How to Deal With Rust Spots on Cannabis Plants?

Finding rust spots on your cannabis plants is a lot like spotting rust on your car — a sign that something's not right. Just as rust can spread and damage your lovely car, these reddish-brown patches signal trouble for your cannabis. 

Whether it's rust fungus, nutrient deficiencies, or another issue, ignoring the problem can lead to stunted growth, disease, or even a dead plant. The good news is that you can tackle these rust spots and keep your plants thriving with the proper knowledge and timely action. Let's learn how. 

What are Rust Spots?

Rust spots on your cannabis leaves are exactly what they sound like — reddish-brown patches that look suspiciously like rust and sometimes even feel a bit rough to the touch. These blemishes can pop up for several reasons, including:

• Rust Fungus
• CalMag Deficiency
• Nutrient Lockout or Burn
• Pests

Out of these culprits, rust fungus and nutrient deficiencies are the usual suspects behind those rusty patches.

No matter the cause, these spots are more than just a cosmetic issue — they're a sign your plant is in trouble. At first, it might seem like just a bit of rust here and there, but ignore them, and you're inviting a host of problems. 

We're talking about withered leaves, stunted growth, an overall unhealthy plant prone to disease, and, in extreme cases, even death of your cannabis crop. However, with the right knowledge and action, you can protect your yield and keep your plants thriving.

Sound alarming? Don't worry — we've got you covered. Let's learn more about what causes these rust spots so you can tackle the problem head-on.

Causes of Rust Spots in Cannabis Plants

Rust fungus infection is a sneaky fungal parasite (scientifically known as Pucciniales or Uredinales) that loves living plants. However, it's not the only cause of rust spots. Let's take a look at them all:

1. Fungal Infections

Cannabis plants can get rust spots due to fungus-based pathogens in humid and/or poorly ventilated environments. Out of these, rust fungus (Puccinia spp.) is one of the most common fungal infections of cannabis. This disease flourishes in high humidity and low airflow and often shows up during the blooming stage of your cannabis plant, leaving behind brown, rusty spots that can wreak havoc on your yield if left unchecked.

But it's not just about the brown spots on the leaves. It can also mess with your plant's ability to photosynthesize and absorb nutrients, leaving it weak and struggling to grow.

Unlike other fungi, which can thrive on both living and dead plant material, rust fungus is picky — it feeds only on living plants. It spreads through spores carried by water or air, so you might notice more spots appearing right after watering your plants. 

This fungus can move in if your plant stays damp for too long. Rust fungus thrives in warm, moist conditions. It loves temperatures between 68°F-86°F(20°C-30°C) especially when the leaves are wet. The fungus spreads quickly during sunny, warmer parts of the day, but if you leave your plant wet overnight, it's basically an open invitation for this fungus to take over.

Overwatering or watering your plant late in the day can also increase the risk. To keep rust fungus at bay, you should water your plants early in the morning, ensuring the topsoil is dry before adding more water. 

This way, any excess moisture has time to evaporate throughout the day. Instead of soaking the foliage, focus on watering the soil directly to keep the leaves dry and the canopy fungus-free.

Symptoms

• The upper leaves of the plants can develop rusty spots. 
• The underside of leaves, where spores form, has a powdery or fuzzy feel.
• The spots on leaves may grow and merge to cause yellowing, wilting, or fall off. 

Note that the spores that are in the air can waft between your plants. The infection can also be spread by contaminated tools or water splashes. 

Solution

• First, cut off the lower leaves and use oscillating fans to improve the airflow.
• Lower humidity, especially during the flowering stage. 
• Use organic fungicides, such as neem oil or copper to treat.

Other Fungal Pathogens

Some other fungal diseases can develop leaf septoria or botrytis (bud rot), which causes rust-like spots.

Symptoms

• Yellow or brown spots that aren't regular in shape.
• Areas that are dirty or dead during flowering stages.

Solution

1. Throw away the infected leaves right away. 
2. Make sure your room has adequate ventilation. 
3. Use fungicides as a preventative measure.

2. Nutrient Deficiencies

Nutrient deficiencies are one of the main reasons for cannabis plants to develop rust spots. These deficiencies often occur due to improper feeding, a pH imbalance, or an inability to uptake nutrients due to the roots.

