Garden & Greenhouse


Humidity Control for Pathogen Prevention in a Greenhouse or Indoor Garden

Posted May 28th, 2019 by Garden & Greenhouse in ,

Pathogenic fungi can be very destructive to an indoor garden or greenhouse. Much like pest insects, pathogenic fungi are capable, under the right circumstances, of quickly destroying an otherwise flourishing crop. To effectively defend a garden against such an attack, indoor horticulturists and greenhouse hobbyists should know how to prevent, positively identify, and treat the most common fungi pathogens. As with most garden disturbances, prevention is the most powerful defense. However, chances are good that even the most diligent growers will experience a hiccup or two when cultivating plants indoors or in a greenhouse. The two most common fungi pathogens to plague indoor horticulturists and/or greenhouse hobbyists are powdery mildew and botrytis (fruit or flower rot).

Powdery Mildew

Powdery mildew is a fungal disease that can affect a wide variety of plants. Powdery mildew is a general term for the plant disease caused by multiple pathogenic fungi; all found in the order Erysiphales (a subcategory of the division Ascomycota).


Plants infected with powdery mildew look as if they have been sprinkled with white flour. Powdery mildew usually starts off as small, circular spots on the leaves, but can also be found on the stems or flowers. In some cases, powdery mildew can cause the leaves of a plant to twist, break, or become distorted. The white spots eventually spread and cover the majority of the leaf’s surface.


Although systemic fungicides are effective against powdery mildew, they should only be used on ornamental plants. For food crops or other consumables, the best treatment option for powdery mildew is some sort of organic-based fungicide. The most commonly used organic fungicides are sulfur-based fungicides, copper-based fungicides, neem-based fungicides, bicarbonate-based fungicides, botanical-based fungicides, and biological fungicides. Even when using an organic fungicide, it is of the utmost importance to follow the manufacturer’s application instructions for safety.


A key to preventing powdery mildew is to make sure the spores never enter a garden in the first place. Perhaps the most common way pathogenic fungi spores enter a garden is through the fresh air intake. By using an intake air filter, a grower can remove many of the spores and pest insects that could otherwise end up in the grow room. A HEPA filter on the ventilation’s intake can be a valuable tool to lessen the ability of powdery mildew spores to enter the garden. Fungi spores are microscopic and, even with intake filters, are almost impossible to stop from entering the grow space.

With this in mind, a grower should focus his or her attention on humidity control as another preventative measure. Maintaining proper atmospheric conditions will help prevent the humidity levels conducive for unwanted visitors. In layman’s terms, humidity levels are affected by the moisture content and the temperature in the garden area. This is why the temperature variance from the lights on cycle to the lights off cycle is an important factor to consider. Keeping the temperature variance between 10-15 degrees F from the lights on period to the lights off period will reduce the likelihood of condensation and unwanted humidity spikes. By controlling temperature variances, a grower automatically has more control over humidity and, therefore, can more effectively prevent pathogens. A general rule of thumb is to maintain a humidity level of 55% or under in an indoor garden or greenhouse.

Botrytis (Fruit or Flower Rot)

Botrytis is a necrotrophic fungus that can affect many different plant species. In horticulture, it is commonly referred to as bud rot, fruit rot, flower rot, or gray mold.


Botrytis mainly affects tender tissues, such as flowers, fruits, and seedlings, but can enter the plant’s tissue through pruning scars or other distressed or wounded tissue. Lower, shaded sections of a plant are usually the first to show signs of a botrytis infection. The first sign shown by a plant with a botrytis infection is a water-soaked, browned area. The distinctive browning is universal, regardless of the type of plant affected. After the initial browning, a silvery-gray fuzzy mat develops on or around the browned tissue. In extreme cases, or in cases where high humidity is prevalent, a brown, slimy substance can appear; this is actually the decimated plant tissue.


Botrytis-infected sections of a plant should be removed immediately in order to prevent it from spreading to other areas of the garden. If possible, bag the affected section of plant before cutting it. This should be done to limit the spreading of spores as the infected area is disturbed. After the infected sections of plant tissue have been removed, the rest of the garden should be treated with a biological fungicide. To prevent future outbreaks, it is a good idea for indoor and greenhouse growers to disassemble the grow room after the garden cycle and disinfect everything with a 5-10% bleach solution or a food-grade hydrogen peroxide solution. This will kill any remaining viable spores and reduce the chance of a future outbreak.


