreads pathogens quickly to many clones through irrigation. Change solution frequently and use sterile techniques when mixing nutrients to avoid mold growth.

8. Unhealthy mother plants

Clones from mothers with mold issues often develop the same problems. Take cuttings only from the healthiest, most vigorous mothers. Avoid overcrowded, stressed, or diseased donor plants when selecting material to clone. Always check mother plants closely for any mold before taking cuttings.

9. Slow root development

Until clones root and start actively growing, they are vulnerable to mold. Rooting hormone, proper media, and optimum propagation conditions help clones root faster. Quickly established clones better resist diseases. But slow growing, stressed, or unhealthy clones allow mold to gain a foothold before growth takes off.

10. High media nutrient levels

Media with high levels of nitrogen and phosphorus feed mold growth. These nutrients often accumulate in reuse media as roots break down after each crop. Test old media and discard or amend as needed to lower nutrient levels for cloning. Use lower strength fertilizer formulas to avoid excess nutrients.

How to prevent mold on clones

The best way to prevent mold issues on clones is to control the environmental conditions. Here are some tips to avoid clone mold:

  • Keep room temperature around 75°F and use heating mats only when needed.
  • Maintain steady airflow in the room and inside domes.
  • Keep humidity under 60% RH by providing good ventilation and air circulation.
  • Avoid condensation by monitoring domes and venting regularly.
  • Use an enclosed chamber or fan filter to provide positive pressure filtered air.
  • Give adequate lighting over 70 watts per square meter for 16-24 hours per day.
  • Water clones carefully and allow the media to dry out between waterings.
  • Make sure media pH stays between 5.5 and 6.5.
  • Sterilize equipment and propagation area thoroughly before each use.
  • Change nutrient solution frequently to avoid contamination.
  • Take clones only from the healthiest mother plants.
  • Use rooting hormone and a proper media mix to speed root growth.
  • Test media for old nutrients and discard excessively salty or nutrient-loaded mix.

With good propagation practices, most mold problems can be avoided. But even in near perfect conditions, spores may eventually land on some clones and grow when plants are stressed or micro-environments favor mold.

How to treat mold issues on clones

If mold occurs despite preventative steps, treat it quickly to avoid spreading. Here are tips to fix mold problems in clones:

  • Remove any visibly affected clones from the propagation area immediately.
  • Prune off any Moldy leaves, stems, and roots and discard in sealed bags.
  • Spray remaining plants lightly with hydrogen peroxide or sulfur solution.
  • Increase air circulation and ventilation to dry plants and media.
  • Use a dehumidifier or AC unit to lower humidity.
  • Treat clones preventatively with fungicides like RootShield or Actinovate.
  • Correct any environmental issues like temperature, humidity, or poor drainage.
  • Monitor closely for more mold and remove any new growths ASAP.
  • Disinfect all propagation areas and equipment thoroughly before reuse.

With quick removal of infected clones and treatment of remaining plants, many batches can still be saved. But mold must be controlled promptly before it spreads out of control.


Mold growth is a serious threat to clone health and causes significant losses for propagators. The high-humidity, enclosed environment clones need also promotes fungus and other pathogens. By controlling humidity, providing adequate airflow, sterilizing equipment, monitoring irrigation, and creating a clean propagation area, most mold issues can be avoided. Keep a close eye on all clones, and be ready to quickly treat any mold outbreaks that occur before they spread. With good prevention and rapid response, even the most susceptible clones can thrive free of mold problems.