That perfect environment that you worked so hard to curate for your high value crops, is also perfect for the development and nurturing of a wide range of diseases that can appear in your veg room. Many diseases of cannabis do quite well in the same warm and moist environment that your girls are thriving in.
It doesn’t help that most plant species have on their surface or in the media they are grown in, microbes of common diseases, just waiting to be activated. Worse yet is that many of these species have evolved to have some resistance to fungicides and other types of treatments. But being present is not a guarantee that they will actually emerge and cause damage to your plants. It is important to know what some of the more common diseases of cannabis are and how to prevent or treat them if and when those pathogenic microbes decide to “wake up”.
Cannabis plant diseases may be distinguished by either the type of pathogen that they are caused by, or they may be categorized by the area of the plant that is being affected such as roots, leaves, flowers, or other tissue. Further classifications may be made by when in the life cycle of cannabis the diseases affect, such as with damping-off disease which affects plants in their early seedling stage or post-harvest mold which sets in as the name implies, after the flower has been harvested.
Regardless of how they are classified, there are several diseases that are more common than others and it is important to be able to conduct disease identification in order to know exactly which cannabis plant disease is affecting your plants. Disease identification is the most important step in treatment. Let’s take a look at how to identify and treat some of the more common fungal, cannabis plant diseases.
Botrytis
One of the most common of all fungal pathogens which has caused disease to a wide range of crops of all kinds for probably as long as humankind has been growing crops on a large-scale is Botrytis cinerea. Botrytis cinerea causes several different types of issues and damage profiles on cannabis depending on the stage of growth. It can in fact affect cannabis plants anywhere except on their root systems.
On seedlings, it is one of the causes of Damping-off disease. This occurs post germination and usually appears suddenly. Seemingly healthy seedlings collapse, and wither rapidly. Bud rot, which is often called either gray mold or Botrytis blight, can occur as your plants are beginning to transition from the Veg phase to the bloom phase. As they begin to form buds this disease will appear as gray specks and buds will shrivel and fail to develop.
Fungal pathogens like Botrytis cinerea, thrive in moist environments. One of the best ways to prevent its appearance and to lessen its impact is to maintain proper humidity and temperature levels in the greenhouse or growing area and to make sure that there is sufficient air circulation. There are hundreds of humidifiers, dehumidifiers, and circulation fans on the market that are specifically designed for the greenhouse and grow room environments which will help to keep the incidence of Botrytis cinerea, and other fungal pathogens down immensely.
Other methods of Botrytis treatment and prevention include adding a biological control product during the rooting stage and removing infected portions of plants or fully affected plants as soon as they are detected to try to protect the other plants in the crop. Specific strains should be chosen that do not have large, water-trapping buds if Botrytis has been a problem in a growing area before.
Fusarium
Fusarium is a particularly ruthless fungal pathogen of cannabis. There are several different species of it that can wreak havoc on your plants. The more commonly encountered strains of this disease-causing pathogen are: Fusarium oxysporum, Fusarium proliferatum, and Fusarium solan . Spores of this fungi, and all of its subspecies, travel easily through water and air. Once it is present in a growing area, like a veg room, it can spread quickly if not addressed immediately.
This pathogen, or range of pathogens, like Botrytis cinerea, are causes for Damping-off disease and also bud rots. Careful environmental controls of humidity, temperature, and air flow are likewise the best course of action that a grower can take to reduce the chance of seeing this happen to seedlings.
Unlike Botrytis cinerea, Fusarium ssp., can cause root rot of cannabis plants, specifically causing Fusarium root and crown rot disease. Prevention should start by testing the mother stock to make sure that clones do not start off already infected. If the disease is detected before symptoms are too far along, the application of a fungicide during the vegetative stage can effectively eradicate the disease.
Fusarium wilt, is a generic name for a disease caused by Fusarium ssp., as each host has a specific strain of Fusarium that affects only that species of plant. Regardless, the symptoms are the same whether it is cannabis, or any other food or ornamental crop. On seedlings, plants will turn yellow, be stunted, and then ultimately die. With older plants, lower leaves will yellow first, but eventually all plant tissue will turn brown and the plant will die. It will cause root rot, but not during the earlier phases of the disease, so it may be misdiagnosed.
There is no currently required action by any government that allows for the sale of cannabis products, including seeds, plants, and clones, to be certified as clean from Fusarium ssp. Growers need to be vigilant if they obtain clones or stock from propagators that do not have a reputation for the quality of their stock, or worse, have a reputation for selling diseased stock.
One species in the genus Fusarium that gets some occasional press is Fusarium venenatum. This fungus is not a disease-causing species of concern for cannabis, but is instead intentionally produced for the health food and clean-living markets. It is marketed as a protein source for those who eat a vegetarian or vegan diet. It is known commercially as “Quorn”.
Golovinomyces
While not nearly as commonly heard by its genus, but rather by the type of disease it causes (powdery mildew), Golovinomyces and all of its variants that affect cannabis crops, G. cichoracearum, G. ambrosiae, or G. spadiceus spread easily through the air and contact with other infected plants. Cloning of infected plants will also spread the disease.
Once contracted, the disease is not immediately noticed. It can take up to two weeks after contact for symptoms to appear. A gray or white powdery material will appear on the leaves, stems, and if contracted later, it can appear on flowers as well. Ultimately the leaves will turn brown and begin to drop off. If left untreated, plants will be stunted and produce poor quality, if any, flowers.
