Brown spots on cannabis leaves: Full diagnosis guide

Brown spots on cannabis leaves are a clear sign your cannabis plant is in trouble. 

Cannabis growers who ignore the warning signs risk stalled growth, reduced yields, and full-on crop failure. 

In this guide, you’ll learn how to diagnose every major cause, from nutrient deficiencies to fungal infections, and get proven fixes for each. 

Whether you're growing cannabis plants in veg or flower, soil or coco, you’ll get expert, actionable advice to restore your plant’s health and keep brown spots from coming back. 

Disclaimer: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis, you’re doing so by the law and subject to appropriate permissions and licenses of the applicable country.

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What causes brown spots on cannabis leaves

Brown spots on cannabis leaves signal a disruption to your plant’s health. 

This might be a nutrient deficiency, nutrient burn, pest damage, fungal infection, or a pH-related lockout preventing absorption. 

Each type of damage affects plant cells differently, and each leaves its own trace on the leaf surface. 

Whenever you see brown spots on cannabis plant leaves, early diagnosis is critical to prevent stunted growth, yield loss, or systemic issues.

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1. Calcium deficiency: Irregular spots and distorted growth

Look for irregular brown spots—often small at first—on newer growth near the top of the plant. 

You may also see curling or twisted leaf edges and poor development of new leaves with a calcium deficiency.

Calcium is crucial for plant structure, especially in rapidly dividing cells, and a lack of it quickly affects overall plant growth and the plant’s ability to maintain strong, healthy tissue.

Calcium deficiency is one of the most common nutrient deficiencies found in cannabis plants grown in coco or low-buffering growing mediums.

It often shows up just before or during the flowering stage and is sometimes misdiagnosed as light burn or a general nutrient problem. 

If left untreated, calcium deficiencies cause leaves to eventually turn brown and dry out, weakening your plants.

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Causes

Soft water (low mineral content), overly filtered RO water with no remineralisation, poor growing medium (especially coco without supplementation), or acidic conditions (pH <6.2) all reduce calcium uptake. 

Cannabis calcium deficiency can also appear if too many other nutrients are present, leading to a subtle form of nutrient lockout in the root zone.

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Fix

In most cases, to address calcium deficiency, increase your base nutrient dosage slightly rather than relying on a CalMag product—particularly in coco grows, where overuse of CalMag can cause nutrient lockout. 

Only resort to a CalMag supplement if your base nutrients are already at optimal levels. 

Check pH and adjust to 6.2–6.5 in soil or 5.8–6.2 in hydro. 

Maintaining a well-balanced medium with proper buffering will support healthy plant development throughout the lifecycle. 

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2. Magnesium deficiency: Rust spots between veins

Rusty brown spots begin between the veins on older fan leaves, often followed by interveinal yellowing. 

Unlike calcium deficiency, this causes brown spots on older cannabis leaves first and often progresses if left unchecked.

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Causes

Magnesium is mobile within the cannabis plant, so deficiencies show on older cannabis leaves first. 

Common magnesium deficiency triggers include soft water, incorrect pH, or a high concentration of other nutrients—especially potassium or calcium. 

It can also arise when using concentrated nutrients without considering balance. A compromised grow medium, nutrient lockout, or prolonged stress during flowering can worsen the issue.

Magnesium deficiency is often mistaken for light burn or early fungal disease, but carefully inspecting affected leaves helps distinguish it.

If conditions aren’t corrected, leaves may eventually turn brown, curl at the leaf edges, or fall off—impacting plant health and flower development.

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Fix

Apply Epsom salts (magnesium sulphate) directly to the root zone via foliar spray. 

Maintain pH between 6.0–6.5 in soil and ensure your water source has sufficient magnesium. 

Supporting health with a stable feeding schedule and avoiding nutrient burns will promote healthy growth.

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3. Phosphorus deficiency: Blotches and darkened leaves

You’ll notice brown or purple blotches on older cannabis leaves. Leaves that are affected often darken, curl under, and may eventually turn brown or grey. 

This is one of the more visually striking nutrient deficiencies and is often mistaken for fungal problems or severe nutrient burn.

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Causes

Phosphorus uptake is highly sensitive to conditions at the roots. 

Cool root temperatures, anaerobic pockets in the growing medium, or low microbial activity all limit availability. pH that’s too high or low can trigger nutrient lockout. 