Calcium Deficiency

Calcium contributes to the structural formation of your plant's cell walls, root health, and plant growth, and a lack of it causes rusty brown spots.

Symptoms

• Small rusty spots on the older or lower leaves.
• Leaves start curling or distorting or turning yellow close to the spots.
• Stems become weak leading to slow growth.

Causes

• Calcium cannot be taken up by plants when soil pH is less than 6.0 or hydroponic pH is less than 5.5.
• The fertilizer is of poor quality, or the nutrient mixture lacks calcium.
• Using soft or distilled water without calcium supplementation lowers calcium levels in plants.

Solution

• Regularly test and adjust pH levels.
• Calcium can be added from calcium nitrate or cal-mag products.
• Make sure your water source has enough minerals.

Magnesium Deficiency

Magnesium helps in making chlorophyll which is responsible for photosynthesis and manufacturing energy. 

Symptoms

• Get rusty brown spots between veins of lower leaves.
• Yellowing or fading green, especially between veins of the leaves. 
• Older leaves turn brittle and may fall off early.

Causes

• Too much potassium or calcium in the soil can stop the absorption of magnesium.
• The plants cannot take up magnesium once the pH drops below 5.5 in hydro and 6.0 in soil. 

Solution

• Use magnesium supplements such as Epsom salts (magnesium sulfate).
• Adjust the pH levels to ensure the plants can take up magnesium. 
• Don't fertilize more than needed with competing nutrients. 

Potassium Deficiency

Potassium helps with energy transport between plant tissue for plant vigor and water regulation. Thus, potassium deficiency can make the plant weak or sick.

Symptoms

• Rusty patches on leaf margins.
• The tips and edges of the leaves turn brown and curl upwards.
• Leaves turning yellow in between and stunted growth. 

Causes

• The soil you're using doesn't contain enough potassium or the fertilizers you're feeding don't have sufficient potassium. 
• The growing medium has pH imbalances.

Solution

• Use a balanced fertilizer with enough potassium.
• Make Sure the pH stays within the ideal levels to avoid nutrient lockout. 

3. Environmental Factors

Environmental stressors can make cannabis plants weak, so they are more vulnerable to rust spots and other problems. Now, as you know already, excess moisture creates high humidity that helps fungal pathogens to breed and thrive easily. Rust fungus and similar pathogens grow and spread in moist conditions. Also, poor airflow usually exacerbates these poor conditions and makes it a wonderful home for fungi. 

Solution

• Keep humidity levels between 40 - 60% in the vegetative stage and 30-50% in the flowering stage. 
• Use dehumidifiers or exhaust fans to control moisture levels.

• To ensure air circulation, try using oscillating fans.
• Avoid overcrowding by providing adequate space for your plants.

Temperature Stress

Rapid temperature changes or extremely cold temperatures could stress the plants and lead to rust spots. For instance, extreme heat or cold can interfere with nutrient absorption and disease resistance. To prevent this, your grow room temperature should preferably stay between 70–85°F (21–29°C).

Watering Practices

Bad watering habits can cause diseases or nutrient deficiencies. In fact, overwatering causes the soil to become inundated, making it hard to oxygenate the roots. 

In addition, when you splash water from above on your plant's leaves, it can make your plant wet. This increases the chances of fungal infections.

Solution

• Water the plants at the bottom and let the soil dry a bit in between.
• Do not wet the leaves, especially after late afternoon or evening.

By fixing the nutrients and improving the environmental conditions, you can fix rust spots on cannabis plants. Just make sure you check the plants frequently to ensure they are healthy. 

Identifying Rust Fungus on Cannabis Plants

Rust spots on your cannabis leaves can have multiple causes, so it's crucial to pinpoint whether rust fungus is the real culprit. Here's what to look for:

• Early signs: White or yellow spots on the lower branches or both sides of the leaves. As the problem gets worse, they will become larger. The spots can develop irregular shapes or look like rings, giving the leaves a mottling appearance. Over time, the spots join together and a larger part of the leaf will look discolored. If you ignore this, the problem may transfer to the upper leaves and infect the whole plant.

