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Avoid The Heartache Of Bud Rot And Powdery Mildew (part 2)

Avoid The Heartache Of Bud Rot And Powdery Mildew (part 2)

Ed Rosenthal photo by Lizzy Fritz

In part II of my “resolutions” series, I address something no grower wants to experience – powdery mildew that ultimately gives rise to bud rot. Only by truly understanding how your enemies operate can they be defeated. Don’t let it happen to you! Let 2019 be the year you take steps to avoid powdery mildew.

“Fungus grows when it finds the right moisture level, temperature, acidic conditions, and a reliable source of food.”

Millions of gardeners all over the world suffer from the heartache of bud rot and powdery mildew, just as they are about to harvest, although these diseases can strike marijuana plants at any stage. A garden’s susceptibility to disease is often traceable to environmental conditions such as temperature, moisture, light, airflow, pH, and nutrients. Fungus grows when it finds the right moisture level, temperature, acidic conditions, and a reliable source of food. There are several things that you can do to protect plants from these diseases.

Gray mold on cannabis bud
Gray mold on cannabis bud

Gray Mold

[Botrytis Cinerea, Botryotinia Fuckeliana (Bud Rot)]

Gray mold spores float in the air so they are always in contact with plant leaves, their possible hosts. However, they require a moist, acidic environment to thrive. The infected area first appears soggy and browned. Then the mold develops a silvery-gray, fuzzy covering composed of thousands of grape-like clusters of spores. The cool, wet conditions the mold prefers occur most often in early spring and fall harvest time outdoors, or in under-ventilated and humid areas indoors.

Botrytis cinerea often develops in the tight floral clusters of cannabis where airflow is restricted. Mold in the center of a bud may not be immediately apparent, and a nice-looking large bud may in fact be “hollow” due to mold damage. This gives rise to its common name “bud rot,” although the proper name, Botryotinia fuckeliana, seems appropriate for this pathogenic fungus.

Close up of Botryotinia Fuckeliana, aka Bud Rot, on cannabis bud

Outdoors, thick buds ripening in cool, damp weather can be affected by an outbreak. Wounded or diseased foliage is also a suitable host for the mold. Indoors, poor air circulation results in still pockets where dampness collects, encouraging fungal growth.

B. fuckeliana is a parasitic fungus that can destroy a plant or buds when left unchecked. It needs no light to grow and prefers cool, damp, acidic, underlit locations. If the spore lands in a moist acidic spot it will sprout and grow strands of hyphae. As these strands fill in the area, the colony becomes easily visible.

“Botrytis cinerea’s common name is “bud rot,” although the proper name, Botryotinia fuckeliana, seems appropriate for this pathogenic fungus.”

The mass of hyphae strands develops into mycelia—the gray-white thread covering that is the obvious indication of a colony. The mycelia absorb nutrients from the host-plant material, destroying leaves and buds in the process. Enzymes are released that break down the plant matter into a nutrient slime that the mold can absorb. This causes the most damage.

As long as the conditions remain favorable and there is plant-host material, it continues to spread. At the same time, it grows reproductive structures that release airborne spores. A B. fuckeliana spore can germinate and mature to produce more spores in a matter of days.

Close up of Botrytis Cinerea on marijuana bud

A single colony releases billions of spores, and each is capable of starting a new colony. If a spore lands in a suitable environment, it will germinate and continue the cycle. Spores that land in an environment not conducive to germination remain dormant until the environment changes. (In a dry environment they can stay dormant for years.)

“ A B. fuckeliana spore can germinate and mature to produce more spores in a matter of days.”

Spores can enter the indoor garden through the ventilation system, on clothing, or on contaminated plants. Proper housekeeping can reduce the number of suitable host sites in the garden, but preventing all the spores from entering the garden is unrealistic in most situations.

How To Control Gray Mold

Photo: OG Growlite reflector Inside a grow tent

The best way to avoid gray mold is to not give it a suitable home. Since the mold develops only in overly moist conditions, the first line of defense is controlling humidity to keep moisture levels under 50 percent.

