Diagnosing Brown Spots on Cannabis Leaves

Brown spots on cannabis leaves appear as small, discolored patches that can range in size and severity, commonly during the veg and flowering stages. These spots or splotches are often accompanied by other symptoms such as yellowing cannabis leaves (not always but very likely) and curling. During the flowering stage, these spots may look purple, especially when examined under LED lights.

What’s the culprit behind all these symptoms? While there could be a list of factors that contribute to brown spots on cannabis leaves, the biggest to blame is calcium deficiency. Calcium aids in the uptake and transport of essential nutrients that your cannabis requires. Without adequate calcium, cannabis plants struggle to absorb nutrients like nitrogen and potassium, leading to deficiencies and poor growth. Let’s have a breakdown of it. 

Calcium Deficiency

Calcium deficiency doesn’t come out of nowhere. It often results from several specific growing conditions and practices when you grow cannabis. You are likely doing one of the following things that trigger the calcium deficiency: 

Low pH Levels

Cannabis plants thrive in a slightly acidic to neutral pH range. When the pH of the growing medium drops below 6.2, calcium can become blocked, meaning the plant cannot absorb it even if it is present in the soil. 

Using Filtered or Reverse Osmosis (RO) Water

Many growers use filtered or RO water to feed their plants. While this water is free from toxicity, it also lacks essential minerals like calcium. Some tap water contains enough calcium but filtered and RO water often requires supplementation to meet the plants' needs.

Hydroponic and Coco Coir Growing

Growing cannabis in hydroponic systems or coco coir without adding extra calcium can lead to deficiencies, particularly during the vivid-growing veg stage. These mediums lack natural calcium, making supplementation a must-have item. 

Soil Without Calcium Supplements

Similarly, when growing in soil, especially if it hasn’t been fed calcium-rich materials like dolomite lime, calcium deficiency occurs and causes brown spots on weed. Soil naturally contains varying levels of calcium, but it may not be sufficient for cannabis plants, particularly during the nutrient-intensive flowering stage.

Excess Potassium

Too much potassium in the growing medium can interfere with calcium absorption, leading to deficiencies. Balancing nutrient levels is essential to prevent competitive uptake issues.

Nutrient Deficiencies: Nitrogen, Magnesium, Potassium. etc.

As mentioned above, lacking calcium can prevent your marijuana plant from taking up other nutrients as well. If you notice rust spots on cannabis leaves during veg, it’s the nutrient factor that’s causing the scene. Here are some examples: 

• Nitrogen Deficiency: Nitrogen is vital for leaf growth. A deficiency often starts with yellowing leaves and progresses to brown spots.
• Magnesium Deficiency: Magnesium is crucial for photosynthesis. Its lack often leads to yellowing between the veins and brown, necrotic spots.
• Potassium Deficiency: Potassium is crucial for your cannabis plants, acting like a dual-purpose immune booster and bloom enhancer. When your cannabis shows brown edges, leaves curl up or turn yellow, it's a clear sign of potassium deficiency.

Pest Invasion & Fungal Infections

Pests can wreak havoc on your plants, often leaving behind tell-tale brown spots. These insects include spider mites, aphids, whiteflies, and more. Notably, they often hide on the undersides of the leaves. 

However, if you notice circular patterns, to be exact, a whitish center surrounded by a dark border emerging at the base of the weed plants, you might be dealing with fungal or bacterial invaders such as Septoria leaf spot. Additionally, powdery mildew and botrytis are also commonly caught on cannabis leaves. 

Light Burn

It makes sense to add a little lighting boost, particularly the red light, in the final flowering stage for higher yields. However, if not implemented properly, it’s often this act that kills your plants. 

When cannabis plants are exposed to too much or too intense light, the overwhelming energy can damage the chlorophyll in the leaves, which leads to the scorching of leaf tissue and manifests as brown spots. The spots can start as yellowing and progress to brown, dry, crispy patches on the uppermost leaves of the marijuana as the damage worsens. In severe cases, the entire leaf may turn brown and die off. 

