There are more than 50 different species of botrytis, also known as gray or brown mold, or bacterial soft rot, which can wreak havoc on numerous plants, including ornamentals, fruits and vegetables.

Asparagus, beans, carrots, celery, eggplants, grapes, lettuce, peppers, raspberries, strawberries, and tomatoes are just some of the plants commonly affected by the dreaded botrytis.

Botrytis is a cool-season disease that affects both outdoor and indoor gardens. It is common in every climate and geographical location, aside from possibly the Arctic and Antarctic regions.

This destructive fungus can quickly destroy an otherwise healthy garden due to its ability to spread rapidly. In fact, many gardeners refer to this fungus as bud rot or fruit rot because of its ability to quickly rot those sections of the plant.

Both indoor and outdoor horticulturists should be on the lookout for botrytis, especially during the cooler months of the year.

A better understanding of identifying the conditions favorable to botrytis, and being able to catch any infection in the early stages, gives growers a fighting chance against one of the worst pathogens to plague gardeners.

Identifying Botrytis

Botrytis mainly affects tender tissues, such as flowers, fruits, and seedlings, but it can enter the plant’s tissue through pruning scars or other distressed or wounded tissues. 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 an infection is a water-soaked browning area. The distinctive browning is universal, regardless of the type of plant affected. Once a grower has seen this distinctive browning, it becomes fairly easy to identify later.

After the initial browning, a silver gray fuzzy mat will develop on or around the brown tissue. Upon closer inspection, the fuzzy mat will look like thousands of tiny balls.

These tiny balls are actually spores that fly up like dust if the area is disturbed. The rest of the plant may show signs of illness such as yellowing leaves or buds.

In extreme cases, or when high humidity is prevalent, a brown, slimy substance, which is actually decimated plant tissue, can appear. The time of the year will play a part in the identification of the botrytis beast.

As mentioned earlier, botrytis commonly occurs during the cooler parts of the season. Growers should be on the lookout during fall and winter months, especially in a greenhouse or indoor garden where fresh air for the garden is taken from the outside.

Outdoor growers need to pay extra close attention to their plants during late summer and early fall.

Botrytis commonly affects outdoor crops when temperatures turn cooler and rain is more prevalent. Moisture from rain, artificial waterings and dew can all accelerate the growth of this fungus.

Preventing Botrytis in Your Growroom

As with many garden pathogens, prevention is the key to avoiding botrytis. Keeping a clean room and removing any dying or dead plant material is a good first step for any grower. In a sense, botrytis is an environmental disease, meaning it can only develop when the environmental conditions are conducive to its growth.

Prevention of botrytis is easier for indoor horticulturists because they have more control over environmental conditions.

Humidity is the largest contributing variable to 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 to 70°F. However, once the fungus has developed, it can withstand a larger range of temperature and humidity.

Botrytis grows most rapidly in lower temperatures with high humidity. For the indoor grower, the best prevention is to maintain a lower humidity in the growing environment, especially during fruiting and flowering stages.

To be extra safe, an indoor grower should keep temperatures on the warmer side and monitor the dark-cycle temperature.

A heater may be needed during the dark cycle to minimize the temperature variance and reduce the possibility of condensation.

A ventilation system with atmospheric controllers allows an indoor grower to ensure environmental conditions are in check and are not inviting infection.

Oscillating fans that create good air flow within the room will help keep the room’s humidity uniform.

Another way an indoor grower can prevent botrytis is by using a filtration system or other air purification system. HEPA filters enable growers to filter out many spores commonly found in the air.

This will greatly reduce the chance of developing botrytis and other pathogens. Be sure to place the filter in a bag and remove it from the growing area before gently removing it for cleaning.

Filters trap many spores that are still viable and, in some cases, growers inadvertently expose their gardens to all sorts of nasty things when they remove the intake filter for cleaning.

Air purification systems offer another way for growers to eliminate spores from the growing space. Photocatalytic air purifiers are the best option for indoor growers looking to eliminate air-borne molds because they completely destroy the spores and do not trap any viable pathogens.

