Fungal Diseases That Affect Cannabis: A Comprehensive Guide To Preventing And Treating Them

The cannabis industry has grown significantly in the past decade. A sector that was shunned earlier is now one of the fastest growing ones as a lucrative business for many commercial cultivators. 

However, despite the advancement in technology, many cultivators face several challenges. Among them, fungal infections emerge as the stuff of nightmares as they can undermine your efforts and render your plants useless if you don’t pay attention. 

Fungal diseases that affect cannabis include powdery mildew, gray mold, and fusarium wilt which are very aggressive. They don’t just affect the visual appeal but weaken the plants and reduce the yields significantly. 

Whether you’re a home grower who wants to enjoy a few buds or a commercial cultivator with acres of space, there’s no doubt that fungal diseases can keep you awake at night. If you recognize it early, though, you can still save your plants. Thus, this guide aims to help you with some of the most common fungal diseases that can affect your cannabis plants. 

Common Fungal Infections in Cannabis Plants

Although cannabis is known for its resilience it can still fall victim to a variety of fungal infections that affect it in several ways. From weakening the plant to reducing the yield, fungal infections can put a dent in your plans if you fall short of managing them. 

That’s why, it’s important to recognize these diseases so you can implement various strategies to help your plants. Let’s take a look at some of these dreaded fungal issues:

1. Powdery Mildew

Powdery Mildew, caused by species of the Podosphaera genus, is an easily recognizable fungal infection that can affect your cannabis plants. You can identify it with the appearance of a white powder that coats the leaves stems and even developing buds. This fungal pathogen thrives in environments with high humidity and temperatures that create ideal conditions to spread rapidly. 

Powdery Mildew goes beyond affecting just the plant’s appearance. If you don’t treat it on time, it can impact the health and the yield ultimately. The fungus first forms a layer on the plant's surface and blocks sunlight from reaching the leaf cells. As you can expect, this interference disrupts the process of photosynthesis which is crucial for the plant to survive. At this point, the plant struggles with energy production, resulting in stunted growth and lower yields. 

As the fungus spreads more, it snatches away the nutrients necessary for growth and development away from the plant. Thus, you may notice that your plant shows stunted growth and less vigor. 

In addition, powdery mildew affects the cannabinoids and terpene composition by disrupting the metabolic processes responsible for producing them. So, if you don’t treat the problem immediately, the buds will not produce the effects you want and won’t smell good either.

One problem with powdery mildew is that it spreads rapidly under favorable conditions. Airborne spores can easily move from one plant to another, so you must spot them early. If you don’t, one affected plant can spread the spores and destroy your entire operation. 

Considering how all your plants may be at risk, you should implement various strategies aggressively. 

2. Gray Mold (Botrytis Cinerea)

Gray mold, caused by the fungus Botrytis cinerea, is another threat to cannabis plants. This notorious pathogen can infect many parts of your plants, but it particularly loves buds — the one part we love so much. Known for its peculiar appearance, it thrives in environments with high humidity levels, making it a major concern for growers, especially during the flowering stage.

The impact of gray mold goes beyond its signs on plant surfaces. Thus, you must understand the implications of Botrytis cinerea to safeguard your plants. 

When plants are affected by this mold, it kickstarts a decaying process that weakens the structural integrity. Flowers and buds become especially vulnerable to its damaging effects. As the decay progresses further, it renders the plant useless as you can’t use the buds anymore. 

Typically, gray mold attacks the plant during the flowering stage when it directs its resources toward flower development and the production of cannabinoids and terpenes. If you’re growing in a very humid environment, the infestation spreads even more rapidly, making it super challenging to contain it.  

Again, if you don’t take care of this issue soon, you can expect it to destroy the buds. However, the problem with this mold is that treating it is very difficult once the infection sets in. Therefore, prevention is your best bet. 