‍This is especially problematic during the flowering stage when phosphorus demand spikes. 

A poorly aerated medium or excess moisture can also lead to root rot, further impacting uptake. 

You may see additional signs like stunted plant growth or dull, brittle leaf edges with phosphorous deficiency—especially if the plant is already compromised by other nutrient issues.

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Fix

Switch to a bloom feed with sufficient phosphorus and check that your nutrients aren’t overly diluted.

Ensure the root area stays warm and oxygenated, and consider adding beneficial microbes to the medium to improve uptake. 

Improving air circulation and allowing for clean air exchange can also support robust growth during recovery. 

Catching this deficiency early helps prevent long-term impact on flower development.

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4. Potassium deficiency: Burnt edges and spots

Look for scorched or burnt leaf edges that progress into brown spots, particularly on older leaves. 

In some cases, the entire leaf may turn yellow before dying off. 

In cannabis plants, potassium is essential for regulating water movement, nutrient transport, and enzyme activation—especially during the flowering stage.

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Causes

Potassium becomes unavailable when pH exceeds 7.0 or when calcium and magnesium levels are too high. 

This often follows attempts to correct a calcium deficiency. Nutrient lockout from salt buildup or imbalanced feeding is common in bottled nutrients. 

Potassium uptake can be further restricted if the growing medium is too dry or compacted. 

Potassium deficiency may also contribute to stunted growth and affect how the plant manages stress or resists fungal disease.

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Fix

Flush with balanced pH water to reset the root zone, then apply a potassium-rich nutrient. 

Monitor your nutrient regime carefully, especially during flowering, when demand increases.

Keeping your medium well-oxygenated will promote healthy growth and help the plant recover. 

Improved airflow and consistent air supply also reduce the risk of secondary issues.

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5. Nutrient toxicity (Nutrient burn): Overfeeding damage

‍Tips curl and turn brown or bronze with nutrient burn. You may see brown spotting and clawing leaves. 

Growth may stall, and the plant takes on a dark, glossy appearance with reduced vigour and stunted growth.

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Causes

Overfeeding with nutrients from bottles can lead to salt accumulation in the root area, particularly in coco or compacted soil. 

This disrupts nutrient uptake and causes multiple forms of nutrient lockout.  

The result? A cascade of nutrient issues, visible as leaf tips turning brown, yellow leaves, or even rust spots in more advanced cases. 

Excessive feeding often overwhelms how the plant processes other nutrients efficiently, affecting plant health and leading to leaves becoming brittle or dying off prematurely.

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Fix

Flush your medium thoroughly with pH-balanced water and monitor EC levels. 

Reduce feed strength and revise your nutrient regime.  

Ensuring good air circulation and access to clean air can help the plant recover. Avoid feeding again until signs of healthy looking growth return. 

If nutrient burns persist, assess whether you're using an overly concentrated mix or duplicating elements across products. 

Preventing toxic buildup is critical for maintaining balance in cannabis plants and avoiding further damage.

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6. pH lockout: Invisible barrier to nutrients

Even if you're feeding correctly, your plant might still show brown spots, curled leaves, or deficiency-like symptoms. That’s often a sign of pH lockout.

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Causes

Nutrient uptake is highly pH-dependent. Even a slight shift outside the 5.8–6.2 range can block key nutrients if you're growing in coco or hydro. In soil, aim for 6.2–6.8. 

Left unchecked, pH lockout can lead to chronic nutrient deficiency and stunted growth. 

As nutrient absorption fails, leaf tips may yellow, curl, or show brown or black spots, particularly on older cannabis leaves. 

Inconsistent watering or poor drainage may also disrupt root conditions and trigger lockouts in cannabis plants. To avoid these issues, use a soil moisture meter.

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Fix

Check the pH of your water source and runoff. If necessary, use a pH-up solution or pH-down to bring levels into range. 

Adjust gradually over a few days to avoid shocking your cannabis roots. 

Maintaining balanced pH supports plant health and helps restore robust growth. 

If symptoms persist despite corrections, inspect your medium and nutrient regime for additional nutrient issues or interactions. 

Good airflow and a stable environment also reduce stress on affected leaves.

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7. Brown rust spots from fungal infection

Circular brown spots with a yellow halo, mostly on the lower fan leaves. 