• Underside clues: Yellowish or orange spots or streaks on the underside of the affected foliage. When the cause is a fungal rust, the underside of the leaves may also show powdery or fuzzy patches. If you touch it, it can spread to other plants, which means there are spores of fungus present there.
• Advanced symptoms: Orange to brown blister-like bumps (also called pustules) under the leaves. In addition to the rusty patches, plants can show other signs of stress like slow growth, nutrient deficiency, or weakened stems. 
• Powdery appearance: Reddish-brown, powder-like spots that look like tiny rust patches.
• Leaf Damage: As rust spots appear, they will weaken the leaves, causing them to curl, wilt, or dry out. If you don't rectify the issue early, the leaves become weak and drop off early. 

Keep in mind that symptoms vary based on how advanced the infection is. Orange spots, for example, usually show up in later stages.

Want to confirm it's rust fungus? Try this quick test: Rub your finger over the spots. If they smudge and leave a residue on your finger, you're dealing with rust fungus (essentially a mildew problem). But if the spots don't budge, the issue might be related to a nutrient deficiency instead.

Why Should You Get Rid of Rust Spots on Cannabis Plants?

Addressing rust spots on cannabis plants is very important, although many growers seem to ignore them. In fact, they are often a sign of something problematic going on in the plant.  Let's take a closer look at why you should address those rust spots right away.

• Prevent Yield Loss

Rust spots often show that cannabis plants are missing important nutrients or have pests and fungi. If you do not treat it, things can get worse, and this can cause: 

• Stunted growth: As the plant cannot conduct photosynthesis properly due to the rust spots, the plant loses its vitality. 
• Low-quality buds: Plants that are damaged or unhealthy might not produce the dense, resinous buds that you expect. As the plants are struggling, they produce poor-quality buds that are low in potency and flavor as well. 
• Plant death: Plants with rust infections develop spots that may limit the plant's capabilities and, in the worst cases, ultimately kill it. When the disease spreads to other parts of the plant, naturally, it will die. 

• Avoid Spreading Issues

Causes of rust spots have the potential to spread from the affected plant to others around it. 

• Fungal infections: Rust fungi grow faster in conditions that are very humid and poorly ventilated and can easily spread via spores. If they are not contained, they can infect the entire crop eventually. 
• Nutrient imbalances: If one plant is deficient in some macro or micronutrient or impacted with toxicity, other plants growing in the same growing medium or feeding scheme can be affected if they are sharing the same growing medium. 

If you tackle this issue early, it won't turn into a full-blown crisis. 

• Optimize Plant Health

If you see rust spots on your plant, it could indicate that it's being stressed out by its environment, feeding habits, pests, etc. When you address these issues, you can expect: 

• Better stress resistance: Healthy plants can fight pathogens either because they're resistant to them or they're less susceptible to them.
• Better Recovery: Intervening early means the plant will recover quickly from damage and get back to being healthy and vigorous.

• Protect your Investment

Growing cannabis, whether for personal use or commercially is a time-consuming and expensive process. You'll need to spend quite a bit for the entire process, right from the seeds to the nutrients, lighting, and the setup. 

Ignoring rust spots as harmless eyesores can take a toll on the plants, meaning a lot of your money and effort will be wasted. 

• Improve Bud Quality

Your plant should have healthy leaves as it's super important for photosynthesis, which ultimately helps in flowering. If the leaves have rust spots, it could lead to: 

• Decreased cannabinoids: Unhealthy leaves will reduce the plant's ability to produce THC, CBD, and other cannabinoids.
• Reduced flavor and aroma: Stress from rust spots can interfere with the plant's terpene production, causing buds to have weaker aroma and flavor.

When you treat rust spots in time, you save your plants while maximizing the yield and reducing risks. Make sure you're proactive and take preventative measures to make sure that the cannabis plants remain healthy.

Preventing Rust Fungus

Rust fungus might be a tough opponent, but you can stop it in its tracks with the right precautions. Start by creating the perfect environment for your cannabis plants. 

The first essential steps are maintaining optimal temperature and humidity levels, ensuring good ventilation, and cleaning your grow area. Crowded plants are more prone to fungal infections, so give your crops enough space to breathe.

You should also choose fungi-resistant seeds from reputable seed banks, which can also make a significant difference, as strong genetics help reduce susceptibility to infections. 

Plenty of light exposure is another key factor, as it helps evaporate excess moisture that fungi thrive on. 

