Powdery mildew is a common fungal disease that can seriously damage indoor and outdoor crops. Powdery mildew can rapidly infect crops in both vegetative and flowering stages, coating leaves, stems and buds in fungus.
Powdery mildew typically thrives in cool, damp, shaded and poorly ventilated areas. Airborne spores brought into the grow room land on leaf surfaces and will germinate given favorable conditions. High night humidity levels often trigger the growth of mildew spores. Powdery mildew can attack indoor crops year round.
Powdery mildew is almost impossible to stop in late flowering, so early detection and control is essential. Perpetual harvest, dense scrog/sog systems, and damp basement grows are particularly vulnerable to powdery mildew. Note: strains vary in their susceptibility.
This FAQ focuses on indoor prevention and treatment options. Please read
Bongaloid’s Powdery Mildew FAQ for more information.
Identification, Symptoms and damage:
Early signs of powdery mildew include white powder/fuzzy patches on leaves (usually low in the canopy) and a fuzzy white coating on lower stems. Note: powdery mildew can be wiped off the leaves for a quick visual check.
(photo provided by turtle power) (photo provided by just one blunt)
These fuzzy mycelium patches produce airborne spores that rapidly attack adjacent plants; mildew will eventually coat leaves and entire plants, reducing photosynthesis, plant vigor and bud quality.
Plants on the edge of a garden, in corners and under stress are attacked first; infection usually starts in the lower canopy where conditions are optimal. As infection progresses, mildew will spread to the top of the plants and finally attack the buds.
Infected buds may appear normal; but are internally dusted with white powder (which cannot be removed by drying), and have a stale, musty/moldy smell when dry. Smoking or trimming infected buds can cause sickness and lung infections, and is not recommended. Infected leaves should be discarded. Lower buds are the most susceptible.
Powdery mildew is difficult to 100% eradicate; control requires prevention, early detection, and pro-active measures.
Preventive gardening:
Preventative gardening techniques can be effective in defending against powdery mildew.
Maintain healthy plants. Stressed plants are often attacked first, so it is important to monitor and remove unhealthy plants.
Detection. Inspect corners, edge and lower portions of the garden frequently. Remove infected leaves, or move infected plants out of the main garden.
Dont water plants at night. Reduce or stop watering before the lights have gone out to help evaporate and reduce room humidity (thnx dutchmaster).
Reduce plant density. Spread plants apart to improve air circulation. Don’t place plants directly against walls or into corners, typically areas of poor air circulation. Pull plants 6”-1’ away from walls or reflective surfaces, and blow air to these areas.
Pruning. Remove the lowest leaves as the plants mature and prune the bottom 1/3 of the plant during veg to increase airflow inside the lower canopy. Remove all unnecessary growth. Put an oscillating fan down low to blow through this pruned area.
Foliar feeding. Foliar feeding can sometimes cause excessive nighttime humidity levels. Discontinue if mildew appears.
Harvest early if mildew is a problem.