These may increase rapidly, especially in the flowering stage or in humid, stagnant air.

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Causes

Leaf septoria thrives in moist or damp conditions with poor air circulation and limited fresh air exchange. 

Dense foliage, wet feet, and warm, still grow rooms create ideal environments for fungal spores to spread. 

‍Stressed cannabis plants or those with existing nutrient issues are particularly vulnerable. 

Unlike light burn or nutrient burn, septoria leaf spot spreads quickly across affected leaves, turning them yellow and then brown and causing them to drop. 

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Fix

Remove all infected leaves and dispose of them away from your grow room. 

Improve airflow using fans and open up the canopy to reduce humidity. Apply neem oil or a mild copper fungicide as needed. 

Boost health by checking your nutrient schedule and maintaining conditions that support strong roots. 

Consider spacing out your plants to prevent rust fungus and other fungal issues from jumping between nearby cannabis plants or other plants. 

Keep your grow medium well-aerated and avoid letting leaves stay wet for extended periods. 

A clean environment and early action can protect your plant’s ability to recover and resume strong growth.

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8. Environmental stress: Light burn and excessive heat

Upper leaves closest to your light source may show brown spotting, crisp leaf edges, or bleaching due to too much light or excessive heat. 

This is often mistaken for calcium or potassium deficiencies, especially in early stages.

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Fix

Check the distance between your light and canopy. If you are using full—spectrum or intense light, keep LEDs at least a few inches higher. 

Monitor for excessive heat around the canopy, which can cause brown or yellow leaves to turn brown and lead to long-term plant stress.

To prevent this, use grow room sensing tools like the Grow Sensor, which tracks temperature, humidity, and VPD in real time. This helps you keep your environment within safe thresholds and avoid heat-related damage. 

Maintaining stable conditions improves how your plants manage stress and supports growth.

Improve airflow, avoid sudden swings in humidity, and ensure your grow medium isn’t drying out too quickly under intense light. 

Keeping a consistent environment is one of the easiest ways to protect your cannabis plants from unnecessary stress.

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9. Pest damage: Mites, gnats, miners and hoppers 

Brown dots, speckling, or chewed patterns on the leaves usually indicate pest activity. 

Spider mites leave tiny rust-coloured specks and webbing, fungus gnats target the roots, while leaf miners and plant hoppers chew distinctive trails or holes through cannabis leaves.

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Fix

Use a jeweller’s loupe to inspect the undersides of leaves and catch the problem early. 

Treat infestations using neem oil, sticky traps, or natural predators. 

A pest infestation can weaken plant cells and increase susceptibility to fungal or nutrient issues. 

Improve air circulation and maintain a clean grow medium to discourage pest breeding.

Regularly inspect your plants, especially during the flowering stage, as pests are harder to spot once foliage thickens. 

Some pests can also spread fungal spores or transmit disease to other plants, so isolation may be necessary. 

Supporting healthy plants with proper feeding, airflow, and hygiene helps reduce risk. 

A balanced environment limits pest outbreaks and supports your plant’s recovery. 

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Stage-specific brown spots

Brown spots on cannabis seedling leaves

Brown spots at the seedling stage often signal early stress from poor water quality, overly strong nutrients, or incorrect pH. 

Young plants have underdeveloped roots and limited nutrient tolerance, making them especially vulnerable to root zone imbalances or overwatering. 

Heat stress or low humidity can also play a role. If your seedlings show brown spots this early, act quickly—delays can severely impact plant growth.

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Brown spots on cannabis leaves in veg

During early veg, brown spots usually relate to calcium or magnesium deficiency, light stress, or pH problems. 

Leaf septoria is less common unless the humidity is high.

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Brown spots on cannabis leaves during flowering

In flower, your plant becomes more sensitive to environmental stress and pathogens like septoria. 

Nutrient burn is also common if your bloom booster is too strong.

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How to confirm the problem

Before treating your cannabis plant, confirm the issue with a few key checks:

• Leaf location: Top = calcium, bottom = leaf septoria or phosphorus.
• Leaf age: Older = magnesium or potassium; younger = calcium.
• pH and EC tests: Runoff pH can reveal lockouts; high EC can indicate salt buildup.
• Visual patterns: Rust spots = magnesium. Large blotches = phosphorus. Yellow halos = fungus.