Gray mold is likely to be most severe within the thickest part of the canopy where air circulation is poorest. Thick outdoor plants can be spread with twine or netting to improve airflow. Remove interior and weak branches to improve circulation. Fans assist in mold prevention by removing pockets of high-humidity air surrounding the leaves and flowers.

“The best way to avoid gray mold is to not give it a suitable home.”

Don’t foliar feed plants during the last 30 days of flowering because moisture may become trapped inside the buds. Foliar feed early in the day during vegetative and early flowering so moisture on leaves or buds has a chance to dry before darkness. Watering the garden at the beginning of the light period or early in the day will give any excess moisture on the leaves an opportunity to evaporate.

Cut out and remove the affected portions of the plant or bud, or remove it entirely to reduce potential host sites. Destroy any infected material, and sanitize any tools to prevent them from spreading infections. Moldy weed should be thrown out, although unaffected buds from the same plant may still be consumed.

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After removing all signs of infection, correct the environmental conditions. If circumstances do not allow the environment to be controlled, use preventive measures such as applying Bacillus subtilis (QST 713, known by the brand name Serenade), or a fungicide like Ed Rosenthal’s Zero Tolerance Herbal Fungicide, which includes potassium bicarbonate. Fungicides are more effective when used preventively, so apply at the beginning of the wet season. Some gardeners use sulfur to treat at-risk or infected plants. It can be applied as a powder, in sulfur dioxide mats, or from sulfur pots. Potassium bicarbonate prevents mold germination; sodium bicarbonate (baking soda) can be used as a substitute. Cinnamon, seaweed, D-limonene (citrus oil), and chamomile sprays have antifungal properties and will help control an outbreak.

Powdery Mildew

Powdery mildew on marijuana plant
Powdery mildew on marijuana plant

Powdery mildew (caused by several fungi) is the bane of modern marijuana growers. It looks like confectioners’ sugar dusting leaves. At first it appears as an irregular circle on just a small portion of the leaf. But it quickly spreads onto the surrounding tissue, covering the entire leaf. Colonies soon develop on the surrounding vegetation and in other areas of the garden. Powdery mildew is most likely to attack young leaves, up to two or three weeks old.

“Powdery mildew (caused by several fungi) is the bane of modern marijuana growers.”

The plant becomes infected when a spore, called a conidium, lands on a leaf and germinates. It soon grows an appressorium, a swollen structure that forms at the tip of a strand of hyphae that attaches it tightly to the leaf surface. Using the appressorium as a guide tube, the fungus pierces the plant’s cell wall and membrane and inserts a haustorium, a projection from the hyphae used to suck nutrients from inside the plant cell. The haustorium sucks up plant nutrients and sends them to the fungus, weakening the leaf and slowing growth. Within a week the fungus produces tiny mushroom stalks that release millions of spores, ready to infect more leaf surfaces. The fungus also produces another kind of structure, a cleistothecium, which contains spores. It overwinters outdoors and may also hide in a greenhouse or growroom after the crop has been harvested.

There Are At Least Two Different Fungi That Cause Powdery Mildew In Cannabis

Powdery mildew on marijuana plant
Powdery mildew on marijuana plant


L. taurica is more likely to attack warm gardens. It prefers a temperature of about 77°F (25°C) and can germinate in low humidity, but functions well in the 40 to 60 percent relative humidity usually found in indoor gardens. It is inhibited by moisture such as water spray, which destroys its spores.


S. macularis prefers a cooler temperature, but the virulent race found in both indoor and outdoor gardens today is more tolerant of heat and bright light. It thrives in moderate humidity and its spores can live in water for short periods.

“Within a week the fungus produces tiny mushroom stalks that release millions of spores, ready to infect more leaf surfaces.”

In the unheated greenhouse and outdoors, the temperature preferences of both L. taurica and S. macularis overlap, so plants are susceptible from around 60 to 85°F (15 to 30°C). They do quite well in moderate, rather than especially humid, weather.

Spores of both species are airborne. When they land on suitable vegetation, they germinate, and start robbing the plant’s nutrients. The S. macularis spores also migrate with moving water such as drops of water falling from leaf to leaf, or blown by the wind to other plants.

