Breaking the Mold

Preventing and Eliminating Greenhouse Mold

mold, mildew, greenhouse, Growing Spaces, mold prevention
Powdery Mildew on a Turnip Leaf

Greenhouse mold and mildew are both types of fungi that grow best in warm, humid environments. Most fungi reproduce and spread via spores. Spores emerge from fruiting bodies of fungi and are most commonly dispersed via breeze or transported by water. Much like plant seeds, spores can survive environmental conditions that aren’t conducive to fungi growth, only to begin growing when the time is right. Once spores land on a growing medium, they germinate and penetrate the medium, beginning to grow and mature.

According to the United States Centers for Disease Control and Prevention (CDC), no one knows how many species of fungi exist, but estimates range from tens of thousands to over three hundred thousand. Generally speaking, molds and mildews become prolific in damp, warm conditions, like those often maintained in greenhouse environments. Knowing optimal growth conditions for common greenhouse fungi allows farmers greenhouse gardeners to prevent growth of harmful fungi by maintaining a greenhouse environment that doesn’t optimize growth conditions for fungi. So, the best solution to mold is prevention!

Optimum growth of common greenhouse fungi including grey moldpowdery mildewand black sooty mold occurs when localized relative humidity is above 85%, temperatures are warm, air circulation is minimal and standing water exists on plant foliage, soil or other greenhouse surfaces. Prolific mold and mildew growth in a greenhouse environment can damage plants, infect soils and other growing mediums and create an unpleasant and even unhealthy environment for people.

The effects of greenhouse molds and mildews on people is highly varied and depends primarily on the sensitivity of individuals to mold spores. According to the CDC, those more sensitive to molds and mildews may experience stuffiness, eye irritation, skin irritation an wheezing when in the presence of varied types of fungi. People with more serious allergies may experience more severe reactions.

When investigating a fungal issue in a greenhouse environment, it is important to look for both signs and symptoms of excessive fungal growth. Symptoms include leaf spots, blight, cankers, rot, and damping off among other plant reactions a gardener may notice as a result of fungal infection. Observation of signs means actually seeing the fungus or fungi that are causing damage or disease. Have you noticed any of these symptoms or signs in your greenhouse?  Of course, it is best to prevent excessive fungal growth, so there is no need to investigate a problem in the first place.

The CDC doesn’t recommend routine mold sampling from affected environments because it is expensive and the reactions of individuals to fungi aren’t always specific. Additionally, there are currently no established standards for judging what is an acceptable quantity of mold or mildew in an environment based on human health concerns. If you are interested in paying for environmental sampling of molds, be sure to establish with your consultant criteria for interpreting test results, asking what they will do or what kind of recommendations they will make based on the results. When sampling molds, always consider the environment from which they are taken.

Mold and mildew can be a troublesome in a greenhouse, but fungal problems are actually fairly easy to prevent. The best way to avoid problems with mold and mildew is to maintain conditions that aren’t conducive to the growth and reproduction of fungi. Keep general and localized relative humidity below 85%, avoid over-watering, and be sure to adequately thin plants for good air circulation.  Tightly packed plants not only compete for light water and nutrients, they create an ideal localized environment for the growth and spread of fungi.  To learn more about adequate plant spacing, check out our blog post about thinning vegetable crops.

Keep crops thinned and well spaced

If you do see signs of a fungal issue in your greenhouse, ask yourself:

Could I be over-watering my plants, or getting the leaves too wet?

Are my plants far enough apart to allow for air movement?

Does my greenhouse have adequate ventilation?

Is the humidity in my greenhouse really high?

Greenhouse ventilation and good air circulation can be achieved by opening greenhouse vents and adding fans.  If your greenhouse is located in a humid location, a dehumidifier or air conditioning unit may also help to facilitate air movement and decrease humidity. Maintaining healthy soil or clean growing medium in a greenhouse is also key to preventing the growth and spread of fungi. In the event that mold and mildew do take hold, infected soil and plant material should be removed from the greenhouse and any other surfaces affected by fungi should be thoroughly cleaned.

Ultimately, fungal growth in any enclosed area where plants are grown is inevitable.  Prevention of fungal growth and proliferation is key to long term growing success in greenhouses. Armed with a good understanding of potential fungal issues, a good greenhouse set up and continued garden sanitation and maintenance, a greenhouse can be a comfortable and healthy environment for both plants and people.

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