Additionally, you should use preventative treatments like sulfur fungicides or neem oil as a protective barrier. These treatments have been trusted for ages — sulfur fungicides date back to ancient Greece  —making them reliable options for keeping rust fungus at bay.

Treating Rust Fungus on Cannabis

If rust fungus has already appeared on your plant, you need to act quickly. Start by carefully pruning and removing infected leaves, ensuring you dispose of them properly to avoid spreading spores. Clean the entire grow area and your pruning tools with a bleach solution (mild, not strong). 

For treatment, start applying a copper-based fungicide that can effectively kill the fungus on your plants, but remember to avoid spraying it on the buds to preserve the quality of your harvest. 

To prevent your cannabis plant from future infections, dust your plants weekly with neem oil or sulfur. If an infection has completely overrun your plant, sometimes the best course of action is to remove and dispose of it entirely to safeguard the rest of your garden. Always discard infected material properly. Burning infected leaves is the safest way to ensure spores don't spread through compost, water, or air.

DIY Fungicide Solutions

Store-bought fungicides aren't your only option. You can easily create effective treatments at home. A simple baking soda solution, made by mixing four teaspoons of baking soda with a gallon of water and a splash of horticultural oil, can be sprayed on plants as a safe and natural remedy. 

Another option is an aspirin spray; just crush two aspirin tablets and mix them with a quart of water to help boost your plant's immunity. For a more traditional approach, the Bordeaux mixture — a blend of copper sulfate and lime water — is also a reliable and time-tested fungicide.

Preventing and treating rust fungus doesn't have to be overwhelming. With a proactive approach and consistent care, you can keep your cannabis plants thriving and rust-free. 

Common Misconceptions about Rust Spots on Cannabis Plants

Many people have wrong assumptions about rust spots. Some growers may believe these myths, which means their plants will suffer at the end of the day. To prevent this, let's take a look at some common myths and debunk them. 

1. Fungal infections are always responsible for rust spots

Many people think that rust spots are exclusively caused by fungal rust. Though fungal infections are a possible cause, there are numerous other possibilities for cannabis plants developing rust spots, and they are:

• Nutrient Deficiencies: If the plant lacks calcium, magnesium, or potassium, the leaves can develop rust-like spots on them. An imbalance in the pH of the root zone or a lack of nutrients may cause these deficiencies.
• Overwatering: Excessive watering can create a lack of oxygen in the root zone that stresses the plant and causes rust spots as an early sign. 
• Environmental Stress: Extreme conditions like a hot humid climate, poor airflow, or air circulation can add to rusty spots on the leaves as well. 

If you misdiagnose this, you might think the rust spots are a fungal infection when the real reason is based on nutrition and the environment. Getting the diagnosis done right is key to sorting it out.

2. Rust Spots are Harmless 

Many growers think that they won't do any damage to the plant as they are just cosmetic issues. But, this complacency can cause the situation to worsen. The truth is that rust spots are often an early sign that something more serious is going on.

• Weakens the plant: Rust spots can weaken the plant by inducing damage to its leaf tissue, which prevents the plant from conducting photosynthesis. 
• Reduced Yields: Plants damaged by rust spots will not be able to produce massive yields. The spots will worsen and cause the leaves to curl, dry out, and drop off from the plant. Ultimately, this will limit their growth.

If you assume that these rust spots are harmless, you'll be harming your plants. 

3. Spraying Fungicides Solves All Rust Spot Problems

Though fungicides can work on fungal rust, not all rust spot problems are fungal in nature. If the spots have developed because of a lack of nutrients, too much watering, or environmental issues, do not apply fungicides. In short, if the cause of the problem isn't a fungus, then fungicides wouldn't work.

Some fungicides might hurt the plant or disrupt the microbes in the soil, which could cause harm and make it even harder to recover.

Thus, you should take some time to assess the plant's symptoms to determine what caused the rust spots instead of throwing fungicides at it.

4. Rust spots can't spread 

A common misunderstanding is that rust spots are isolated and won't spread to other plants. In reality, rust, if caused by fungal pathogens, can be very contagious. It can spread rapidly in humid or poorly ventilated environments. Spores can travel with the aid of water and air and infect adjacent plants. 

