Brown spots on cannabis leaves: Identification guide

Adam Parsons

Cannabis Journalist & Staff Writer at Zamnesia

Adam Parsons

Last updated: September 25, 2025

Brown spots on your cannabis leaves? Don't panic. Here's how to tell what's really going on.

Seeing unwelcome brown or dark spots on cannabis leaves, especially on large fan leaves, can be a worrying sight and throw even the most seasoned cultivators off track. However, these symptoms often signal manageable issues that require the right diagnosis and prompt action.

This guide is designed to help you quickly identify the root cause of brown spots on cannabis leaves. Using clear visuals, checklists, and quick reference cues, we'll walk you through the most common reasons these symptoms appear and where to turn next for solutions.

What do brown spots mean on cannabis?

Brown or dark spots on cannabis leaves aren't just cosmetic; they're signals that something in the plant's environment, nutrients, or health isn't quite right. If you catch them early, you can prevent more damage and save your harvest.

Brown spots are typically areas of localized damage (essentially dead or dying tissue) that develop in response to stress. These spots are your plant's way of sounding the alarm, often pointing to issues like nutrient deficiencies, pH imbalances, pest infestations, or environmental factors such as excessive heat or humidity. They can show up as small, neat dots or expand into larger, irregular patches depending on the root cause. Recognizing these marks early is key to preventing further damage and preserving the health of your plants.

If left untreated, these dark spots can reduce your plant's ability to photosynthesize, leading to weakened growth, poor health and ultimately a lower-quality yield. Whether it's a nutrient issue causing brown spots on fan leaves or early signs of mold, acting fast allows you to address the problem before your whole plant is affected.

Causes of brown spots on cannabis leaves

As mentioned, brown or dark spots on cannabis leaves don't just appear at random; they're the result of specific stressors affecting your plant's ability to thrive and flourish. The key to treating them is identifying where the symptoms appear, what they look like, and what conditions triggered them. From mineral deficiencies to environmental stress and pest invasions, here are the most common causes of brown spots on weed leaves and how to identify them:

Calcium deficiency

Calcium is vital for strong cell walls and healthy root development. When it lacks, plants can develop patchy brown spots, especially on new growth.

How to spot it:

Look for irregular brown spots on young leaves, often paired with curling, twisted edges, or yellowing between veins. The spots may appear dry or slightly sunken.

Where it appears:

Brown spots from a calcium deficiency typically show up on the upper parts of a cannabis plant, especially on newer leaves that are still forming. For more information on how to fix calcium deficiency in cannabis, check out our complete guide.

pH imbalance

Incorrect pH levels at a plant’s root zone can keep it from absorbing nutrients even if they’re present, resulting in symptoms like brown or dark spotting.

Visual traits:

Brown spots caused by a pH imbalance may be accompanied by yellow leaf edges, discoloration (chlorosis), or even crispy leaf tips.

Common triggers:

Over-fertilization, unfiltered tap water, and a lack of regular testing are the main culprits of pH imbalances. However, diagnosing a pH issue is simple with pH Test Strips.

Light burn

Strong grow lights can help boost yields, but too much light intensity or incorrect positioning can lead to scorched leaves and brown spots similar to those caused by nutrient deficiencies.

How to identify it:

Light burn causes brown spots or fading mainly on the leaves directly beneath your lights. The leaf edges may curl up or feel brittle.

How it differs from deficiencies

Unlike calcium or magnesium issues, light burn symptoms won't usually affect lower leaves. The rest of the plant may look perfectly healthy.

Pests and fungal infections

Insects and pathogens are another common source of brown spots on cannabis leaves, especially when humidity is high or airflow is poor.

What to watch for:

• Tiny bite marks or pinprick dots (characteristics of spider mites and thrips)
• Brown spots with fuzzy edges or white mildew nearby (indicative of a fungal infection)
• Sudden spotting on lower leaves that spreads quickly

Be sure to check the underside of the leaves and use a microscope to check for pests, their larvae, and fungi. Also, browse our catalog of mold-resistant strains.

Overwatering and root problems

It's easy to over-love your plants with too much water. But overwatering cannabis can cause oxygen-starved roots, nutrient lockout, and brown spotting from the bottom up.

How to spot it:

Leaves may look droopy, soft or yellow. When brown spots appear, they usually show on older fan leaves first and may be accompanied by a musty smell in the grow space.

Where it starts:

Overwatering affects the root zone first and is caused by soggy soil, poor drainage, and a lack of dry periods between waterings. For a full guide on how to water plants properly, check out our cannabis watering guide.

Where and when spots appear matters

Not all brown spots are created equal. Spotting where and when they appear on your plant can reveal a lot about what’s causing them and helps to properly diagnose the underlying issue.

Fan leaves vs sugar leaves

Fan leaves are the large, broad leaves that act as your plant's solar panels and are crucial for photosynthesis. Sugar leaves, on the other hand, are smaller, resin-covered leaves that grow out of your buds.

If you notice brown spots on fan leaves, it usually points to systemic issues like nutrient imbalances, pH fluctuations or watering problems. Conversely, brown or dark spots on sugar leaves, especially during flowering, may indicate mold, light burn or environmental stress.

Upper vs lower plant signs

Where the brown spots appear vertically on the plant is just as important. Upper leaf spotting is commonly linked to light burn, calcium deficiency or pH lockout; all problems that affect new growth first. If you're seeing dark or crispy spots on the top leaves directly under the grow light, it's likely due to light intensity or nutrient mobility issues.

Lower leaf spotting, on the other hand, often results from overwatering, root problems, or mobile nutrient deficiencies like magnesium. These tend to hit the oldest leaves first as the plant pulls resources upwards. In either case, tracking the progression of symptoms can help confirm the underlying cause and point you to the right solution..

Diagnostic checklist

Use this quick reference guide to help identify the likely cause of brown spots on cannabis leaves. Simply match the symptom of your plant, then follow the linked guide to take the appropriate action:

Symptom: Small brown spots on new upper leaves, often with twisted or curling edges

• Likely cause: Calcium deficiency
• First check: Inspect new growth, test pH and water source
• Next step: How to fix calcium deficiency guide

Symptom: Brown or dark flecks on older leaves, sometimes with yellowing between veins

• Likely cause: pH imbalance or nutrient lockout
• First check: Measure pH in substrate and runoff
• Next step: How to adjust pH level when growing cannabis

Symptom: Crispy brown patches on leaves directly under lights, with upward-curling edges

• Likely cause: Light burn
• First check: Review light distance and intensity
• Next step: How to fix light burn

Symptom: Speckled spots or clusters, possibly with webbing or white fuzzy edges

• Likely cause: Pests or fungal infection
• First check: Inspect the undersides of leaves and check humidity levels
• Next step: Read our pests and fungal infection guide

Symptom: Drooping or yellowing first, followed by brown spots on lower fan leaves

• Likely cause: Overwatering or root stress
• First check: Check soil drainage, aeration and watering frequency
• Next step: How to water cannabis plants properly

Identify and deal with brown spots like a pro

Brown spots on cannabis leaves may look intimidating, but with the proper diagnosis, they're almost always fixable. Whether it's a calcium deficiency, pH issue, pests or light stress, each cause leaves behind visual clues; you just need to know what to look for.

From there, head to our cannabis diseases hub for treatment-specific solutions tailored to your issue. You'll find in-depth guides for watering, nutrition, mold prevention and more.

Finally, to take your grow even further, browse the full Zamnesia grow guide for step-by-step cannabis growing instructions and expert insights to keep your plants healthy from seed to harvest, indoors or outdoors.