Botrytis (Grey Mould) in Hydroponic Flowering Plants: Identify, Treat and Prevent

Botrytis cinerea, commonly called grey mould, is one of the most damaging fungal diseases a flowering or fruiting crop can encounter indoors. Unlike powdery mildew, which can germinate on healthy dry tissue, botrytis specifically needs two things to establish: high humidity and dead or damaged plant matter to colonise first. Understanding this distinction is the key to preventing it, because it means botrytis risk is largely a housekeeping and airflow problem rather than something you spray your way out of.

Dense flower clusters on tomatoes, strawberries and many ornamental flowering plants are particularly vulnerable, since the tightly packed structure traps humid air at the centre where light and airflow struggle to reach. A spent flower petal or a small patch of dead tissue caught inside that cluster is often all botrytis needs to get started, and from there it spreads into surrounding healthy tissue rapidly.

Why Dense Flower and Fruit Clusters Are High Risk

Any crop that produces tightly packed flower or fruit clusters creates a microclimate at the centre of that structure that is warmer, more humid and less ventilated than the surrounding canopy air. Strawberries, tomatoes and many flowering ornamentals all share this vulnerability during their fruiting or flowering stage.

As these clusters mature, some tissue naturally dies back, spent petals drop and get caught in surrounding growth, and moisture from transpiration and any overhead watering collects in the gaps between tightly packed structures. This combination, dead tissue plus trapped humidity plus poor airflow, is exactly what botrytis spores need to germinate and spread.

The Climate Conditions That Let Botrytis Establish

Botrytis becomes a serious risk once relative humidity holds above 60 percent for extended periods, particularly during the later stages of flowering or fruiting when clusters are densest and most vulnerable. Temperature is a secondary factor, botrytis is most active between 15 and 25 degrees Celsius, which unfortunately overlaps with the temperature range most indoor growing environments are held at.

The conditions that create the highest botrytis risk are:

Overnight humidity spikes. As with powdery mildew, the temperature drop that occurs when lights switch off causes relative humidity to rise. If daytime RH is well managed at 50 percent but climbs to 70 percent overnight, that extended humid period is exactly when botrytis spores germinate on any available dead tissue.

Still air within dense clusters. Even a well ventilated grow space can have stagnant microclimates at the centre of a tightly packed flower or fruit structure. Airflow at the canopy surface does not necessarily reach the interior of these clusters.

Delayed removal of dead or damaged tissue. Spent flowers, yellowing lower leaves and any damaged fruit or flower tissue left in place are colonisation points. The longer dead matter sits in a humid environment, the more likely it becomes a botrytis entry point that then spreads to healthy surrounding tissue.

Water on flower or fruit surfaces. Any watering method that wets the flower or fruit clusters directly, rather than the root zone, increases botrytis risk substantially. Free moisture sitting on dense tissue for extended periods is a strong contributing factor.

Building a Ventilation System That Prevents Botrytis

The prevention approach for botrytis overlaps significantly with powdery mildew prevention, since both diseases are ultimately humidity and airflow problems, but botrytis specifically rewards prompt removal of dead plant matter alongside environmental control.

Extraction and humidity control

A grow space should be sized to turn over its full air volume every 1 to 3 minutes under load. The Revolution EC Inline Fan allows precise speed control to match extraction to actual conditions rather than running at a fixed rate. In climates or seasons with high ambient humidity, a dedicated Ora 60L Commercial Dehumidifier running through the lights-off period directly addresses the overnight humidity spike that is the most common botrytis establishment window.

Canopy-level airflow

Extraction fans manage the room. They do not reach the still air pockets inside dense flower or fruit clusters. A Heller 40cm oscillating fan sweeping across the canopy, or clip fans positioned within the canopy itself, keep air moving through the interior of dense clusters rather than just across the surface of the plant.

Automated control

The GAS Enviro V2 controller responds to humidity readings automatically, increasing extraction the moment RH climbs, which is particularly useful for catching the overnight humidity rise before it has time to establish conditions botrytis needs.

When Botrytis Is Already Present

Once grey, fuzzy fungal growth is visible on flower or fruit tissue, the affected material should be removed and disposed of away from the grow space immediately, rather than treated in place. Botrytis spreads through spore release from infected tissue, and leaving infected material in the canopy continues seeding new infections even if you treat the surrounding healthy growth.

After removing visibly infected material, Micro Kill Concentrate can be applied as a foliar spray to surrounding healthy tissue as a preventative barrier against further spread. Apply during the final hour of the light period when temperatures are lower, and correct the underlying humidity and airflow conditions at the same time, since treating the visible infection without fixing the environment it thrived in generally results in recurrence.

What to Buy to Prevent It

Prevention comes down to the same combination as powdery mildew, airflow and humidity control, with the addition of prompt plant hygiene. An oscillating clip fan positioned to reach the interior of dense flower or fruit clusters, paired with a properly sized inline exhaust fan and a dehumidifier if ambient humidity runs high, addresses the environmental side. The rest comes down to removing dead or spent plant material before it becomes a colonisation point.

Other Common Grow Room Pests and Diseases

Botrytis often gets confused with powdery mildew since both are fungal and both are humidity driven, though they behave differently. See our guide on powdery mildew for the key differences, along with our guides on root rot, spider mites and thrips. For the full range of treatment products, browse the Pest and Disease Management collection. Need a fast answer rather than the full guide? Our Troubleshooting Index covers botrytis and other common problems in a quick-answer format.

Frequently Asked Questions

What is the difference between botrytis and powdery mildew?
Botrytis (grey mould) produces a grey, fuzzy growth specifically on dense flower or fruit tissue and needs both high humidity and dead or damaged plant matter to establish. Powdery mildew produces a dry white powder on leaf surfaces and can germinate on healthy tissue without needing decaying matter first. Botrytis is most common in late flowering or fruiting stages under high humidity, powdery mildew appears earlier and across a wider humidity range.

Can botrytis spread from one plant to another?
Yes. Infected tissue releases spores that travel through the air and settle on nearby healthy plants, particularly in canopy areas with poor air movement. Removing infected material promptly and improving airflow both reduce the chance of spread to neighbouring plants.

Why does botrytis keep appearing on my strawberries or tomatoes even with good ventilation?
Check airflow specifically at the interior of dense flower or fruit clusters, not just at the canopy surface. A grow space can read as well ventilated on a hygrometer positioned at the tent wall while the centre of a tightly packed cluster stays stagnant and humid. Clip fans positioned to reach inside dense growth address this directly.

Is it safe to eat fruit from a plant that had botrytis if the infected parts are removed?
Remove and discard any fruit or flower tissue showing visible grey mould growth, along with surrounding tissue that may carry spores even without visible symptoms. Unaffected fruit from the same plant, harvested and inspected carefully, is generally considered fine, but when in doubt discard affected material rather than risk consuming contaminated tissue.