There isn’t much that is more disheartening than a pathogen attack on a flourishing marijuana garden. All cannabis cultivators will eventually encounter a pathogen in his or her garden. Fungi, bacteria, and viruses are all pathogens that can utterly damage a marijuana garden if left unchecked. Although fungi are the most common pathogens to cause headaches for cannabis growers, horticulturists need to be aware of all the potential threats to their gardens, how to properly prevent them from taking hold, and how to combat them when necessary. In most cases, the introduction of pathogens into the garden is preventable. Armed with a little knowledge, most growers can prevent or rectify a problem before it becomes catastrophic.
The most common pathogens to affect cannabis crops are fungi. There are many different fungi that can affect cannabis, but the most common fungal pathogens found in a marijuana garden are pythium, powdery mildew, and botrytis.
Pythium, the major contributor to root rot, is the most common root disease to plague the cannabis horticulturist. This damaging, parasitic root fungus is capable of quickly spreading and destroying otherwise healthy crops. Pythium thrives in anaerobic conditions and dies off in well oxygenated conditions. Planting containers that stay saturated eventually become stagnant. This stagnant environment, which is depleted of much of its dissolved oxygen content, is perfect for pythium. Put another way, overwatering is a common cause of pythium. Pythium is also common in hydroponic systems that have an insufficient amount of dissolved oxygen in the nutrient solution.
The first signs of pythium can mimic a nitrogen deficiency, with the lower leaves turning yellow. Eventually these leaves may also have brown spots, again mimicking other nutrient deficiencies. The entire marijuana plant will ultimately appear wilted and, in extreme cases, will die. Upon inspection of the roots, a marijuana grower can quickly identify pythium. Brown, slimy roots are a dead giveaway that pythium lurks in the grow room.
If caught early enough it is possible to recover from a pythium attack. There are many oxygen boosting additives available in hydroponic stores. Diluted hydrogen peroxide is one way growers can increase the dissolved oxygen content, but it should be used sparingly. A hydroponic grower must maintain a consistent temperature of 65-70 degrees F in the reservoir. When the temperature of the water rises, the dissolved oxygen content decreased. Pythium is notorious for showing up in hydroponic systems when the water temperature reaches above 75 degrees F. Hydroponic gardeners can use a water chiller to maintain temperatures in the desired range.
Powdery mildew is another common pathogen that causes trouble for cannabis growers. Powdery mildew spreads quickly and can destroy a healthy garden in a matter of days. Powdery mildew can be detected by a white, powdery coating on leaves and shoot tips. This fungus spreads fast via airborne spores.
Powdery mildew is a fungus that can only thrive when the atmospheric conditions are right. Keeping temperature and humidity levels in check are the best preventative measure against powdery mildew. Air filtration devices on the ventilation system’s intakes are also a good way to reduce the likelihood of powdery mildew. If powdery mildew is contracted, sulfur burners or organic-based fungicides can be effective during the vegetative and early stages of flowering. Treating a powdery mildew outbreak in the late stages of flowering can be very difficult for a cannabis grower. Most of the time, when contracted in the later stages of flowering, a grower is forced to harvest early and cut his or her losses.
Botrytis (bud rot) 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. The first sign of a botrytis infection is a water-soaked browned area. After the initial browning, a silvery-gray fuzzy mat will develop on or around the browned tissue. Upon closer inspection, the fuzzy mat will look like thousands of tiny balls. These tiny balls are actually the spores which can 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 in cases where high humidity is prevalent, a brown, slimy substance can appear, which is actually the decimated plant tissue. Outdoor growers need to pay extra close attention to their plants during the 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 a botrytis infection.
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. For the indoor grower, the best prevention is to maintain a lower humidity in the growing environment, especially during the fruiting and flowering stages. 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.
Outdoor growers must rely more on Mother Nature and do not have as much control over temperature, humidity, or airflow as indoor horticulturists. However, 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 continuously shaded or does not get adequate airflow. This will reduce the chance of botrytis developing on the lower sections of a plant and spreading 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; a perfect breeding ground for botrytis.
Sections of a cannabis plant that have contracted botrytis need to be removed immediately in order to avoid the pathogen spreading to other sections of the garden. If possible, bag the infected section of the plant before cutting it. This is to reduce the possibility of spreading the spores as the area is disturbed. In fact, all sections with botrytis should be slowly and carefully removed to reduce spreading the fungus. Make the cut at least two to four inches below the infected area so that all of the botrytis is removed. It is a good idea for indoor and greenhouse growers who have experienced a botrytis infection to disassemble the 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 future infections.
Although much less likely than a fungal attack, pathogenic bacteria can also prevent a marijuana garden from flourishing. Bacterial blight caused by the bacteria pseuedomonas syringae is the most common bacterial related problem for cannabis growers. The symptoms are similar to those of brown leaf spot fungus. Pathogenic bacteria infections are almost always misidentified as a fungus or other pathogen.
Preventing pathogenic bacteria is a marijuana gardener’s best defense. Sanitizing gardening equipment and keeping an organized, clean grow room will go the farthest in preventing a pathogenic bacteria from taking hold. Air filtration devices with UV lights or a photo-catalyst will remove bacteria from the air and reduce the chance of the plants getting an infection. Plants that have contracted a bacterial infection should be removed from the garden to reduce the chance of the infection spreading to other plants.
Most cannabis growers who have experienced plant viruses in their gardens were probably completely unaware it happened. This is because viruses rarely kill cannabis plants. Viruses can only exist and replicate within a living plant. However, viruses can seriously reduce the crop’s yield. Viruses can infect every part of the plant’s living tissue which means pollen and seeds can carry viruses into the next generation of plants. Clones taken from mother plants with a viral infection will also have that virus.
Unfortunately, once a virus is acquired, it is nearly impossible to eradicate. Prevention is the best way to ensure a crop stays protected from viruses. Most plant viruses are transmitted by a vector. Cannabis growers need to be concerned about the two vectors that commonly transfer viruses: people and insects. Growers who do not clean gardening tools between uses run the risk of spreading a virus between plants. This is just one of the reasons why it is so important for cannabis growers to always disinfect pruning shears and other tools that cut live tissue. Although people can be vectors for plant viruses, the most common culprits are insects. Pest insects that feed on the live tissue of the plants can be vectors for viruses and can quickly spread a virus around the entire garden. Viruses are just another reason to make sure pest insects don’t make their homes in a marijuana garden.
Cannabis plants have natural enemies that marijuana horticulturists need to be aware of. Pathogens, including fungi, bacteria, and viruses, can reduce yields, hinder growth, and, in extreme cases, cause death. For all pathogens, prevention is the best defense. However, once a pathogen has made it into the garden, a grower must make quick decisions regarding treatment or plant removal. Experienced cannabis growers know that a pathogen attack may mean cutting their losses to negate further damage. After experiencing a pathogen attack, a cannabis grower should diligently and thoroughly disinfect his or her growing space and any equipment used for cultivation. Marijuana growers armed with the knowledge of prevention and treatment of pathogens will be more capable of correcting the garden when a pathogen attempts to steer it off course.
Eric Hopper is a Professional Marijuana Grower Senior Editor.
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