Rust spots caused by nutrient deficiencies don't “spread” in this way. However, they will worsen with time impacting more leaves and ultimately damaging the plant if you don't treat the plant in time. 

5. Rust spots on plant leaves often appear on the top of the leaves

Some growers assume that all rust spots are only visible on the upper surface of the leaves. However, fungal rust, for instance, can appear on the underside of leaves first.

Remember that fungal infections will make the underside of the leaves look like they have spores. So, if you check only the top part of the leaves and ignore the undersides, it can be problematic as you'll not be able to diagnose the issue. 

Common Mistakes to Avoid When Treating Rust Spots

When dealing with rust spots on your cannabis plants, avoiding these common mistakes can be the biggest difference between robust regrowth and progressive deterioration. 

• Over-Treating the Plants

Many inexperienced growers immediately treat problems without assessing the cause first. Though it may make sense to over-treat to sort the problem more quickly, excess or unnecessary treatments can harm the plant.

• Excessive Use of Fungicides

Growers typically choose fungicides immediately when they suspect a fungal infection. However, it's also possible that the problem is caused due to something else, which means that fungicides are of no use. 

If you repeatedly use a lot of fungicides, it may cause plant stress, damage beneficial microorganisms in the soil, and leave behind chemical residues that affect the plant.

Therefore, make sure you spray the fungicides after confirming that the plant has a fungal infection. You can also use organic or less harsh fungicides if possible.

• Overfeeding Nutrients

Some growers will overfeed nutrients if rust spots are caused due to nutrient deficiencies. While this may seem like the most obvious solution, you shouldn't forget that overfeeding nutrients can only cause more harm. Moreover, the nutrients can start competing and the plant may get nothing due to a lockout. 

Thus, to prevent these issues, start with smaller doses and see if the plant improves before adding more.

• Frequent Spraying

Spraying (nutrients or treatment solutions) can help in some situations but is often overdone. Water on the leaves can lead to fungi growth; fungal spores don't like water, but they love moist conditions. It may also cause leaf suffocation, which causes blockage of the pores.

To avoid damage, try treating your crops when it won't rain for a few days and it won't be too wet. 

• Ignoring Early Signs

Many growers ignore the first few rust spots forming as they think it's not a big deal. After all, the leaves and the plant itself look fine, so why bother, right? Well, you're wrong because this negligence often allows the problem to worsen until it causes more damage.

Sure, the spots look harmless at first but if you don't take action, the damage escalates. For example, fungal rust can spread to other plants, and nutrient deficiencies can stunt growth if not treated.

If nothing is done, yield can drop, bud quality can be poor, or it can kill the plant, so take immediate action rather than waiting for the issue to resolve itself. 

• Overlooking Environmental Factors

Many environmental issues can cause rust spots. For example, airflow, humidity, and how wet someone makes the plant. If you do that it will let the root cause exist even if you treat the symptoms. Spraying fungicides would result in temporary rust fungus control, however, with too much humidity, the fungus might return.

Therefore, check and control the growing conditions regularly. Keep the air moving, maintain proper humidity (40-60%), and avoid overwatering.

• Not Monitoring All Plants

It's easy to concentrate on symptomatic plants while neglecting the other plants. However fungal rust problems can spread with spores, water splashes, and using tools. When other plants are affected, it may be too late to stop the problem.

Check all plants regularly even if they look healthy. You can take care of problems before they get bigger.

• Ignoring pH Levels

Many deficiencies are not the result of a lack of nutrients but a pH problem that makes it impossible for the roots to absorb them. If you're just adding more nutrients and not fixing pH, it won't help and it may even cause toxicity.

Therefore, make sure to frequently test the pH of your growing medium and water. Change the pH levels to the right level for cannabis (6.0–7.0 for soil and 5.5–6.5 for hydro).

Best Fungicides for Rust Spots on Cannabis

• Natural and Organic Fungicides

These choices are good for growers who don't want toxic residues and want peace of mind while using the buds.

• Neem Oil

Neem oil is a natural antifungal agent that prevents the growth of fungal spores and their spread. This also helps to control the insects that stress the plant. 

To mix neem oil, add 1-2 tsp. of neem oil to 1 quart (1 lt.) of water and a few drops of mild dish soap to emulsify the oil. For fungicide spray, it is best to spray on the underside of infected leaves. Use every 5-7 days for best outcomes.

