Gray mold (Botrytis) and powdery mildew in greenhouse tomatoes: what conditions favor them and what growers can do
Reviewed: · SeedMatchGroup
Gray mold (Botrytis cinerea) is favored by cool, humid conditions and free moisture on plant surfaces, often entering through pruning wounds and old flowers. Powdery mildew does not need free water on leaves and can develop under drier conditions. Because they respond to different conditions, confirm which one you have, then combine humidity and condensation control, air movement, irrigation timing, spacing and pruning, sanitation and regular monitoring.
Identification aid, not a diagnosis. Symptoms of different problems can look alike; confirm with a local extension service or plant-health laboratory before acting.
How do gray mold and powdery mildew differ?
Gray mold, caused by Botrytis cinerea, can affect stems, leaves, flowers and fruit. Typical signs include tan to gray lesions, often starting at pruning wounds or on dying flowers, and a fuzzy gray growth of spores under humid conditions. On green fruit it can leave small pale rings sometimes called ghost spots.
Powdery mildew on greenhouse tomatoes is caused by different fungi. One type produces white, powdery patches on the upper leaf surface; another (Leveillula taurica) shows as yellow patches on the upper surface with sparse growth mostly underneath. Powdery mildews generally do not need free water on the leaf to infect.
What conditions favor each disease?
- Gray mold: high relative humidity, condensation and leaves or flowers that stay wet, especially with cool nights and limited air movement.
- Gray mold: wounds from pruning, de-leafing or handling, and dead or senescent plant tissue left in the crop.
- Powdery mildew: can develop at moderate humidity without leaf wetness; dense canopies and poor air movement can still increase risk.
- Both: weak monitoring that lets early infections spread before they are noticed.
Why does accurate diagnosis matter?
The two diseases respond to different conditions, so a measure that helps one (for example reducing leaf wetness) may not be enough for the other. Other diseases and nutrient or environmental problems can also look similar. Send samples to a local diagnostic service or ask your extension advisor when you are unsure.
What integrated prevention steps can growers take?
- Monitor humidity, condensation and leaf wetness, and adjust heating and venting to reduce long wet periods, especially overnight.
- Keep air moving through the canopy with ventilation and circulation fans.
- Time irrigation so foliage and floors are not wet going into the night, and avoid wetting leaves where possible.
- Use spacing, pruning and de-leafing practices that open the canopy; make clean cuts and keep tools clean.
- Remove infected leaves, fruit and debris and dispose of them outside the growing area in closed containers, following local rules.
- Scout plants regularly and record where symptoms appear so you can link them to environment data.
What about crop protection products?
Only use products registered for tomatoes in your country and growing system, and follow the exact label. Registrations differ by country. Ask your local extension service or a licensed plant-health professional which options apply; this guide does not recommend products or rates.
Weekly scouting sheet: gray mold and powdery mildew
Print or copy this sheet. Check the same marked plants each week and add random plants across the house.
| What to check | Where | Notes / count |
|---|---|---|
| ☐ Tan or gray lesions on stems, especially at pruning wounds | ||
| ☐ Fuzzy gray growth on old flowers, leaves or fruit | ||
| ☐ Pale rings (ghost spots) on green fruit | ||
| ☐ White powdery patches on upper leaf surfaces | ||
| ☐ Yellow patches on upper leaf surface with growth underneath | ||
| ☐ Dead leaves, fruit or debris left in rows or on the floor |
Greenhouse environment observation checklist
- Record temperature and relative humidity at canopy height, morning and evening.
- Note condensation on roof, walls or leaves, and how long leaves stay wet.
- Check that fans and vents run as set and air moves through dense canopy areas.
- Record irrigation start/end times and whether foliage or floors stay wet into the evening.
- Note cold spots, drips from structures and areas with poor drainage.
- Write down any change in heating, screens or ventilation settings this week.
Frequently asked questions
How can I tell gray mold from powdery mildew?
Gray mold usually causes tan to gray lesions with fuzzy gray spores, often at wounds and on flowers, and favors wet conditions. Powdery mildew shows as white powdery patches or yellow patches with growth underneath, and does not need wet leaves. Confirm with a local diagnostic service.
Does lowering humidity prevent both diseases?
Reducing condensation and leaf wetness is especially important for gray mold. Powdery mildew can develop without free water, so air movement, canopy management and monitoring still matter.
Should infected plant material be removed?
Removing infected leaves, fruit and debris and disposing of them outside the growing area reduces spore sources. Handle it carefully and follow local disposal rules.
Can this guide recommend a fungicide?
No. Check which products are registered locally for tomatoes and follow the exact label, with advice from your extension service or a licensed professional.
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