The problem with gray mold is that the impact of mold extends beyond cultivation as it doesn’t go away even after you harvest the buds. Flowers and buds that are contaminated can introduce the fungus into your drying and curing process, putting your other healthy buds at risk. This is why you have to be extra vigilant if you see signs of this mold. You can do this by controlling the environment, detecting the issue early, and using various strategies to treat it. 

Cannabis buds are a medicinal resource for many people. Thus, many users try to use moldy buds due to desperation or ignorance. Even recreational users sometimes consume moldy buds when they are out of their stash. However, moldy buds pose a serious threat to your health. It could have bad consequences especially if you’re suffering from any type of breathing or lung issues. So, stay away from moldy buds as the effects are opposite to what you desire. 

3. Root Rot 

Root rot is a term that describes a collection of different diseases caused by various fungal pathogens like Fusarium, Pythium, and Rhizoctonia. These pathogens pose a risk to cannabis plants' root system — their very core or structure. The pathogens from this group invade the roots and disrupt functions related to water absorption and nutrient uptake.

Root rot can not only disrupt important processes in the plant and prevent it from growing properly, but it can also kill the plant. Ask any hydroponic grower and he’ll tell you that he fears this fungus the most as saving the plant becomes near impossible once it takes hold of the roots. 

Since the root system is the most crucial part of the plant, you must understand its characteristics and how it affects your plant. Roots are responsible for absorbing water and nutrients in plants, so when they attack, the very life of the plant is in danger and sets off a chain of effects on the health and growth of the plant.

You can get away from most diseases by cutting off parts of the plants that are affected, but what do you do when the roots themselves are infected? You guessed it — you can’t do much other than watching the plant die. Therefore, be cautious, especially if you're a hydroponic grower. 

Since root rot affects the roots, it’s easy to assume that it’s impossible to detect the problem. However, that’s not the case as the signs are often visible above ground, indicating that everything’s not right below. 

Typically, you’ll see that the plants start wilting. The leaves will also start yellowing as the plant struggles to absorb water and nutrients. Another noticeable symptom is stunted growth, where affected plants fail to reach their potential or grow very slowly. 

Now, these symptoms are similar to various other deficiencies and problems that affect cannabis. For example, yellowing leaves is a common issue that can arise due to nitrogen deficiency. However, if the plant starts wilting, you should check the roots or you’ll lose the plant once the infestation escalates. If you see that the roots are mushy, it’s a surefire sign of root rot. They also look like they are decaying and give off a very off-putting odor. 

Root rot can pose a serious risk in hydroponic systems, but that doesn’t mean that your plants are safe in soil. It’s just that hydroponic plants are more susceptible to it as the roots are always in water. Since there is no soil barrier the roots fall victim to the invasion.

If your soil is too wet most of the time, your plants can still get affected by this fungal disease. This is the reason why you should not overwater your plants. Most problems start due to overwatering. Although underwatering also kills the plant, the plant gets back to normal once you start watering properly; however, overwatering comes with many more issues and wastes the plant’s precious time. If you’re growing autoflowers, keep these points in mind and water only when necessary or the yields will be severely compromised. 

4. Fusarium Wilt 

Fusarium wilt, caused by species of Fusarium fungi, is another challenge in cannabis cultivation. This disease affects the plant’s vascular system and disrupts the normal flow of water and nutrients. Its impact can be quite damaging.

Fusarium fungi specifically target the plant's vascular system, which transports water, nutrients, and other vital compounds. When these fungi invade, they block the xylem vessels, thereby restricting the movement of water and the distribution of nutrients throughout the plant. This disruption significantly hampers the plant's ability to maintain its turgor pressure and support necessary functions.

Infected cannabis plants with fusarium wilt exhibit symptoms showing signs of compromised vascular function. The main symptom is that the plant begins to wilt, indicating that it is struggling to maintain a water balance. Like other fungal infections, fusarium wilt can also cause yellowing or chlorosis that occurs due to the plant’s inability to absorb nutrients due to an impaired vascular system. 