Pros

• Organic and biodegradable.
• You can apply at every stage of your cannabis plant's life.
• It also keeps aphids and spider mites away from your plants.

Cons

• Can leave behind an oily residue.
• Using this under direct sunlight or strong grow lights can burn the leaves.

• Copper-Based Fungicides

Copper ions kill fungal spores on contact and stop them from spreading further. They have been used for a long time to control these pathogens.

To make a diluted copper sulfate solution, follow the instructions on the product label. Wet all the infected leaves and those which are likely to get affected. Reapply it every 1 to 2 weeks.

Pros

• Effective in treating various types of fungal diseases
• Stops new infections from attacking your plants.

Cons

• Using it too much can cause copper to build up in the soil and affect microbial life. 
• Not recommended for frequent applications.

• Sulfur-Based Fungicides

Essentially, sulfur creates an environment on the leaf surface that hinders fungal growth and reproduction.

• You can use sulfur dust or a sulfur-based spray as directed.
• Use when the temperatures are below 80°F (27°C) to avoid burning the leaves.
• Avoid mixing with neem oil, as the final product can be toxic to plants.

Pros

• Great defense against fungal diseases.
• Can be used in organic cultivation. 

Cons

• Can irritate you if you breathe it in. 
• Can harm plants in hot weather. 

• Potassium Bicarbonate

Potassium bicarbonate, an alternative to baking soda sprays, changes the pH of the leaf surface so fungal spores can't survive.

Mix 1 teaspoon of potassium bicarbonate with 1 quart (1 liter) of water and add a few drops of dish soap. Spray infected leaves thoroughly. Re-apply every 7-10 days for continual protection.

Pros

• Safe for plants, humans and pets
• Works perfectly against early fungal issues (prevention as well)

Cons

• Has to be applied constantly to work.
• Best to prevent fungus than to cure it.

• Synthetic Fungicides

If the infection is too severe, you may need to use chemical fungicides or synthetic fungicides; however, use them with caution, especially when the plants are flowering.

• Chlorothalonil 

It is an organic compound used in many fungicides that can be used against various fungal pathogens. Use it as a foliar spray on the leaves, according to the manufacturer's recommendation. However, remember that it's best if you use it early in the vegetative stage to prevent outbreaks.

Pros

• Protection from fungal disease for a long-time.
• It works well against rust fungus and leaf septoria.

Cons

• Not good for flowering plants as it leaves a residue. 
• Can harm beneficial insects. 

• Propiconazole Tilt Fungicide 

When applied to plants, this active substance stops the enzyme activity of the fungi, thereby stopping rust fungi from spreading more. Make sure you use very little and avoid the buds completely.

Pros

• Long-lasting effects.
• It helps in killing multiple fungal pathogens. 

Cons

• Not suitable for plants that have just a few weeks of flowering left. 
• Some strains could become resistant if you use it often.

• Biological Fungicides

Friendly microbes that stop pathogens naturally without chemical fungicides.

Trichoderma spp.

Beneficial fungi that outcompete harmful fungal pathogens that cause disease. You can use it as a foliar spray or a soil drench. 

Pros

• It strengthens plants and prevents rust fungus.

Cons

• Works better as a preventive spray than as a treatment.

Best Practices for Applying Fungicides

• Always be sure to check to see if the plants will react in a negative way before applying it to everything.
• It is best to apply fungicides during cooler hours, as putting them under strong grow lights or in sunlight can burn them. 
• Use fungicides as preventive measures as it's tough to get rid of fungal pathogens once they take over the plants. 

If you choose the right fungicide and make the environment better, you can treat and prevent rust spots on cannabis plants. 

Summary: How to Deal With Rust Spots on Cannabis Plants?

Rust spots on your cannabis don't have to spell disaster. By identifying whether the culprit is rust fungus, a CalMag deficiency, or another issue, you can take swift and effective action. 

From adjusting the pH levels of the solution and improving growing conditions to applying fungicides or supplements, there are solutions to restore your plant's health and protect your harvest.

Remember, prevention is always better than cure, so keep your grow space clean, monitor pH levels, and provide balanced nutrients to ensure your cannabis plants remain rust-free and productive.