Watering Issues

Watering cannabis is a delicate task, as both overwatering and underwatering can stress plants, resulting in brown spots. It’s common for growers to confuse themselves with watering issues and nutrient deficiencies. Some tricks to identify the cause include:

1. Check the soil moisture with your finger. How does the soil feel, soggy, dry, or just fine? If the soil feels waterlogged and smells a foul odor, the plant is being overwatered for sure. 
2. Take a closer look at the top of the soil. Is there any algae growing on it? They are the proof of overwatered weed plants. 
3. When plants are drooping and showing marked leaf spots, it typically indicates a watering problem.

Wind Burn

It may not occur to you, but excessive wind or breeze can also lead to brown spots on cannabis leaves. This is especially common when a fan is blowing directly on the leaves, causing them to move around constantly.

When a part of the plant is exposed to too much wind, it can experience a wind burn. This condition can manifest in various leaf symptoms, including curling, signs of nutrient deficiencies, and underdevelopment. Occasionally, brown spots can appear as well. 

Treating Brown Spots on Cannabis Leaves

Encountering brown spots on your cannabis leaves can be concerning, but with a thorough understanding of the causes, you can take effective steps to treat and prevent these issues. Here’s how to address the various causes of brown spots:

1. Nutrient Deficiencies

To remedy nutrient deficiencies, particularly calcium deficiency, flush the system with clean, pH-balanced water containing a regular dose of cannabis-friendly nutrients such as Dolomite Lime. Doing so helps eliminate any nutrient salts hindering calcium absorption and restore the pH to appropriate levels.

Furthermore, ensure a balanced nutrient schedule that includes all essential macro and micronutrients. Adjust based on the plant’s growth stage.

2. Pest Infestations

Look under leaves and in crevices where pests hide. Once you spot any pests, use organic treatments like neem oil or insecticidal soaps to get rid of them asap. These invaders multiply themselves way faster than you can imagine. 

3. Fungal Infections

If fungal already lives on cannabis leaves, remove affected leaves to prevent the spread of the infection first. Then apply fungicides appropriate for the specific fungal infection, such as copper-based fungicides for Septoria leaf spot or sulfur-based sprays for powdery mildew.

The biggest part of mitigating fungal infections lies in maintaining a proper humidity level. Ensure good airflow to reduce humidity levels, which can prevent fungal growth and moldy weed. Use fans and dehumidifiers as needed.

• How to Lower Humidity in Grow Tent

4. Light Burn

There’s a lot to talk about cannabis indoor lighting. Lucky you, we are experts in this industry and we understand what bothers you. Let’s say, the grow light distance, how to hang the grow light so it suffices the lighting without burning the canopy? Or, how much PPFD does the cannabis need? Take it easy, you can explore everything by clicking the highlighted links to avoid brown spots on cannabis leaves. 

5. Watering Issues

For overwatered weed plants, leave them to dry out completely before making any moves. If the plant unfortunately gets root rot, refer to this guide on cannabis root rot for detailed fixes. 

In terms of prevention, overwatering often happens during the early phases such as the seedling stage, so be aware not to repeat the mistake. Use your fingers or moisture meters to check soil moisture levels before watering. Also, ensure proper drainage in pots and containers.

6. Wind Burn

When using a clip fan, position it to create gentle air circulation without directly blowing on the plants. We suggest placing it on the side wall of the grow tent, slightly above the canopy of the plants. The rest is to observe how plants respond to airflow and adjust fan settings to prevent excessive wind exposure.

Conclusion

Brown spots on cannabis leaves are a sign that something is amiss with your plant's health. This post addresses the causes behind these spots and offers insights on the best treatment. By taking proactive steps to treat and prevent these spots, you can ensure your cannabis leaves remain healthy and productive.