Fusarium wilt attacks plants without relenting. As this disease progresses, symptoms become more severe resulting in a decline in plant health. If left untreated this fungus can kill the plants. Remarkably, it can survive in soil for long periods too. They survive even when there’s no host, so it’s dangerous for new seedlings if you don’t treat the soil or discard it before planting them. 

Symptoms of Fungal Infections in Cannabis Plants

Fungal infections are devastating but you can save your plants if you react early. However, sometimes, it can be difficult to identify them. But, still, these infections leave several visual cues and symptoms that can help you determine the problem. Let’s take a look at some of these symptoms:

Symptoms of Gray Mold in Cannabis Plants

Gray mold is not hard to spot if you look closely enough. At first, you may not identify it, but as the disease progresses, it becomes easier to understand what’s going on. However, it may be a bit late to save the entire plant. Here are a few symptoms cannabis plants show when they are affected by gray mold:

1. Fuzzy Growth: One of the biggest indicators of gray mold is the appearance of a fuzzy grayish growth on different parts of the plants, particularly on the buds. This growth consists of clusters of spores that give them a distinct appearance.
2. Necrotic Lesions: Another symptom to watch out for is the development of spots or lesions on plant tissues. In the beginning, these lesions may look like water spots, but they turn brown once the fungus spreads. The fungus extracting nutrients from the plant is the cause of the necrosis. 
3. Bud Rot: Gray mold is infamous for causing bud rot. Infected buds sometimes even seem mushy and brown. The fungus can penetrate the buds and compromise their quality. Thus, the buds lose their aroma too. 
4. Wilting: As the fungus affects plant tissues it can cause the affected leaves to wilt and droop. Typically, this happens because of the decay and breakdown of cells within the plant.
5. Musty Odor: Plants affected by mold give off an unpleasant musty smell. Many growers identify mold primarily because of the odor. 
6. Spore Production: When the conditions are good for them, Botrytis cinerea produces several spores. These spores can appear as a powdery substance on the surface of plant tissues. 
7. Cankers in Stems: The fungus can cause lesions or cankers on stems near the area where the infection started. These cankers sometimes encircle the stems and disrupt nutrient flow. 

Symptoms of Powdery Mildew in Cannabis Plants

You can easily spot powdery mildew if you regularly check the leaves. Remember to take action as soon as you spot it, though. Here are a few symptoms cannabis plants display:

1. White Powdery Substance: Powdery mildew appears as a powdery substance on leaves stems and buds. This dusty appearance is due to the presence of fungal spores. Unlike many other diseases, the powdery substance will be found on both sides of the affected leaves. 
2. White Spots and Patches: The disease that typically starts with spots on leaves progresses with the spots getting larger. Finally, they look like patches of powdery growth. Stems and buds can also be affected by the fungus.
3. Leaf Curling and Deformation: Infected leaves may curl, twist, or show signs of deformation as powdery mildew disrupts tissue development. At times, the leaves could develop purple or red blotches, due to severe infestation. 
4. Stunted Growth: Powdery mildew infection can hinder plant growth, so infected plants may be smaller than healthy cannabis plants. 
5. Yellow leaves: Eventually, the leaves may turn yellow and begin to fall off. Even normal leaves will begin to fall prematurely. As a result., the plant struggles to perform functions necessary for its growth. 

Symptoms of Fusarium Wilt in Cannabis Plants

Unlike other fungal diseases, fusarium wilt is not easy to identify, especially when it’s still in its dormant state. However, once it attacks the plant with all its might, the symptoms get clearer. Here are a few symptoms to watch out for:

