Author: Kira Turner, Master Gardener
What will I need to know for identifying a disease on my cannabis plant?
A disease is the result of a dynamic relationship between an organism and the normal processes of the plant. The organism that causes the disease is referred to as the pathogen. Most plant diseases are caused by fungal or fungal-like organisms. However, they can also be caused by viral and bacterial organisms. There are two important factors to consider when Identifying the disease affecting your cannabis plant. These factors are the signs and symptoms that the plant is experiencing as a result of the pathogen. A sign is physical evidence of the pathogen, it is the visible products or structures produced by the pathogen on or in the plant. Examples of a sign would be mushrooms, reproductive structures, as well as bacterial ooze. While a symptom is the visibles effects of the disease is having on the plant's processes. Examples of symptoms include a change in colour, structure, and overall plant function. As well as being able to properly identify visual characteristics of the present disease, it is also important to know what type of disease it is. This will be helpful for determining the best approach for treatment and prevention. There are Infectious and non-infectious diseases. Infectious diseases tend to be more difficult to control as they are caused by living organisms (bacteria, fungi, virus, etc) and these organisms have the ability to spread. While non-infectious diseases are caused by non-living organisms, and are often a result of unfavourable growing conditions such as extreme temperatures, light, water, nutrients, etc. These diseases do not have the ability to spread on their own and as a result are typically easier to get under control.
Now that you have the knowledge to properly identify signs and symptoms of diseases continue reading for 6 common diseases found in cannabis and the visible characteristics to look for in your plants. If any of these signs and symptoms are noticed in your crop control methods will be provided to help combat against the disease.
What are the most common diseases in cannabis and how do I treat them?
1. Fusarium wilt & root rot (Fusarium sp.)
- No visible signs of fusarium are typically present, it is usually the symptoms the plant experience as a result of disease that are used to ID
- Stunted growth
- Initially plants will wilt at the hottest part of the day and recover at night
- Leaves begin turning pale green/ yellow and will later wilt, wither, and die, dropping off progressively upward from the base of the stem
- The whole plant will eventually become pale green/yellow and begin to wilt
- Roots may become brown and begin decaying
- Vascular tissue on the plant will become discoloured often a red or brown colour
- Leaves will also curl and look like they are going to drop off, but they won't
The best “treatment” for fusarium is prevention. However if this is not possible there are several steps you can take to combat this disease and limit its effects. Unfortunately once a plant becomes infected with fusarium there is no “cure” but you can take measures to make sure the problem does not get any worse. Begin by quarantine any infected plants, dispose of infected plants in a way that does not spread spores. Dispose of any soil the infected plant may have had contact with. Making sure any disposal of material is done away from healthy plants. The last measure you can take is to make sure all tools used are sanitized appropriately.
2. Powdery mildew (Golovinomyces sp.)
- The mycelium of the fungus begins forming patches of white/grey “powder”. Plants often look as though they have been dusted with flour.
- Usually starts off as circular, powdery white spots, which can be found on leaves, stems, and sometimes flowers.
- Typically found on the upper part of the leaves
- These white spots will continue to spread covering most of the leaves of affected areas
- Very early symptoms include small chlorotic spots or blistering on leaves and flowers
- Older lesions will begin turning brown
- Infected leaves often become chlorotic as they are not able to properly perform photosynthesis
- Can cause leaves to twist, break, and become disfigured
- New growth of leaves and buds is often deformed
If powdery mildew is present begin by making sure there is enough space between your plants for adequate airflow. If plants are too close together and leaves begin overlapping this allows the canopy to hold moisture and continues to provide the pathogen with an ideal growing environment. Using a foliar application is extremely effective at helping to combat the disease. Sprays such as neem oil, milk sprays, and baking soda sprays are all effective. If any of the mentioned sprays are not having the desired effect the next step is to apply a fungicide.
3. Bud rot/ Grey mold (Botrytis cinerea)
- Grey masses with a velvety appearance begin developing on the buds of the plant.
- flowers and leaves will have a collapsed and water soaked appearance
- Brown lesions may become present on undeveloped flowers
- Stems and branches infected with grey mold will die back (stem cankers)
There is currently no approved fungicide to combat botrytis, making prevention that much more important. This includes closely monitoring humidity and temperature levels of your grow. Making sure your plants are spaced properly to allow for adequate air flow. If bud rot is discovered the most effective form of treatment is to remove the affected parts of the plant. It is important to remember that while removing these pieces of flower not to let them come into contact with clean material.If the whole plant is infected it should be destroyed to avoid contaminating any other plants in your crop. Disposal of infected plants and material should be done away from clean plants to deter the spread of spores.
