How to Identify Bud Rot, Mold, Mildew in Cannabis Plants

Growing cannabis is an extensive process that requires time, energy, and resources. However, bud rot, mold, and mildew can occur at any stage of growth for indoor and outdoor plants. 

Mold spreads at a rapid pace and can destroy healthy plants in as little as a week. To avoid such a disastrous scenario, growers need to know what signs to look out for and prevent it for successful growth. 

Identifying bud rot, mold, and mildew

Bud rot, also called botrytis cinerea or gray mold, is one of the most common diseases growers face. Bud rot develops in the heart of the cannabis bud, usually in denser buds. In the right conditions, the rot spreads to other areas, and in some cases, transmits spores to other plants. 

The first signs of bud rot are the discoloration of flowers and short leaves protruding from them. Buds may also appear colourful, dark, or dried.

Bud rot affects small areas in indoor cannabis plants and appears as patches on outdoor plants, especially after several days of rain. You’ll notice dead spots develop within a day or two. 

The first signs are small blisters, bumps, or pimples. Fluffy white mold follows quickly behind in the middle or side of the buds. The mold quickly changes to gray or brown. 

The second indicator is the pungent rotting smell, which occurs in the late stages. 

Advanced rot appears as pale, powdery mildew and turns slimy as the rot progresses. Powdery mildew targets young cannabis leaves.

Fully infected buds separate easily to reveal a dusty interior–the dust is spores.


Pathogens affecting cannabis plants 

In the study, Pathogens and Molds Affecting Production and Quality of Cannabis sativa L., Punja et al. confirm plant pathogens infecting marijuana plants reduce growth in the roots, crown, and foliage. Mold (fungi) infecting buds (inflorescences) during development or after harvest reduces product quality. 

Root-infecting pathogens like Fusarium oxysporum, Pythium dissotocum, and Pythium aphanidermatum cause root browning discoloration of the crown and pith tissues, stunting, yellowing, and in later stages, plant death. 

Powdery mildew, commonly caused by Golovinomyces cichoracearum, occurs on the foliage. 

Penicillium is the primary culprit for bud rot. Endophytic fungi such as Chaetomium, Trametes, Trichoderma, Penicillium were found in the crown, stem, and petiole tissues. 

Air samples from indoor environments revealed the presence of Penicillium, Cladosporium, Aspergillus, Fusarium, Beauveria, and Trichoderma. 

Causes of mold

There are several ways cannabis plants can be infected with mold spores. Outdoor plants come into contact with spores through wind, rain, pollinators, or even clones within the same vicinity. Indoor plants are at risk due to spores in the air and poor ventilation. 

In the same study by Punja, research showed mechanical trimming of buds caused the quicker spread of the Penicillium species, presumably by providing entry points through wounds. Airborne pathogens and mold spores also enter through wounds on roots, stems, and bud tissues. 

Mold thrives in wet or humid conditions; a moderate, dry environment helps prevent rot from spreading. Grow Weed Easy provides a great guide on humidity levels for healthy cannabis growth. High humidity, warm temperatures, poor ventilation, and dense foliage make plants more susceptible to infection. 

Can moldy buds be saved? 

At the first signs of fungal infection, the bud, or infected areas should be removed and destroyed immediately. The mold makes it dangerous to consume. If consumed, risk factors include respiratory infections and lung diseases. 

Destroying the infected parts will help prevent further spread to the plant and surrounding crops. Use sterile tools when removing infected areas and clean them often to avoid spreading spores. 

If caught early, the rest of the plant can usually be saved. Unless the entire plant has been infected, it should be removed from the premises and destroyed to avoid contaminating other vulnerable plants. 

How to prevent rot

The best way to prevent any rot, mold, or mildew growth is to control the humidity level and environment around your plants.

Brian Spigott of One Eye’s Weedery had this to say in regards to preventing bud rot: “This is difficult for outdoor growers where the climate is beyond their control. Cultivators can do their part by shielding their plants with a tarp to reduce moisture or placing them in a greenhouse enclosure. Plants should be planted a good distance from each other–leaves should not touch. 

Shake off any dew on your plants during humid days or after several days of rain. During colder days, remove frost as best you can to avoid bud rot when it melts.”

Indoor growers can take several preventative measures to help. Fans, dehumidifiers, temperature control systems, and good filtration are powerful tools to keep humidity low and maintain proper temperatures. 

Maintain a temperature of 21 to 25°C at 40 to 50% humidity. Night temperatures can be 5 degrees lower to let the buds cool down. 

Cultivators can use neem oil, a natural fungicide, to ward off mold. Dilute it and spray it on the plant. 

Outdoor and indoor plants benefit from regular pruning, proper distancing, and adequate watering. Trimming improves airflow underneath the canopy and reduces moisture collection. 

Rot during the drying process

Infections can still occur after harvesting. To prevent any rot, check colas for rot. Anything infected should be discarded immediately. 

For minimal moisture, harvest during dry days. Afterward, pull off fan leaves and trim the buds. When hanging to dry, place each leaf a good distance away for better airflow. 

Leave your harvest in a dark room around 20°C with 45-50% humidity. Slower drying lets the THC convert smoother but if you find mold, increase temperatures and lower humidity to prevent quicker spread. Trim infected areas immediately. 

If you find powdery mildew infection, spray the area with a dilute solution of 3% hydrogen peroxide and remove the mildew. 

Combatting bud rot 

Bud rot acts quickly, ruining a hard day’s work within, well, a day. Cultivators should monitor their plants daily to check for any signs of growth. 

Asides from temperature and humidity surveillance, remove any moisture build-up found on and around the plant. The proper tools will help prevent future infections. Outdoor plants should be protected with a tarp or greenhouse enclosure while indoor plants require fans, temperature monitors, and proper ventilation to prevent spores from spreading. 

The devil is in the details, as people say. Pay close attention to anything that looks out of the regular when you inspect your buds. The earlier you find any signs of rot, the sooner you can act to save your crops and weeks of work. 

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