1. Yellowing leaves: Fungal infections often disrupt the production of chlorophyll in leaves, which is why you can see the leaves yellowing. This is because the fungi interfere with the plant's ability to perform photosynthesis efficiently. Of course, the progression and intensity of yellowing may vary depending on how aggressive the fungal pathogen is and the stage of the infection, but this is one of the first signs that the plant is suffering. Like most other problems that affect the cannabis plant, fusarium wilt can also cause yellow leaves, especially the ones found in the lower parts of the plant. The younger leaves also get affected eventually. You may also spot yellowing between the veins, known as interveinal chlorosis which occurs because the fungus blocks water and nutrient flow within the plant.
2. Root rot: Fusarium wilt can also cause the roots to rot. The roots start turning red and the discoloration progresses upwards.
3. Stunted Growth: As the disease advances, cannabis plants affected by Fusarium wilt may show slow and lethargic growth. As the vascular system is damaged, the plant can’t take in nutrients like it should. 
4. Drooping Leaves in Warm Weather: Fusarium wilt symptoms can be more noticeable during warmer temperatures. Infected plants may droop severely. You may also notice that while the plants wilt during the daytime when it’s hotter, they recover temporarily when the temperatures cool down.
5. Leaf Tissue Death: Once the disease advances, it can make the leaves wilt and fall off. The leaves could also display necrotic areas on their edges or in between the veins. 
6. Leaf Curling or Cupping: Infected leaves may start curling or cupping. The leaves will look distorted as the plant experiences water stress. 
7. Brown Streaks on Stems: Brown streaks may appear on the stems in the tissue indicating the impact of the fungal infection on the plant's transport system.

Symptoms of Root Rot in Cannabis Plants

Root rot attacks the roots of the cannabis plant. The symptoms resemble several other issues including overwatering and nutrient deficiencies. If you feel something’s amiss, examine the root carefully for a better understanding, Here are the symptoms you should look for:

1. Wilting Leaves: Like many other fungal diseases, root rot causes the leaves to wilt even if the soil is moist. This occurs because the fungus doesn’t allow the plant to take up nutrients and water through the roots. 
2. Discolored Roots: A healthy cannabis plant’s roots are almost white. However, root rot can infect the roots in such a way that they appear dull and brown. If the disease progresses more, they can even look black and start decaying. 
3. Leaves Yellowing: Again, the roots can’t take up nutrients properly so the leaves experience deficiencies. 
4. Foul Odor: The roots can give off a strong odor, indicating that they are decaying. 

Identifying and Diagnosing Fungal Infections in Cannabis Plants

Accurate identification and early diagnosis is crucial to treat fungal infections in cannabis. As a grower, you should be able to recognize the signs of infection and use different techniques to pinpoint the specific fungal pathogens involved. You don’t need any special equipment for this (of course, commercial cultivators will use lab tests), but you should develop the skill to understand subtle visual cues. 

When you inspect the plants for any signs of infection, look for changes in leaf color, such as yellowing or browning as well as irregular patterns or spots on the surface. Make sure to inspect both the lower leaf surfaces, stems, and buds for any abnormalities.

To identify these fungal infections, we must understand the characteristics of their growth. For example, powdery mildew appears as a white powdery substance on the upper leaf surface while gray mold manifests as grayish growth on the buds. These visual cues can help distinguish between infections.

Different fungi induce different symptoms on leaves. Fusarium wilt may cause wilting and yellowing whereas gray mold can lead to wilting and a grayish decay of affected tissues. Recognizing these symptoms can narrow down fungal culprits.

Here’s a quick overview to help you identify these issues:

Keep track of how symptoms progress over time. Note the speed at which they spread and which parts of the plant are most affected. Understanding how the disease progresses helps you to determine its severity and develop strategies to eliminate it. 

How To Prevent Fungal Diseases in Cannabis Plants?

As fungal diseases can quickly spread and kill your plants, you should try and prevent them first. Fungi thrive in ideal conditions that promote their growth. Thus, by implementing some measures including controlling the environment, proper ventilation, and maintaining optimal growing conditions you can protect your plants. 

Here are a few ways to prevent fungal pathogens from attacking your cannabis plants:

1. Soil

If you’re growing in soil, you should always prepare it in such a way that it drains well. Drained soil prevents waterlogging, reducing the risk of root diseases caused by fungi. You can also use raised beds or containers with good drainage. 

