Two Fungal Threats Every Grower Faces

Powdery mildew and botrytis (grey mould) are among the most common — and most costly — fungal diseases in indoor and greenhouse growing. Left unchecked, they can move through a crop fast and wipe out weeks of work. Traditional control relies on fungicide sprays, but these come with real drawbacks: chemical residues on the harvest, re-entry waiting periods, increasing fungal resistance, and tightening rules on what you're even allowed to spray.

There is a better way.

Understanding the Threat

Powdery Mildew

Powdery mildew appears as white, dusty patches on leaves and stems. It is favored by warm temperatures and moderate-to-high humidity — but, unlike botrytis, it does not need free water or wetness on the leaf to take hold. Poor air circulation makes it worse. Spores spread rapidly through the air, so early intervention is critical. Once established, it weakens plants, reduces yields, and can make a harvest unsaleable.

Botrytis (Grey Mould)

Botrytis cinerea is a necrotrophic fungus that attacks soft tissue — flowers, fruit, and stems — especially in cool, humid conditions with sustained high humidity. It spreads through airborne spores and infected plant debris. In dense canopies or poorly ventilated spaces, a single infected site can trigger a cascade of losses.

Why Chemical Fungicides Fall Short

• Resistance: Repeated use of the same active ingredients selects for resistant strains that no longer respond to treatment.
• Residues: Spray residues raise food- and product-safety concerns. For cannabis growers in the USA and Canada, this is the big one — residue testing can fail an entire batch and disqualify it from sale.
• Re-entry intervals: After spraying, you have to wait before re-entering the space — disrupting daily operations.
• Shrinking options: Very few fungicides are legally usable on cannabis, and the list of permitted actives keeps getting shorter under regulatory pressure.

UV-C Light: A Proven, Chemical-Free Alternative

UV-C light (wavelengths around 254 nm) damages the DNA of fungal spores, preventing them from reproducing. Because this is a physical process rather than a chemical one, fungi don't build up resistance the way they do to repeated fungicide use. There are no residues and no re-entry waiting periods.

Research has demonstrated UV-C efficacy against both powdery mildew and botrytis in a range of crops, including strawberries, tomatoes, cucumbers, cannabis, and ornamentals. In published trials, nighttime UV-C applications performed as well as — or better than — the best available fungicides.

How to Use UV-C Effectively in Your Grow Room

1. Start Early — Prevention Over Cure

UV-C is most effective as a preventive tool. Begin treatments before disease pressure builds, ideally from the start of the growing cycle. Weekly or twice-weekly passes over the canopy keep spore loads low.

2. A Dark Period Afterward Boosts Results

You can treat at any point in the cycle, but following the treatment with a period of darkness makes it more effective. Fungal spores have a natural, light-activated repair system that reverses some UV damage when triggered by blue and UV-A light. Keeping the plants dark for a few hours after treatment limits that repair, squeezing extra performance out of each pass — a worthwhile edge for relatively little effort. Treating during the existing dark period is the easy way to get this benefit.

3. Maintain Consistent Distance and Speed

The UV-C dose delivered to a surface depends on the distance from the lamp and the speed of movement. Move the device slowly and consistently over the canopy — keeping it close to the leaf surface — for even coverage. Follow the dosing guidance for your specific CleanLight device.

4. Focus on High-Risk Areas

Pay extra attention to dense canopy zones, lower leaves, and areas with reduced airflow — these are where botrytis and mildew establish first.

5. Combine with Good Cultural Practices

UV-C works best as part of an integrated approach. Maintain good ventilation, manage humidity (keeping it well below the levels that favor botrytis — under 70% RH during lights-off is a safe target), remove infected plant material promptly, and avoid overhead watering.

Choosing the Right CleanLight Device

CleanLight offers UV-C devices designed specifically for growers, from hobby cultivators to professional greenhouse operations.

• CleanLight Pro — Designed for professional growers and larger grow rooms. Delivers a high UV-C dose efficiently across wide canopies.
• CleanLight Hobby — The ideal entry point for home growers and small grow tents. Lightweight, easy to use, and effective.
• CleanLight Air — Targets airborne spores continuously, reducing the ambient spore load in your grow space around the clock.

Not sure which device fits your setup? Contact us for a free consultation. 

What Results Can You Expect?

Used consistently as part of a disciplined routine, growers using CleanLight UV-C devices aim for:

• Lower powdery mildew pressure after a few weeks of regular treatment
• Reduced botrytis pressure, particularly in flower and fruit stages
• Cleaner crops with no chemical residues
• Less fungicide spend and fewer spray interventions

Getting Started

The best time to introduce UV-C is before disease pressure appears. If you're already seeing early signs of powdery mildew or botrytis, increase treatment frequency and combine it with removal of visibly infected material.

UV-C is not a silver bullet — but as part of a disciplined growing routine, it is one of the most effective, sustainable, and residue-free tools available to modern growers.

Ready to protect your crop? Explore the CleanLight range and find the device that fits your grow.