How To Control Powdery Mildew

Image: Air cooled lights

Because of this pesky fungus’ mode of transmission, it can be hard to prevent an outbreak, and it is challenging to control once an infection begins. It is in some ways more noxious than insect pests because it ruins any plant matter it touches, rendering it useless. However, infected buds can be processed for their contents to make concentrates such as water hash.


Indoors, it is especially important to quarantine new plants before introducing them to the garden. Air filtration helps prevent powdery mildew from entering the room in the airstream. If there is any uncertainty about a location’s history with powdery mildew, sterilize the area and appliances before starting a new crop.

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An additional precaution for powdery mildew is to add UVC light to the garden. With ventilation, all incoming air should pass through the light. UVC light is germicidal, and delivers a fatal blow to all airborne fungal spores that pass under it.

“Air filtration helps prevent powdery mildew from entering the room in the airstream.”

Once an indoor garden shows signs of powdery mildew infestation, there are many different approaches to eradicate it. If it is caught early, the gardener can prune away affected plant parts. It is important to disinfect tools used in the removal of infected plants and to carefully handle the pruned material in order to avoid accidentally spreading spores.


Powdery mildew is also controlled through close attention to temperature. Both types of fungi that cause powdery mildew are sensitive to heat and stop growing when temperatures range over 90°F (32°C). They quickly perish when the temperature rises to 100°F (38°C). Introducing a temperature spike to kill powdery mildew can be implemented, but if it is done improperly it could be detrimental to the plants.

“By using the preventive measures and solutions, you should be able to control these debilitating diseases that would destroy your buds. You will minimize losses even if the environmental conditions are favorable to the pathogens.”


Adjust the pH of the leaf surface so it is alkaline, with a pH over 8, using potassium/sodium bicarbonate or pH Up. This makes the leaf surface inhospitable for powdery mildew.

Silica helps strengthen the stem, serves as an alkaline adjustor, and has natural fungicidal properties. There are many ways to increase silica in the soil or garden medium, such as adding diatomaceous earth, greensand, pyrophyllite clay, and high-silica fertilizers such as Pro-TeKt.


There are formulations that are sprayed on leaves, and elemental sulfur can be used in burners to create sulfur dioxide. Sulfur is more effective at preventing the formation of powdery mildew than treating an infection. When used improperly or at too high a concentration, it causes leaf damage. Still, it is an effective control method.

Powdery Mildew fungus on cannabis plant
Powdery Mildew fungus on cannabis plant


Foliar sprays can be bought or made to control powdery mildew. Make your own using cinnamon oil or tea, copper, garlic, herbal oils, hydrogen peroxide, D-limonene (citrus oil), milk, neem oil, and vinegar. Some gardeners recommend alternating these treatment options for best results. A spray of 10 percent milk to 90 percent water is also helpful in preventing spore germination.


Serenade, when applied weekly, puts powdery mildew into remission. Another strain of bacteria, Bacillus pumilus (QST 2808, known by the brand name Sonata), produces a compound that disrupts fungal development. Sonata does not eradicate powdery mildew completely, but it works well in combination with Serenade or other solutions to greatly reduce an infection. Zero Tolerance Fungicide and Serenade work synergistically to combat powdery mildew and prevent its recurrence.

Both gray mold (bud rot) and powdery mildew can damage marijuana plants after all your hard work in the garden. Believe your eyes. When you see bud rot and powdery mildew, act quickly and do a thorough job of treating your plants. (For example, when using fungicides, make sure that all infected surfaces are covered: get under leaves and deep inside the foliage, as necessary. It helps to use a spray wand.) However, preventive maintenance is always the best defense.


ABOUT ED ROSENTHAL “THE GURU OF GANJA” Ed Rosenthal is a leading cannabis horticulture authority, author, educator, social activist, and legalization pioneer. He writes the much loved and long-running cannabis column, ‘Ask Ed’, and his seminal cannabis growers guide, ‘Ed Rosenthal’s Marijuana Grower’s Handbook’ has become the definitive marijuana cultivation resource, inspiring millions to learn the best marijuana cultivation techniques. His upcoming book, Beyond Buds, Next Generation, is the indispensable guide for anyone looking for perspective on the next generation of cannabis innovation. You can order your copy HERE today.

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