Like any other fungal pathogen, treatment is best undertaken preventatively by making sure that temperature and humidity levels are not too high. Proper air circulation above, below, around, and between plants is critical too. If powdery mildew does appear though, it can be treated with common fungicides or even baking soda and water at proper concentrations.
Pythium
Another fungus to look out for in the veg room is Pythium and several of its strains. It is a killer of root systems and is synonymous with “root rot”, though it can be the cause of damping off as well as crown rot too. While generally cannabis is most susceptible to this disease in the seedling stage, it can definitely wipe out a crop of mature plants if proper disease scouting and knowledge of symptoms is avoided or taken for granted.
One of the reasons that Pythium is so deadly is that unless you are thoroughly examining your plants’ roots, it can spread throughout your entire hydro system and infect all of the plants. Plants that are seemingly healthy from the crown up, may not get the scrutiny that they need to try to avoid P. myriotylum, P. dissotocum, P. aphanidermatum, and P. ultimum.
Symptoms of an outbreak include both sights and smells. If you are able to see the root system and note brown mass growing in the root system, it is very likely to be a variant of Pythium. Above the root system, leaves may curl, droop and drop. They will have discoloration ranging from white to yellow to brown. Unfortunately, relying on above the crown symptoms may lead to a misdiagnosis since many pathogens and poor practices can cause these symptoms, even over-fertilization.
If the culprit truly is Pythium, there will be quite a foul smell that accompanies its presence. Most would say that this bacterial invasion smells like potent mildew. If your plants are fully into the veg phase when you diagnose that the issue is Pythium, it is probably time to consider starting over. Treatments that are available and work, are only really effective when your plants can be correctly diagnosed and if the pesticide regimen is acted upon swiftly and effectively.
Pythium rots and mildews are most effectively treated at the outset of their discovery. Make sure that substrate or media humidity, as well as environmental humidity levels, are sufficient to both give the plants what they need, yet dry enough to where Pythium and the like can not come into your greenhouse and grow rooms and “set up shop”. Be careful not to overwater as well. If the presence of Pythium is suspected, but the crop has to be grown, even at the risk of total crop failure, treat the rooting area with a preventative fungicide, and then treat the symptoms on the leaves as soon as they are detected.
Sooty Mold
Sooty mold or “mould” is a fungus that is easily prevented. Unlike the other diseases mentioned here, sooty mold only occurs following an infestation from insects, usually being aphids. Leafhoppers, mealybugs, scale insects, and whiteflies can also be the culprit though.
On the host plant for the insects, signs of sucking insects will become apparent. Look for small yellow spots, or stains, which turn to brown. Prior to this damage, inspections of the undersides of cannabis leaves may reveal egg deposits. Look for these on the larger fan leaves.
A dark, sticky substance appears on leaves and stems. This most often occurs on lower leaves or in vertical growing systems, on the lower plants. Insects feeding on upper plants, drop their waste onto lower plants and leaves. They feed on the plant sap, and excrete a substance known as “honeydew”, which is a sticky, sugary substance that sticks to anything it touches. This honeydew is the ideal environment and food source for sooty mold spores. Unfortunately, once it appears, sooty mold spores do spread quickly.
Photosynthesis is obstructed by the presence of sooty old. The dark “cape” of ooze blocks light, preventing plants from creating their own food and energy. Because this is a disease that lives on the surface, it can be washed off with plant soaps. It may take multiple applications and some amount of careful wiping, but sooty mold can be wiped off.
Sooty mold treatment starts with inspection of your plants. Don’t let insects get to problematic levels and you will not get sooty mold. Treatment is two-fold. The population of insects that released the honeydew must be eradicated and the plants covered in the mold need to be treated. Disposal of all affected plants may be advisable, but if the outbreak is minimal, it can be treated.
Insects are often vectors of other diseases too, not just sooty mold. They can inject viruses into plants while feeding on them. This not only harms the host plant, risks surrounding plants, but also renders affected plants unable to be cloned as they will pass along that virus.
The Current State of Cannabis Disease Treatment
With the recent and sometimes disjointed legalizations in various capacities to grow, sell, distribute, and use cannabis in the United States and Canada, subsequent supporting laws, statutes, regulations, and the like have not yet caught up. Growers of Cannabis in either country still cannot access the same range of pesticides available for other crops, even if they are trying to mitigate a disease or pest that is nearly identical to, or in fact the same as,those that affect more “traditional”crops.
Growers of cannabis must largely act to prevent diseases as much as possible, rather than relying on treating them if they arise. The addition of monitoring equipment to accurately measure temperature and humidity levels, as well as the strategic placement of fans in the growing area will go far to prevent all diseases of cannabis in the veg phase, and flowering phase too. This does not mean that there are no pesticide solutions available to combat disease pathogens of cannabis. Depending on where the grower is located, vaporized sulfur has been approved for use in Canada, and pesticides utilizing potassium salts of fatty acids are allowed to be used on cannabis in the US.
Additionally, growers on both sides of the border can use products that reduce the risk of disease spread such as hydrogen peroxide. A slew of microbial biological agents, better known as biopesticides, are also allowed for use on cannabis crops to combat the appearance of many cannabis diseases. In general, and because laws, and regulations surrounding cannabis are changing frequently, growers should check with their respective regulatory agencies to determine what products can and cannot be used to address fungal diseases on cannabis crops. As always also, read the label on every pesticide container fully before use. Information obtained within that label can also tell you if what disease you are trying to mitigate in your veg room or elsewhere, can actually be done with the product in your hand.
