Controlling mold in greenhouses

By Matt Williamson
Info Guru,

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Interior of large greenhouse
The most common disease of greenhouse floral crops is grey mold
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A few tips for keeping mold out of your greenhouse

Adding a greenhouse to your home is a great way to ensure that your plants will grow to their full potential. You can easily build your home greenhouse with a greenhouse kit. However, be careful, because it's once the greenhouse is up and running that many home owners begin to run into problems. By far the biggest problem you may have with your greenhouse is mold. However, with a few simple guidelines, your greenhouse will be mold-free year round.

Gray mold is caused by the fungus Botrytis cinerea, a common fungus with a very wide host range. It can persist in the greenhouse year round as mycelium, conida, or as sclerotia on living or dead tissue. The fungus produces a large amount of spores that move throughout the greenhouse via air currents. Under optimum environmental conditions for growth—relative humidity at or above 85 % with little or no air circulation or with free water on leaf surfaces—the spores land on the plant surface, germinate and penetrate the host plant.


Symptoms of gray mold vary depending on the host and the environmental conditions associated with the host. Under most conditions and with the majority of hosts, the disease is characterized by the production of leaf spots, flower blight, bud rot, stem canker, stem and crown rot, cutting rot, damping off, and in extreme cases, plant death. On all infected plant parts, in high humidity, the fungus will produce characteristic spores. All affected areas become covered with a fuzzy gray fungal growth.

Under magnification, the millions of spores are seen to be formed in clusters that resemble bunches of grapes. When a plant that is covered in the spores is tapped, a cloud of gray "smoke" (spores) is often seen. This fungus also causes damping-off of seedlings.

Botrytis gray mold is a common nuisance and a very expensive one at that. However, it is also one of the easiest diseases to control.

Controlling the environment

If there is one practice that will go a long way towards controlling gray mold, it is controlling the greenhouse environment. Maintaining conditions within the greenhouse that will not permit the fungus to grow and sporulate is a critical step to spore control. By keeping the humidity level below 85%, as well as ensuring good air circulation and adequate plant spacing, you can achieve excellent control.

Whenever possible, you should spread apart plants that are packed closely together; this will allow better air circulation and will reduce the level of relative humidity within the plant canopy. Use fans to provide good air movement above the canopy. Plants with wounds should either be protected with a fungicide or should be removed from the greenhouse. An open or untreated wound is the ideal environment for the fungus to initiate the infection process. 

Sanitation alone is often not sufficient to control Botrytis in the greenhouse, but it is a very, very important part of your regiment to control gray mold.

- Clean the greenhouse thoroughly between crops. All debris, including prunings and dead leaves, should be removed from the greenhouse immediately.

- Infection of plants usually begins at wounds. Plants should be handled carefully to avoid wounds. Plants should not be pinched or pruned more often than necessary. Avoid allowing leaves to come into contact with potting medium. Remove spent flowers as soon as possible.

- Prevent chilling injury.

- Keeping foliage dry is extremely important to control this disease.

- Ventilate the greenhouse to decrease relative humidity and prevent dew from falling on plants at night.

- Avoid watering late in the day and at other times than do not promote fast leaf drying.

- Avoid overhead watering and over watering. The most overlooked component of a greenhouse that encourages algae growth is the irrigation system."

- Space plants adequately on the bench, and use open-mesh benches to improve airflow.

- Keep the relative humidity in the canopy below 85%.

- For bedding plant production, time seeding and transplanting to avoid holding plants longer than necessary.

- Major genetic resistance has not been identified, although cultivars of the same plant species differ in their resistance to this fungus.

- Contact your local Extension agent regarding current recommendations for chemical control measures.

Other ways to controlling mold in greenhouses

Keep seedlings healthy and vigorous and avoid injuring the foliage. Fertilizer-burned or frost- damaged foliage is particularly susceptible to Botrytis infection.

- Avoid overly dense seedling growing levels by selecting a container that allows adequate spacing for seedling development. Containers can also be placed at a wider spacing to allow better air circulation during periods when seedlings are especially vulnerable.

- Reduce the time that seedling foliage is wet by encouraging air circulation, irrigating early in the day, using surfactants in the irrigation water, providing under bench heating, or force-drying foliage with fans.

- Follow a strict sanitation policy that includes removal and destruction of all plant debris, prompt rouging of infected seedlings, and sterilization of containers and growing area surfaces between crops.

- Chemical. All fungicides registered for the control of grey mold are protectants that must be applied before infection takes place. There are a number of chemicals registered for controlling B. cinerea on ornamental plants, but not all of these are registered for tree seedlings. New chemicals are also continually being developed, so growers should monitor trade publications and check with a certified pesticide specialist or your local Extension agent for up-to-date information as to the type of FDA approved fungicides.

The timing of protective fungicide applications is important. These chemicals must cover susceptible plant tissue before the Botrytis spores germinate and penetrate the foliage. The medication should be applied at regular intervals (1 to 2 weeks) during the susceptible period.

Botrytis may develop tolerance to fungicides that are used repeatedly so be sure to use them periodically during the growing season.

Regardless of the effectiveness of pesticides, chemical control of grey mold is virtually impossible without a corresponding and coordinated program of the other practices mentioned above.

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