Quick Answer:
BC Greenhouse Builders recommends five proven strategies to keep mold and fungus out of your greenhouse:
- Maximize sunlight by removing shade cloth as soon as peak summer temperatures dip
- Maintain proper ventilation with automatic roof vents and supplemental fans for airflow
- Monitor temperature and humidity levels closely through late fall and winter
- Water plants conservatively in winter, delivering water to the roots and never to the foliage
- Clean the greenhouse thoroughly every fall, washing surfaces, pots, and tools with soapy water
- Change out potting soil annually to eliminate lingering fusarium spores
- What Are the Most Common Types of Greenhouse Mold and Fungus?
- How Can Sunlight Help Prevent Greenhouse Mold?
- Why Is Ventilation Critical for Mold Prevention?
- How Should You Monitor Temperature and Humidity in a Greenhouse?
- How Often Should You Water Plants to Prevent Fungal Growth?
- What Cleaning Routine Keeps a Greenhouse Mold-Free?
If you have noticed fuzzy patches on your tomato stems, yellowing cucumber leaves, or a dusty white coating on your favourite plants, you are not alone. BC Greenhouse Builders hears from greenhouse owners every fall and winter who are battling stubborn mold and fungus outbreaks. The cooler, damper months create the perfect conditions for grey mold, fusarium, sooty mold, and powdery mildew to take hold. The good news is that climate control and consistent cleanliness can prevent the vast majority of fungal problems before they start. Greenhouses are not one-size-fits-all, but the principles behind a healthy growing environment are universal.
What Are the Most Common Types of Greenhouse Mold and Fungus?
The four most common fungal problems in a home greenhouse are grey mold, fusarium, sooty mold, and powdery mildew. Each one thrives under different conditions, so identifying which mold you are dealing with is the first step toward eliminating it.
| Mold Type | Primary Cause | Common Symptoms | Plants Most Affected |
|---|---|---|---|
| Grey Mold (Botrytis cinerea) | Cool air with stagnant high humidity | Fuzzy grey coating, brown stems, mushy fruits | Tomatoes, soft fruits, ornamentals |
| Fusarium | Soil-borne spores, root infection | Wilting, root rot, yellow leaf edges, brown stems | Cucumbers, tomatoes, fruits, vegetables, ornamentals |
| Sooty Mold | Sugary honeydew from aphids and scale | Black surface coating on leaves and stems | Lemons and other citrus, ornamentals |
| Powdery Mildew | Cool nights, micronutrient deficiency (especially zinc) | White flour-like dusting on leaves | Wide range of greenhouse plants |
Grey Mold
Grey mold (botrytis cinerea) is one of the most common diseases found in greenhouses and gardens, and it is mainly related to cool air combined with stagnant high humidity.
Grey mold on tomatoes. These should have been removed before spores had a chance to spread.
Fusarium
This common fungus causes a lot of wilting and root rot in fruits, vegetables, and ornamentals. Fusarium triggers wilting on cucumbers or turns tomato stems brown.
Fusarium can start as yellow leaf edges on cucumber progressing to entirely yellow leaves. When fusarium is in the roots, it also causes cucumbers to wilt.
Sooty Mold
Sooty mold is related to insect problems. When aphids and scale feed on plants, they excrete sugary honeydew, but luckily this is just a superficial problem. Control the scale and aphids and you control the sooty mold.
Sooty mold on lemon leaves, stems, and lemons sits on the leaf surface. It is easily washed off with a soft cloth and can be prevented by eliminating scale and aphids.
Powdery Mildew
Powdery mildew, like grey mold, is related to cool fall and winter climates, but it can also relate to low or imbalanced micronutrients including zinc. BC Greenhouse Builders recommends using only the highest quality fertilizers with micronutrients to prevent nutrient-triggered powdery mildew.
Powdery mildew often looks like flour has been sprinkled on leaves. It appears most often when evening temperatures are markedly cooler than day temperatures and is sometimes linked to a lack of micronutrients such as zinc. If in doubt, remove and discard dead or dying leaves as soon as you spot this mold and fertilize with a product that includes micronutrients.
How Can Sunlight Help Prevent Greenhouse Mold?