4. Root rot (Pythium sp.)
- Brown gungy substance growing in and around the roots
Is extremely difficult to detect and is often caught too late, this is because it affects the root system and typically goes unnoticed until the whole plant is experiencing symptoms.
- Roots will become discoloured turning brown, soft, watery, and often smell of mildew or rot
- Plant leaves will start wilting, turning yellow, with brown edges.
- Other leaf symptoms include burning, brown spots, yellow or white colouring, dropping off leaves, and signs of various nutrient deficiencies
Pythium can be treated and eradicated, however, it can be quite costly. Root rot treatments products are available but the ones that are extremely effective tend to be extremely expensive as well. If you are unable to acquire an effective fungicide the best way to control pythium is to destroy any infected plants completely.
5. Yellow leaf spots (Septoria cannabis)
- There are no visible signs for this disease ahead of time
- Yellow leaf spots first start appearing on the lower leaves of your plant
- Leaf spots can vary in colour from white to a grayish brown to a yellow, initially appearing as small circular lesions
- As the lesion grows it will begin to look like a bump
- Affected leaves will dry where the spot is, overtime the spot will break and produce a hole on the leaf.
- Severely affected leaves will become completely yellow, dry out, and fall off.
This can be avoided through crop rotation. This is done by picking a new location to plant at the beginning of each grow. If this is not possible completely destroy any material after harvest, to ensure any remains are gone. You can also apply a fungicide to pre - treat the soil before starting your grow. Applying helpful bacteria to your soil such as trichoderma will also help combat disease.
6. Tobacco Mosaic Virus (TMV)
- There are no visible signs for this disease
- Changes in leaf colour such as: pale colouring, yellowing of new growth as well as lower older leaves (yellowing is typically between the leaf veins), dark or purple leaves,
- Black or gray patches, white powdery patches
- Brown and dark spots
- Mottling and mosaic pattern on leaves.
- Burnt leaf tips as well as death of leaf tips
- Red stem
- Old leaves falling off
- Slowed growth
- Leaves curling under or upwards
- Weakened stem
There is no cure for TMV unfortunately. Viruses simply cannot be controlled once they are transmitted. That's why sanitation is key for controlling this virus. If you do discover an infected plant it should be removed immediately to stop spread.
How do I prevent disease from occuring in my plants?
Prevention is the most important part of keeping your plants disease free. To be successful at preventing disease you will want to develop a disease management strategy. This can be broken down into several steps.
This includes doing daily/weekly inspections of plants. Quarantining plants suspected of disease or new material being brought into your grow space. Disinfecting plants either through heat or chemical. This could be dipping new cuttings into a pesticide before the planting step, or using a foliar spray on larger potted plants. This also includes reevaluating your cultural practices. Is the growing space regularly cleaned? Are the tools you are using cleaned between plants? Is it time to rotate the crop to a different variety? These are all things to consider and evaluate to better your practices and limit chances of disease.
This is the second step to consider if there is no way to keep disease out of the crop. Timing is very important for this step as you need to consider the different stages of the disease as well as the different stages in plant development. As there will be times when said disease is more likely to spread as well as times when the plant may be more susceptible. Use your growing environment to the best of your ability to protect your plants. This could be spacing your plants further apart so the foliage is not touching. Re-evaluating your harvesting practices. Is something you are doing potentially putting your plant at higher risk for disease? Consider adjusting your watering and fertilizing schedule. Watering in smaller doses to avoid over saturating your soil or fertilizing more frequently to avoid nutrient deficiencies that could potentially weaken your plants immune system. The final step in protecting your plants would be to apply preventative fungicides/biofungicides to decrease the chance of infection and spread.
This would be choosing a strain that has been shown to have resistance or tolerance to certain diseases. For example if in the past your crops have always struggled with fusarium, consider for your next grow picking a strain that is known to be more resilient to said disease and will have better chances of survival even if it becomes infected.
The final step in prevention is eradication. This is the step to consider when you have an infected plant. At this point the infection cannot be reversed but you can reduce spread and the potential of infecting other plants by: Killing any pest in your grow space that could be spreading the disease, remove infected parts of the plant, fumigate between crops, complete removal of an infected plant, and once again sanitation! Remove any dead organic material from your grow space.
Remember, prevention is the most effective way to combat disease!