Causes of powdery mildew in cannabis plants

What is powdery mildew and why does it spread so fast?

Powdery mildew is a fungal disease that can infect trees, flowers, and fruits at any stage of life. When contaminated with mildew spores, cannabis plants will typically grow patches of white powder on the upper and lower surfaces of leaves, flowers, stems, trichomes, and other parts, usually within 14 days of infection.

Infected plants absorb fewer nutrients and less water, leading to reduced cell function. Plant health deteriorates as the infection blocks photosynthesis, causing discolored leaves, abnormal growth, and spoiled cannabis resin.

Powdery mildew spreads fast once it enters a facility. In the right conditions, a single infected plant can release spores that travel long distances and trigger new outbreaks within a week. The disease rarely kills its host, but its ability to reproduce and persist makes it essential for commercial growers to track down the source. Identifying the cause is the only way to save plants and prevent future infections.

Golovinomyces – the pathogen most often responsible for powdery mildew in cannabis – releases spores that travel easily through the air, and wind or greenhouse fans accelerate the spread. The fungus favors humid conditions but, unlike most others, is suppressed by direct moisture. It thrives at relative humidity (RH) above 95%, yet can still produce heavily even when RH drops below 50%.

Temperature conditions can also facilitate the spread of powdery mildew. It tends to show up in mid-to-late summer months because temperatures between 68°F and 86°F mark the sweet spot for infection. Higher temperatures are often fatal to spores, but when conditions are humid enough, powdery mildew can thrive even in shady areas.

The frequent combination of high humidity, poor air circulation, and moderate temperatures make commercial indoor and greenhouse facilities particularly susceptible to outbreaks.

Where does powdery mildew come from and how can it be prevented?

The cannabis industry’s frequent use of moms, cuttings, and clones for propagation explains why so many cultivators often battle powdery mildew. Infected clones are a common source of problems – bringing one bad plant into a facility has the potential to infect an entire crop. Five to 10 days after the initial infection, the diseased plant releases spores that spread the disease to other plants.

In optimal conditions, powdery mildew spores can survive in shoots, buds, leaves, and other plant parts. Leaving infected plant debris unattended or adding it to a compost pile can contribute to the pathogen’s spread, resulting in infected soil that, if used, will infect every new plant brought into the facility.

Overcrowded rooms can facilitate outbreaks even further, making healthy plants more susceptible to airborne spores simply due to proximity.

Growers who have conquered powdery mildew typically employ three key tactics:

1. Source and breed cultivars that show resistance to mildew
2. Develop best practices and SOPs for facility cleanliness
3. Adopt an integrated approach to pest and pathogen management

The more growers understand the conditions that can cause powdery mildew, the less likely an infection goes from a nuisance to a nightmare.