Cannabis Pathogens in New Zealand: Identification, Prevention & Treatment

Pathogens in New Zealand and Cannabis

Main Pathogens Affecting Indoor Cannabis

Overview Table

Detailed Information

1. Fusarium

• Description: Soil-borne fungal pathogen causing vascular wilt and root rot.
• Prevalence: Moderate in indoor grows, more common in reused soil or hydroponic systems.
• Symptoms:
• • Wilting despite adequate water
  • Yellowing of lower leaves
  • Brown discoloration in stem vascular tissue
  • Stunted growth
• Favorable Conditions:
• • High soil moisture
  • Temperatures between 20-30°C (68-86°F)
  • Poor drainage
• Impact on Yield: Can reduce yield by 20-50% if left untreated.
• Common Scenario: Introducing contaminated soil or using unsterilized equipment.
• Prevention:
• • Use sterile growing media
  • Maintain proper pH (5.8-6.2)
  • Avoid overwatering
  • Sterilize all equipment
• Treatment:
• • Remove and destroy infected plants
  • Apply beneficial microbes (Trichoderma spp., Bacillus subtilis)
  • Improve drainage and reduce watering

2. Pythium

• Description: Water mold causing root rot and damping-off in seedlings.
• Prevalence: High, especially in hydroponic systems and overwatered soil.
• Symptoms:
• • Brown, mushy roots
  • Stunted growth
  • Wilting
  • Damping-off in seedlings
• Favorable Conditions:
• • Overwatering
  • Poor drainage
  • High humidity
  • Temperatures above 20°C (68°F)
• Impact on Yield: Can cause total crop failure in severe cases, typically 30-60% yield loss.
• Common Scenario: Overwatering combined with poor drainage and high humidity.
• Prevention:
• • Maintain proper water-to-air ratio in root zone
  • Use beneficial microbes as a preventative measure
  • Ensure good drainage and air circulation
• Treatment:
• • Improve drainage immediately
  • Apply hydrogen peroxide solution (3%) to roots
  • Introduce beneficial microbes (Bacillus spp., Trichoderma spp.)

3. Botrytis (Gray Mold)

• Description: Fungal pathogen causing bud rot and gray mold on leaves and stems.
• Prevalence: High, especially during flowering stage in humid conditions.
• Symptoms:
• • Gray, fuzzy mold on buds and leaves
  • Brown, rotting buds
  • Wet, slimy appearance on affected areas
• Favorable Conditions:
• • High humidity (above 60%)
  • Poor air circulation
  • Temperatures between 15-25°C (59-77°F)
• Impact on Yield: Can destroy 20-80% of a crop, often affecting the most valuable parts (buds).
• Common Scenario: Dense canopy combined with high humidity during late flowering stage.
• Prevention:
• • Maintain humidity below 50% during flowering
  • Ensure good air circulation
  • Prune to improve airflow
  • Avoid water on buds and leaves
• Treatment:
• • Remove all infected plant material immediately
  • Reduce humidity and increase air circulation
  • Apply organic treatments (e.g., potassium bicarbonate, Bacillus subtilis)

Key Differences

Best Practices for Pathogen-Free Growing

• Maintain proper environmental controls (temperature, humidity, airflow)
• Use sterile growing media and clean equipment
• Implement a robust Integrated Pest Management (IPM) program
• Regularly inspect plants, especially roots and dense canopy areas
• Optimize plant spacing and prune for good air circulation
• Keep grow room clean and free of plant debris
• Quarantine new plants before introducing them to the grow space
• Use beneficial microbes as a preventative measure
• Maintain detailed records of environmental conditions and plant health

Additional Pathogens in New Zealand

• Rhizoctonia: Causes damping-off in seedlings and root rot in mature plants. Affects potatoes, cereals, and vegetables.
• Phytophthora: Causes root rot and collar rot in avocados, kiwifruit, and native kauri trees.
• Verticillium: Causes vascular wilt in potatoes, tomatoes, and other crops.

Risk of Exposure & Prevention

• Risk: Moderate to high if sanitation measures are not followed.
• Example Exposure Sequence:
• 1. Contaminated soil or tools introduced.
  2. Pathogen spreads in the growing medium.
  3. Roots become infected.
  4. Plants show wilting, yellowing, and stunted growth.
  5. Infection spreads through water splashing or contaminated equipment.
• Treatment:
• • Isolate affected plants
  • Remove and destroy severely infected plants
  • Reduce watering and improve drainage
  • Apply appropriate fungicides
  • Improve air circulation
  • Sterilize tools and surfaces
  • Introduce beneficial microbes

References

• Diversity of Mycoparasitic Pythium Species
• Phytophthora Detection in NZ
• Horticultural Pathogen Guide
• Frontiers in Plant Science
• List of Hemp Diseases
• Cannabis Agronomy Research