Keeping a clean grow room and removing any dying or dead plant material are good first steps for any indoor or greenhouse grower. In a sense, botrytis is an environmental disease. This means it can only develop when the environmental conditions are conducive to its growth. The prevention of botrytis is somewhat easier for indoor horticulturists because they have more control over the environmental conditions. Humidity is the biggest trigger for botrytis in an indoor garden. As long as the humidity is kept below 55%, botrytis is unlikely to develop.

The other contributing environmental factor is temperature. Botrytis can only germinate on damp or wet plant tissue in temperatures between 50-70 degrees F. However, once the fungus has developed, it can withstand a wider range of temperature and humidity. Botrytis grows most rapidly in lower temperatures paired with high humidity. The humidity levels in close proximity to the plants are generally much higher due to the plant’s transpiration processes. This is why air movement within the grow space is so important for maintaining proper humidity levels. To create good airflow, oscillating fans should be used to mix the humid air that is close to the plants, with the air in the rest of the room; this will help keep the room’s humidity uniform.

Optimal Humidity Range for Indoor Gardens and Greenhouses

As previously mentioned, maintaining proper humidity levels in an indoor garden or greenhouse is very important when trying to prevent pathogenic fungi. Put another way, if humidity levels are kept in check, the pathogenic fungi’s ability to establish is hampered. The optimal humidity range for indoor gardens and hobby greenhouses is 50-60%. Even when an indoor garden is climate controlled by a mini-split air conditioner, a dehumidifier may have to be used to maintain the optimal humidity level. Again, it is important to remember how the garden’s temperature also affects the relative humidity levels. Controlling temperature variances will reduce spikes in humidity. A grower who invests in an atmospheric controller, which can be used to automate fans, air conditioning equipment, and dehumidifiers, will have a much easier time maintaining the optimal temperature and humidity. The controlled, consistent temperature and humidity levels are a strong defense against pathogens.

Air Purification Devices

In addition to atmospheric control devices, which help automate the temperature and humidity in the garden, growers who wish to take pathogen prevention one step further can implement a stand-alone air purification device. When combined with an air intake filter and an atmospheric control system, a stand-alone air purification device can give even more protection against pathogenic fungi to an indoor garden or greenhouse crop. Essentially, these devices have an internal fan that circulates the air within the grow space; purifying the air in the process. The technology used in these devices can differ, but, most commonly, they either generate negative ions or utilize some sort of UV lighting. Some of the UV lighting systems will actually produce ozone in the purification process. Devices that produce a detectable amount of ozone can cause the ozone levels to build up in the grow space. This can damage essential oil production or, in extreme cases, become harmful to the gardener. Both the size of the grow room and the amount of detectable ozone should be carefully considered to ensure the air purification unit will be safe for the particular garden application.

Perhaps the biggest draw to indoor and greenhouse gardening is the heightened level of control over the environment. That being said, a grower who fails to control his or her garden’s climate properly will likely have a continuous battle with pest insects and/or fungi pathogens. Ideally, a garden’s temperature (and temperature variances) would be controlled by an atmospheric controller. When the temperature of an indoor garden or greenhouse is automated, it makes it that much easier to control the relative humidity. However, it must be remembered that, plants are made mostly of water and go through a natural transpiration process as they grow. In other words, the plants themselves naturally increase the humidity level in an enclosed area as they grow.

Without proper air movement in the grow space, the humidity levels close to the plants will be much higher than the ambient air. For atmospheric equipment to operate efficiently and effectively, the humidity of the room must be uniform. This is why an ordinary oscillating fan is such a crucial piece of equipment. If the ventilation system or air conditioning unit cannot, on its own, handle the increased humidity produced by the plants, a dehumidifier should be implemented to keep the humidity levels uniform. When humidity levels are kept in check, pathogenic fungi cannot establish. This is why the ultimate prevention and protection against these pathogens is humidity control. Horticulturists who prioritize uniform humidity levels and automate control over the garden’s temperature and humidity will be better equipped to prevent pathogenic fungi, such as powdery mildew and botrytis.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at

Related Articles

Deep Cleaning the Greenhouse

Increasing Microbial Population in a Soil or Medium

Ways You Can Protect Your Garden and Keep It Healthy

What Soil Organisms Actually Do

Click Here for  a Free Garden & Greenhouse Magazine Subscription

Click Here for a Free Garden & Greenhouse Email Newsletter Subscription