Gray mold, caused by the fungus Botrytis cinerea, is the number one nightmare for growers in humid, rainy regions or areas with high relative humidity (RH). In Brazil, especially in the South, Southeast, and Northeast during the rainy season, it can destroy 30-70% of the harvest in just a few days if not controlled.

This article is a complete and practical guide with over 3,000 words to help you understand the enemy, apply the best preventive techniques, and, if necessary, combat Botrytis effectively — all while keeping the focus on plant quality and safety.

1. Understanding Botrytis: the fungus that loves humidity

Botrytis cinerea is an opportunistic necrotrophic fungus. It does not aggressively attack healthy plants; it enters through wounds, old flowers, dead leaves, or parts with micro-lesions.

Ideal conditions for its development:

• RH above 60-65% for more than 48 hours (ideal 80-95%)
• Temperature between 15-25 °C (the “mild climate” of Brazilian autumn/winter)
• Poor air circulation
• Presence of free water on the surface of flowers (raindrops, condensation, excessive irrigation)

Once established, it produces grayish spores that spread through wind, water, and even the grower’s hands. Within 3-5 days, the flower becomes soft, brown, and smells of mold. If not removed, it contaminates the entire plant and neighboring ones.

2. Prevention: the only strategy that truly works

90% of Botrytis losses occur because the grower only acts after seeing the mold. Prevention is 10 times cheaper and more effective than control.

2.1 Ventilation: the most important factor

In humid outdoor grows, the biggest mistake is planting too close together or leaving the canopy closed.

2.2 Strategic defoliation: opening the canopy

Well-done defoliation increases air circulation by up to 60% and reduces humidity inside the canopy.

Recommended technique (flowering week 2-3 and week 4-5):

• Remove all large leaves shading lower flowers
• Clean the interior of the plant (larceny or “lollipopping”)
• Never remove more than 25-30% of the foliage at once
• Always do it at night or with lights off

2.3 Silicon: the plant’s “invisible shield”

Silicon strengthens cell walls and makes the plant less palatable to fungi.

Most effective forms:

• Silicic acid (0.5-1 ml/L in all waterings during veg and early flower)
• Potassium silicate (1-2 ml/L)
• Diatomaceous earth or burnt rice husk in the substrate

2.4 Relative Humidity (RH) Monitoring

Buy a digital hygrometer and record:

• Day: 40-55% (veg) / 45-60% (flower)
• Night: never above 55% during flowering

Tips: automatic dehumidifier, morning watering, avoid watering flowers directly.

2.5 Resistant strains: genetics is your biggest ally

At Seeds Genetics Co. you will find proven resistant options:

1. Gelato 41 Autoflower
   Mold resistant, THC 24%, flowering 9-10 weeks. Ideal for humid climates.
2. Purple Punch Feminized
   Resistant to fungi and cold. Great performance in high RH.
3. Northern Lights Autoflower
   Classic strain famous for mold resistance. Finishes in 8 weeks.

3. Early detection and combat

Inspect daily with a flashlight. If mold appears:

1. Remove infected parts immediately
2. Use Bacillus subtilis or neem + baking soda
3. Increase ventilation and lower RH

Conclusion

Botrytis is not invincible. Combine resistant genetics from Seeds Genetics Co., ventilation, defoliation, silicon, and RH monitoring to harvest safely even in rainy climates.

Grow with science, harvest with safety. 🌱