Also, if you’re growing outdoors, make sure that you choose a spot that gets a lot of sunlight. If growing indoors, choose full-spectrum lights with enough coverage so that all plants get enough light in the grow tent/room. Light is important not just for photosynthesis but also to help the foliage dry and reduce humidity. Essentially you’re creating the conditions the fungi hate. 

2. Maintain Cleanliness

Regularly remove fallen leaves and any other plant debris from the grow tent. Decomposing matter can harbor fungal spores and serve as a breeding ground for infections. Properly dispose of them so it doesn’t affect other plants. Compost plant debris to maintain cleanliness.

Just like you water your plants on time and develop a routine, do the same for cleaning your tools, containers, and equipment. Fungal spores can linger on many of these surfaces and lead to cross-contamination. Regular cleaning reduces the risk of spreading pathogens between plants.

3. Maintain Proper Plant Spacing

Allow space between plants to promote airflow. This helps reduce humidity around the foliage by preventing moisture buildup, making it less favorable for fungal growth. Avoid overcrowding plantings as this creates the perfect conditions for rapid fungal spread. Providing space between plants ensures air circulation and minimizes pathogen transfer.

4. Environmental Control 

Monitor humidity levels using hygrometers to manage moisture. You can also use dehumidifiers if you live in a humid environment to deter the growth of fungi that thrive in such conditions. In addition, temperature regulation is crucial for cannabis cultivation. Plants love consistency so the temperatures shouldn’t fluctuate too much as it can stress them and make them more susceptible to infections. 

That said, proper ventilation helps to create a proper growing environment. Installing exhaust fans and intake vents facilitates air exchange and reduces the risk of fungal spore buildup. It's also important to ensure air circulation within and around the plant canopy as this helps the leaves dry after you water them. 

5. Choose Mold-resistant Cannabis Strains

Choose varieties known for their resistance to pathogens. Many seedbanks offer such strains, so you won’t have trouble searching for them. In addition, conduct some research on the strain you want to grow to understand its characteristics and protect it from fungal diseases. 

6. Biofungicides

Apply sprays or biofungicides before the disease starts. These treatments create a shield on plants and protect them against fungal infections. Alternatively, you can introduce beneficial microorganisms like mycorrhizal fungi to the soil as they establish relationships with plants and boost their defenses against harmful fungi.

How to Treat Fungal Diseases in Cannabis Plants

1. Pruning and Thinning

Once you realize that your plants have been attacked by fungal pathogens, you should remove the infected leaves of the cannabis plant. Don’t hesitate to remove all sections that have been affected, or you’ll risk losing the entire plant as these pathogens spread quickly. Make sure your tools are sanitized. Clean them before using them on other plants. 

Pruning is particularly effective in controlling powdery mildew as removing infected leaves helps contain the spread of powdery mildew spores. It's crucial to prevent the disease from moving to parts of the plant or neighboring plants.

In addition, pruning reduces crowding and increases air circulation around the remaining leaves, which in turn decreases humidity within the canopy. It helps to open up the canopy allowing more sunlight or artificial light to reach the remaining foliage. Sunlight naturally possesses fungicidal properties that can hinder powdery mildew development.

2. Beneficial Fungi (Trichoderma) 

After you’ve removed the infected leaves, you can start by introducing trichoderma — a fungus that competes with other pathogens for nutrients and space — to the plants. These beneficial fungi produce secondary metabolites that fight against fungal infections like Fusarium wilt, gray mold, and powdery mildew.

Essentially, trichoderma creates a shield on the plants against the fungi. To use, start by spraying it directly on the leaves and stems. Of course, do this during early morning or evening or when your lights are off so that the leaves don’t get burned. 

The best part is that you can use trichoderma for both prevention and treatment. Many home gardeners growing vegetables use it early on to prevent issues. 

3. Neem Oil

Neem oil is used to treat a variety of issues attacking many plants, including cannabis. This is because of its antifungal and antibacterial properties that help the plants fight against diseases. It acts as a fungicide by inhibiting the growth of pathogens such as fusarium and botrytis cinerea. It also works to fight powdery mildew. 