Read More: How to Maintain Good Grow Room Air Quality Using Air Purification Devices

If you choose to use a photocatalytic air purifier, be sure to note if it produces ozone. High levels of ozone can be harmful to people and plants when it becomes concentrated in an enclosed area like a growroom. This can be counterproductive to the plants and dangerous to the grower.

Preventing Botrytis in Your for Greenhouse

During the late summer and early fall, greenhouse growers should use a dehumidifier and/or heater during the night to minimize the temperature variance and reduce humidity.

A large temperature variance from night to day can cause significant condensation, which raises the environment’s humidity and could potentially promote botrytis growth.

In most cases, heating the greenhouse at night will not be necessary until the last month or so of the growing season.

Greenhouse growers should continuously monitor temperature and humidity levels so they can, if necessary, make the appropriate corrections to the atmospheric conditions to better avoid a botrytis infection.

If the greenhouse is relatively sealed, a HEPA filter can be used on the air intake to reduce the chance of pulling in spores from outdoors. A photocatalytic air purification system can also be used in a relatively sealed greenhouse to eliminate air-borne pathogens, including botrytis.

Pruning the lower branches that are shaded will allow for better air flow and reduce the chance of developing botrytis as well. Oscillating fans that create adequate airflow within the greenhouse are imperative to preventing botrytis and other pathogens prone to high humidity conditions.

Preventing Botrytis in Outdoor Gardens

There are a few preventative measures outdoor growers can implement to avoid a problem with botrytis.

One good preventative measure is to thin out vegetation that is being continuously shaded or that does not get adequate airflow. As with greenhouse crops, this will reduce the chance of botrytis developing on the lower sections of a plant, which would eventually spread to other places in the garden.

Outdoor plants should also be spaced far enough apart so there is adequate ventilation between the plants. Crowded plants will overlap each other and create pockets that harbor moisture and potential fungal infections.

Another preventative measure is to create temporary shelters placed above the plants during late summer or early fall rain showers to help reduce the amount of moisture on the fruits or flowers.

These can be as simple as draping some painter’s plastic over the plants during a heavy rain storm or more complex, such as large stakes placed around the perimeter of the plants that can support a temporary rain shelter.

Outdoor growers with irrigation systems in place should avoid overwatering when conditions are favorable for botrytis.

Treatment Options for Botrytis Outbreaks

Once a garden has developed a botrytis infection, it is imperative immediate action is taken. When conditions are favorable, botrytis has the capability of wiping out an entire crop within a matter of days.

Sections of a plant infected with botrytis need to be removed so the infection doesn’t spread. If possible, bag the section of the plant with the botrytis before cutting it to reduce the possibility of spreading the spores as you disturb the area.

In fact, all sections with botrytis should be slowly and carefully removed to reduce spread. Make the cut at least 2 to 4 in. below the infected area so all infected parts are removed. Be sure to sterilize the pruning shears in rubbing alcohol after each cut and before they are used again anywhere else in the garden.

Once the infected sections of plant tissue have been removed, the rest of the garden can be treated with a biological fungicide. It is difficult to completely rid a garden of botrytis once it has developed.

All a grower can do is carefully cut the infected sections out and monitor the rest of the garden closely. For many growers, a botrytis infection means they have to harvest early to cut their losses.

As previously mentioned, a photocatalytic air purification system can kill the spores. Even with a photocatalytic air purifier, it is important for the grower to carefully remove any infected areas to reduce the chance of the fungus spreading.

It is a good idea for indoor and greenhouse growers who have experienced an infection to disassemble the room after the garden cycle and disinfect everything with a 5 to 10% bleach solution or a food-grade hydrogen peroxide solution.

This will kill any remaining viable spores and reduce the change of future infections.

Botrytis is a horrible fungus that can turn a season’s worth of hard work into a mess. Botrytis spores are virtually everywhere, so prevention is the key to keeping this horrendous fiend at bay.

Sterilizing equipment, controlling environmental conditions, and implementing air filtration or purification devices are a grower’s best defense.

Want more? See Maximum Yield's full collection of grow room sterilization articles.