Quick answer: Most humidity-related greenhouse diseases start with water on the plant surface, not with a high RH number on its own. Water condenses on leaves, flowers and fruit when they cool below the air's dew point, usually from dusk to dawn and after cloudy days. Keep the canopy dry at night: hold the RH a safe margin below saturation, keep air moving through the crop, and remove moisture with heating and venting or a dehumidifier when vents are closed. Then match the control to the disease, because Botrytis, downy mildew, powdery mildew and edema respond differently.
Quick answer
- Key goal: no condensation on the crop from dusk to dawn.
- Night RH setpoints that leave a margin: about 83 % at 10 °C, 89 % at 16 °C, 91 % at 20 °C (UMass Extension).
- Powdery mildew is the exception: it can infect without leaf wetness.
How humidity causes disease: the dew point
Air at a given temperature and humidity has a dew point. Any surface colder than it gets wet. Leaves lose heat to a cold roof or clear night sky and can be a degree or more colder than the air.
UMass Extension gives two examples: at 16 °C (60 °F) and 85 % RH, leaves become wet if they cool about 3 °C below the air; at 95 % RH, a leaf only half a degree cooler than the air is enough (UMass Extension, Reducing humidity in the greenhouse).
Night setpoints that avoid leaf wetness
| Air temperature | RH setpoint (UMass) | Humidity deficit |
|---|---|---|
| 10 °C (50 °F) | 83 % | about 1.6 g/m³ |
| 16 °C (61 °F) | 89 % | about 1.5 g/m³ |
| 20 °C (68 °F) | 91 % | about 1.6 g/m³ |
| 30 °C (86 °F) | 95 % | about 1.5 g/m³ |
The pattern behind the table: each setpoint leaves the air about 1.5 g of water vapour per cubic metre short of saturation. That is why a fixed RH setpoint is wrong across temperatures; a fixed humidity deficit (or VPD) is a better control target. Our VPD chart guide explains the conversion.
Disease by disease
| Problem | What water it needs | When risk is highest | Main controls |
|---|---|---|---|
| Botrytis (gray mold) | Free water or condensation on tissue; spores germinate in it | Cool, humid nights; after cloudy weather; dense canopies; dead or wounded tissue | Keep canopy dry dusk to dawn; air movement; remove dead flowers and debris |
| Downy mildews | Leaf wetness for infection; high night humidity for spore production | Long wet periods, cool nights | Minimize leaf wetness duration; lower night humidity; inspect leaf undersides at dawn |
| Powdery mildews | Do not need free water; infection can start at humidity below typical greenhouse levels | Moderate humidity with dry leaves; development favored by higher humidity later | Resistant varieties, monitoring, fungicide programs; humidity control helps but does not stop it |
| Edema (oedema) | Not a disease: roots take up water faster than leaves can lose it | Cool, cloudy, humid, poorly ventilated weather with warm, wet growing media | Raise transpiration with air movement and lower humidity; avoid over-watering in dull weather |
Botrytis
Botrytis colonizes dead, dying and wounded tissue first, then attacks healthy tissue. Infected petals that fall onto leaves start new lesions, and changes in humidity trigger spore release (PNW Plant Disease Handbook). In tomato, it enters through leaf scars, cracks and pruning wounds in humid conditions, and survives on plant debris (NC State Extension). UMass reported a pansy case after over-watering and a cold, cloudy day in a poorly ventilated house (UMass Greenhouse Update).
So: dry canopy at night, prune in the morning so wounds dry during the day, and clear dead flowers and leaves from benches and floors (UConn Extension).
Downy mildews
"Downy mildew" covers different pathogens on different crops. Basil downy mildew sporulates mainly on the lower leaf surface during the dark period, so early morning is the time to inspect; Cornell notes that minimizing leaf wetness and reducing humidity can suppress it in greenhouses (Cornell Vegetables). Cucurbit and bedding-plant downy mildews look different on each host (NC State: cucurbits; NC State: bedding plants).
Powdery mildews
Powdery mildew is the reason "keep RH low" is not the whole answer. Cornell explains that infection can begin at humidity below levels commonly found in greenhouses, while later development may be more vigorous at higher humidity (Cornell Greenhouse). On cucurbits, high RH favors infection while dry conditions favor spore spread (Cornell). Dry leaves do not rule it out.