Tracking your feed schedule and water source helps pinpoint nutrient issues quickly.

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Prevention: Keeping cannabis plants’ leaves spot-free 

To prevent brown spots on cannabis in future, focus on stable, balanced growing conditions:

• Water quality: Avoid overfiltered RO water without remineralisation.
• pH stability: Test your water and runoff regularly.
• Balanced nutrition: Don’t overfeed. Stick to a proven feeding schedule.
• Fresh air: Improve airflow and reduce humidity with fans or ventilation.
• Cleanliness: Remove dead plant matter and check for early signs of disease.

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Quick reference table

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Takeaways

Brown spots on cannabis leaves are your plant’s way of signalling distress—and as you’ve seen, those signals can mean many different things. 

From nutrient deficiencies and toxicities to pH issues, fungal problems, and pest pressure, every cause leaves its own unique fingerprint on your leaves.

The key is learning to observe, diagnose, and respond methodically. 

Use the leaf location, growth stage, and symptom pattern as your clues. 

Don’t jump straight to CalMag or drastic changes—start by checking pH, reviewing your feed schedule, and examining the overall environment.

With the right knowledge and a calm, scientific approach, you can treat your plant, restore strong growth, and prevent further damage—without compromising your harvest.

Take your time, make one change at a time, and trust that your plants will bounce back.

Happy growing!

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FAQs

Why do my cannabis leaves have brown spots?

Brown spots on your cannabis leaves usually point to an underlying issue affecting your plant’s health—most often a nutrient deficiency, nutrient toxicity, pH imbalance, pest damage, or fungal infection. 

The exact cause depends on factors like your growing medium, pH levels, nutrient routine, and environmental conditions. 

For example, calcium or magnesium deficiency tends to affect newer or older leaves. In contrast, fungal issues like leaf septoria usually start as brown spots on cannabis fan leaves further down the plant. 

To fix the problem, you’ll need to identify the cause based on symptoms, growth stage, and leaf location—then take targeted action to restore healthy, vibrant foliage.

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How to get rid of brown spots on canna leaves?

To get rid of brown spots on cannabis leaves, start by identifying the cause—whether it’s a nutrient deficiency, pH lockout, fungal infection, pest damage, or light stress. 

Check the pH, review your feeding schedule, and examine leaves closely. 

Flush your medium if you suspect overfeeding or salt buildup, and adjust nutrient levels accordingly. 

Improve airflow and humidity control to stop fungal spread and remove infected leaves where necessary. 

In many cases, increasing your base nutrient dose is more effective than adding CalMag. 

Correct the underlying issue, and new growth should come through clean and healthy. 

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How do you treat brown spots on leaves?

To treat brown spots on cannabis leaves, first identify the cause by examining leaf location, growth stage, and overall symptoms. 

Check your pH levels—imbalances often lead to nutrient lockout. 

If the issue is a deficiency, adjust your base nutrients rather than reaching straight for additives. 

For overfeeding, flush the medium with pH-balanced water. 

Improve airflow, lower humidity if fungal infection is suspected, and remove any infected leaves. 

Pest damage may require neem oil or biological controls. 

Treat the root cause, not just the symptoms, and monitor new growth for signs of recovery.

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Can overwatering cause brown spots on cannabis?

Yes, overwatering can cause brown spots on cannabis leaves, though it’s usually an indirect effect. 

Constantly wet roots lead to poor oxygen uptake, root rot, and microbial imbalances—all of which stress the plant and disrupt nutrient absorption. 

This stress can cause deficiencies, especially calcium and phosphorus, resulting in brown rust spots on cannabis leaves. 

Overwatering also encourages fungal problems like leaf septoria, which appear as circular brown spots on lower cannabis leaves.     

If you suspect overwatering, let the medium dry out, improve drainage, and ensure proper air circulation. 

Always water based on your marijuana plants’ needs, not a fixed schedule. 

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Can brown spots recover?

Not fully. Damaged tissue doesn’t regenerate; new growth should be spot-free if the issue is resolved.

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Should I remove affected leaves? 

Yes, especially with fungal infections like septoria. In other cases, remove them only if they’re severely damaged.

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Can I still harvest with brown spots?

Yes, if the issue is under control. Avoid using infected leaves in any extracts or edibles.

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