Warm, dry growing conditions are unfavourable to grey mold growth, which is why letting the sun in is one of the simplest mold prevention strategies. Remember to take shade cloth off as soon as peak temperatures dip in late summer. Sunlight helps dry out plants and keep temperatures warm enough in the greenhouse to discourage spore growth on damp surfaces.
Why Is Ventilation Critical for Mold Prevention?
Strong airflow is one of the most effective tools against fungal growth because it lowers humidity and keeps spores from settling on damp leaves. A well-ventilated greenhouse allows for the air circulation needed to keep mold from taking hold in the first place.
All BC Greenhouse Builders greenhouses come with automatic roof vents (unless the customer requests manual roof vents), which really sets them apart from cheaper greenhouses. Adding fans or other ventilation systems on top of roof vents helps keep air circulating and discourages mold spores from settling on damp leaves.
How Should You Monitor Temperature and Humidity in a Greenhouse?
Regular temperature and humidity monitoring is essential to keep conditions within the appropriate range for your plants and out of the danger zone for mold. Extreme temperature and humidity fluctuations are more common in the late fall and winter greenhouse, and the best way to control this is to use a heater and fans during the cool season.
BC Greenhouse Builders recommends avoiding water puddles or sitting water on floors in winter, as standing moisture quickly raises humidity and creates ideal conditions for grey mold and powdery mildew.
How Often Should You Water Plants to Prevent Fungal Growth?
Watering as little as possible in winter is the single best way to prevent fungal growth and a secondary problem, fungus gnats. Over-watering during cool months invites trouble, so use a watering system that delivers water directly to the roots, and avoid getting water on the leaves and foliage.
Unless you heat the greenhouse and add supplemental light, plant growth and water use are greatly diminished in winter. As a personal example, I water my potted lemons late in November and then not again until late February because I allow my lemon and lime pots to dry out completely between waterings in winter.
What Cleaning Routine Keeps a Greenhouse Mold-Free?
A thorough end-of-season cleanup is the single most important habit for long-term mold control because botrytis spores can survive in dried plant material for up to 20 years. According to Greenhouse Canada, a commercial growers magazine, all infected sporulating tissues are a source of invisible spore clouds that move throughout the greenhouse on any air current, especially during routine work. A thorough cleanup at the end of the crop cycle is necessary because B. cinerea produces survival structures called sclerotia that can live up to about 20 years in dried plant material.
Home Gardener Greenhouse Cleaning
Botrytis thrives in a dirty greenhouse, including all surfaces, pots, and tools. BC Greenhouse Builders recommends keeping your space clean and free from plant material to prevent the buildup of fungal spores. Home gardeners rarely rely on sprays and fungicides. Instead, clean and wash with soapy water every fall and remove all dead plants and their materials as you garden. If you see brown stems on tomatoes or fuzzy fruits in fall or winter, remove them at once.
Grey mold coating my tomato plant. These were promptly removed from the greenhouse to keep all my other plants thriving.
I have stopped growing cucumbers in my greenhouse soil and only grow them in their own pots with fresh soil every season. After losing a whole crop to fusarium once, I now change out soil in pots annually for reliable cucumber success.
Final Thoughts on Preventing Greenhouse Mold and Fungus
The worst types of mold and fungus found in a greenhouse are botrytis and fusarium, both of which can damage plants and reduce yield significantly. By implementing effective climate control measures, cleaning as you go, and taking steps to prevent fungal growth, you can reduce the risk of these and other fungal problems in your greenhouse.
If you are planning your next growing season, prioritize these actions in order:
- Inspect your ventilation system and add fans if airflow is weak
- Schedule a deep clean of all surfaces, pots, and tools before winter sets in
- Adjust your watering schedule to match the season
- Replace potting soil annually for cucumbers and other fusarium-prone crops
- Remove infected plants and fuzzy fruits the moment you spot them
BC Greenhouse Builders greenhouses are designed with automatic roof vents and durable construction that supports healthy airflow and climate control year-round. If you are ready to upgrade your growing space or replace an aging greenhouse, BC Greenhouse Builders offers a full range of hobby and commercial greenhouses built for Canadian conditions.