Edema
Edema shows as small blisters or corky bumps, often on lower leaf surfaces, and can look like mite or thrips damage. The Connecticut Agricultural Experiment Station links it to humid, dim, cool, poorly ventilated conditions with warm, wet media and cool night air (CAES fact sheet). It does not spread from plant to plant. UConn notes that responses differ by crop and cultivar, for example ivy geranium versus tomato intumescence (UConn Extension).
Why cloudy weather raises risk
After a cloudy spell, plants transpire little during the day, the media stay wet, and the evening brings cool, saturated air. Vents are often closed to hold heat. These are the hours when condensation forms. Plan for them: start moisture removal before sunset, not after the crop is already wet.
Measuring leaf wetness, not just RH
A leaf wetness sensor reports how long a surface stays wet. It is a proxy for the leaf it sits next to, not for the whole crop, and wetness varies across a house and within one plant. Record the sensor location and crop zone, and keep RH-based wetness estimates separate from direct sensor data (UF/IFAS, Leaf wetness sensor installation and maintenance). Place sensors in the canopy, in the coldest and least-ventilated zones.
Air humidity is not root-zone moisture
RH describes water vapour in the air; a substrate moisture reading describes water in the growing medium. They have different units and answer different questions. Wet media plus humid air is the edema combination; dry-looking media can still be wet below the surface (UMN Extension). Track both.
How plants respond to humidity
Stomata respond to the air's drying power. In a summer cucumber greenhouse study, high VPD increased transpiration and was followed by stomatal closure, especially when solar radiation fell quickly (PubMed). Leaves grown for long periods at very high humidity, such as rose, can develop stomata that close poorly later, which shortens vase life (Wageningen University & Research). Very high humidity is not only a disease issue.
How to keep the canopy dry at night
- Heat and vent at dusk and dawn. UMass recommends venting and heating two or three times an hour after sunset and around sunrise: exchange the humid air, then heat the drier outside air.
- Move air through the crop. Horizontal airflow fans running continuously keep leaf surfaces close to air temperature.
- Avoid evening irrigation and wet floors before night.
- Space plants and remove lower leaves where the crop allows, so air reaches the canopy.
- Dehumidify when venting is costly or does not work. When outside air is humid, or screens are closed and heat must be kept in, a greenhouse dehumidifier removes water directly and returns the heat of condensation to the house. See how to choose a greenhouse dehumidifier.
FAQ
Frequently asked questions
What humidity causes Botrytis in a greenhouse?
Botrytis spores germinate in free water. The risk is high whenever leaves, flowers or fruit get wet, which happens at high RH when tissue cools below the dew point. Keep night RH below about 85–90 % depending on temperature, and keep air moving.
What humidity prevents powdery mildew?
None on its own. Powdery mildew can infect at moderate humidity without leaf wetness. Use resistant varieties and monitoring; humidity control reduces, but does not stop, its development.
Is edema caused by high humidity?
High humidity is one part. Edema develops when roots take up more water than leaves lose, typically in cool, cloudy, humid weather with wet media.
When is greenhouse disease risk highest?
From dusk to dawn, in spring and autumn, and after cloudy days, when vents are closed and the crop cools below the dew point.
Should RH be the same day and night?
No. Use a humidity deficit or VPD target so the margin to condensation stays similar as temperature changes.
Sources
- UMass Extension, Reducing humidity in the greenhouse.
- PNW Plant Disease Management Handbook, Greenhouse ornamentals: gray mold.
- NC State Extension, Botrytis gray mold of tomato; Cucurbit downy mildew; Downy mildew of bedding plants.
- UConn Extension, Botrytis on bedding plants; Non-infectious disorders: oedema and intumescences.
- Cornell University, Powdery mildew (greenhouse); Cucurbit powdery mildew; Basil downy mildew.
- Connecticut Agricultural Experiment Station, Edema fact sheet.
- UF/IFAS, Leaf wetness sensor installation and maintenance (AE538).
- University of Minnesota Extension, Soil moisture sensors for irrigation scheduling.
