Quick answer: Strawberries face two humidity-related molds that need opposite conditions. Gray mold (Botrytis) infects flowers and berries that are wet with free water in cool weather, so keep flowers and fruit dry, especially overnight. Powdery mildew infects dry leaves under high humidity, so drying the air helps but does not stop it. In tabletop and gutter systems, the humid air sits in the fruit band below the plants: move air there, hold a night margin below saturation, and remove infected and ripe fruit promptly.
Quick answer
- Gray mold: no free water on flowers and berries; night RH below about 85–89 % depending on temperature.
- Powdery mildew: high humidity with dry leaves; needs monitoring and resistant cultivars as well.
- Air movement in the fruit band matters more than the RH at the room sensor.
Two molds, two conditions
| Gray mold (Botrytis fruit rot) | Powdery mildew | |
|---|---|---|
| Where | Flowers, then berries (often from infected flowers); rot appears as fruit ripens or after harvest | Leaves (curling, purple blotches), flowers and fruit |
| Conditions | Free water on tissue in cool weather; dry, windy conditions interrupt it | Dry leaf surfaces, high RH, cool to warm air; severe in greenhouses and tunnels |
| Humidity control | Very effective if flowers and fruit stay dry | Helpful but not sufficient |
| Other controls | Remove dead tissue, ripe and rotten fruit; upright cultivars; good airflow | Resistant cultivars, scouting, protectant programs |
Sources: UC IPM, Botrytis fruit rot and Powdery mildew of strawberry; Penn State Extension describes gray mold as favored by moisture, high humidity and relatively cool conditions (Penn State Extension).
Because Botrytis infections often start at flowering and stay hidden until the fruit ripens, humidity control during bloom protects the harvest weeks later.
Where moisture stays in strawberry houses
- Under tabletop gutters and in the fruit band: fruit hangs below the leaves where air moves least.
- Around flower clusters in dense canopies.
- Near cold glazing and under screens at night, where drips fall onto plants.
- After irrigation and cleaning, when trays and floors are wet into the evening.
Place sensors in the fruit band, not only above the crop. See greenhouse humidity sensor placement.
Night humidity targets
Keep the air a margin below saturation so flowers and berries do not get wet:
| Night air temperature | RH setpoint that leaves a margin (UMass) |
|---|---|
| 10 °C | about 83 % |
| 16 °C | about 89 % |
| 20 °C | about 91 % |
These values come from UMass Extension guidance on reducing greenhouse humidity. In dense or poorly ventilated fruit bands, aim a little lower, and raise the temperature gradually at sunrise so cold fruit does not collect condensation.
Airflow for tabletop and gutter systems
| Layout | What to do |
|---|---|
| Hanging gutters or tabletops | Run horizontal airflow fans or perforated air tubes along or just below the gutters, so air passes through the fruit band |
| Long bays | Several smaller air sources along the length rather than one at the end |
| Dense canopies | Remove old leaves, space plants, choose more open cultivars |
| Indoor or vertical farms | Air delivered to each tier; check the top and bottom tiers separately |
Hygiene that humidity control cannot replace
- Remove dead leaves, old flowers and infected fruit from plants and the floor, and take them out of the house.
- Harvest ripe fruit promptly; overripe fruit is a source of spores.
- Avoid wetting flowers and fruit during irrigation.
- Cool harvested fruit quickly and keep condensation off it when it moves between cold and warm areas.
Dehumidifiers for strawberry houses
- Refrigerant dehumidifiers suit heated houses with night temperatures above about 15 °C, which covers most strawberry production.
- Desiccant units are only worth considering for very cold houses or dry-air storage.
- Size for the night peak at the house condition, not at the 30 °C / 80 % RH rating.
- Distribute dry air into the fruit band with air tubes or several units; avoid blowing directly on flowers.
- Stage with vents and heating from the climate computer, starting before sunset.
See our greenhouse dehumidifiers, how to choose a greenhouse dehumidifier and greenhouse humidity and disease.
FAQ
Frequently asked questions
What humidity causes gray mold on strawberries?
Gray mold needs free water on flowers or fruit in cool weather. Keep night RH a margin below saturation, for example below about 89 % at 16 °C, and keep air moving through the fruit.
Does low humidity stop powdery mildew on strawberries?
No. Powdery mildew infects dry leaves under high humidity. Lower humidity helps, but resistant cultivars and monitoring are needed too.
What is the best humidity for greenhouse strawberries?
Moderate daytime humidity with active transpiration, and a night target that keeps flowers and fruit dry.
Why do my strawberries rot after harvest?
Botrytis infections often start at flowering and develop as fruit ripens or in storage. Control humidity at bloom, remove infected fruit, and cool harvested fruit quickly.
Sources
- UC IPM, Strawberry: Botrytis fruit rot; Powdery mildew.
- Penn State Extension, Strawberry disease: gray mold.
- UMass Extension, Reducing humidity in the greenhouse.
- UF/IFAS, Leaf wetness sensor installation and maintenance (AE538).