To use neem oil, make sure you buy cold-pressed neem oils. Next, dilute the oil according to the instructions from the manufacturer. You can increase the effectiveness by pairing it with insecticidal soap. Then, apply it as a foliar spray on the leaves, ensuring you cover all the leaves. 

Again, you can use neem oil as both a preventive measure and treatment. If the grow tent gets too humid, you can use it to stave off fungal infections. Finally, use neem oil for both diseases and pests that attack cannabis plants. 

4. Bacillus Subtilis

Bacillus Subtilis is a bacteria found in the soil. Its primary aim is to increase the plant’s resistance against fungal pathogens. By producing compounds that hinder the growth of fungal infections, bacillus subtilis is usually very effective. 

Similar to most other fungicides, you can use this bacteria as both a foliar spray and soil drench to establish a shield against diseases like fusarium wilt. You can use it during the vegetative stage to prevent fungal attacks, especially if there’s high humidity in the grow tent. 

5. Copper-based Fungicides

Copper-based fungicides act as broad-spectrum fungicides that inhibit the growth of fungal pathogens. There are various fungicides with copper as an ingredient, so read the label to choose the right product. Copper fungicides help protect the plant against both gray mold and powdery mildew as a foliar spray. However, for gray mold, you can use it as a soil drench. 

A note of caution — be careful while using copper fungicides as using them excessively can do more harm than good. Don’t use concentrated solutions; instead, dilute them according to the manufacturer’s instructions. 

6. Chemical Fungicides

Chemical treatments play a big role in combating diseases in cannabis plants. Essentially, they are compounds specifically designed to disrupt cellular processes to impede the fungi’s growth or cause cell death. There are various types and brands of chemical fungicides in the market. Typically, the main ingredient includes Tebuconazole, Myconazole, or Propiconazole for fungal diseases. These products are super effective against many types of fungal pathogens. What’s more, they also control the spread and severity of diseases.

However, there are drawbacks associated with them as well. One such disadvantage is that the pathogens could get resistant to them if you use the same type for a long time. Therefore, you should use different types of fungicides to prevent the pathogens from getting immune to them. 

Another disadvantage is that certain chemical fungicides can impact the environment negatively. They also tend to damage beneficial organisms and contaminate water resources, ultimately destroying soil health. To minimize these risks, you must always follow recommended application rates and practices.

Another concern is that since you’ll be smoking cannabis buds or consuming them in some manner, you should be aware that these chemical fungicides may remain on cannabis buds even after you harvest them. Avoid the buds completely if you decide to use these fungicides. However, some fungal infections like gray mold attack the buds primarily. In such cases, simply get rid of the affected buds as you can’t use them once they are infected anyway. 

7. Sulfur

Sulfur acts as a fungicide by interfering with the metabolic processes of fungi. This makes it effective against many types of fungal diseases. You can use them as foliar sprays to control powdery mildew or use them as a soil drench to treat fusarium wilt in the root zone.

However, note that different cannabis varieties may have varying sensitivity levels to sulfur. So, use it only on a section of the plant as a patch test and continue if there are no adverse effects. You can also rotate between copper and sulfur to prevent the fungi from getting immune to the treatment. Finally, adjust the strategy based on the severity and stage of infection. 

Summary: Fungal Diseases That Affect Cannabis: A Comprehensive Guide To Preventing And Treating Them

Fungal diseases including fusarium wilt, powdery mildew, and gray mold can ruin your harvests and even kill the plants, leaving you with nothing, so you should act immediately to control them. Most importantly, you should know how to identify them. Once you do, try biological fungicides and then switch to chemical fungicides if everything fails. 

You should also do your best to prevent fungal diseases as it could be too late to treat them in many cases. By maintaining a clean grow tent/room, regular monitoring, controlling humidity, and creating an environment that discourages infections, you can greatly reduce their impact.