During a regular inspection of your cannabis plants, you might have noticed some brown spots appearing on the fan leaves at some point. Urgh! What gives?
The good news is that the brown spots are usually caused by two conditions that are easily fixed: a nutrient deficiency or an infestation by pests like spider mites or fungus.
The most common nutrient deficiencies that make leaves turn yellow or have brown spots are caused by a lack of calcium, magnesium, and potassium. In addition, one of the most frequently reported yellow leaf spot diseases in some parts of Australia is the contagious fungal infection known as Septoria leaf spot.
In this guide, we’ll dig into the reasons why brown spots appear when you grow cannabis, what they mean, and, most importantly, how to fix them to keep your plants looking and feeling their best!
Key Takeaways
- Brown spots on weed leaves can indicate various issues, including nutrient deficiencies, pests, underwatering, diseases, and environmental stress.
- Calcium and magnesium deficiencies are common nutrient-related causes of brown spots, while nitrogen deficiency is the leading cause of yellow leaves.
- Pests like spider mites and fungal infections in your grow medium can also lead to brown spots on your cannabis leaves.
- Environmental factors like overwatering and heat stress are other potential culprits.
- Solutions include nutrient management, pest and disease control, and making environmental adjustments.
What Are Brown Spots On Weed Leaves?
Brown spots on weed leaves are exactly what they sound like – discolored brownish, yellowish, or light burn areas on your marijuana leaves that can range from small specks to larger patches.
You might notice that small, irregularly shaped spots with bright yellow margins form on leaves. These spots expand to about ¼ inch in diameter and appear brown with a prominent yellow halo.
These unwanted markings can appear on the edges, tips, or in the middle of the leaves. Sometimes, they’re dry and crispy, other times, they’re more like blotches.
Why It’s Important to Address Brown Spots
Ignoring brown spots in the vegetative or flowering stage is a bad idea. They can indicate underlying problems that, if left unchecked, can spread and affect your entire crop. In addition, brown spots can stunt growth, reduce the quality of your yields, and in severe cases, kill your plants.
Catching and fixing brown leaf issues early is crucial for keeping your cannabis plants healthy and productive.
Common Causes of Brown Spots
What causes those pesky brown spots? There are various possibilities. Let’s examine the most common causes.
Nutrient Deficiencies
Nutrient deficiencies are a biggie when it comes to brown spots appearing on plant leaves. Calcium, magnesium, and potassium are the most common deficiencies cannabis growers should be worried about.
Your cannabis plants need a balanced diet of nutrients to thrive, and when they don’t get enough of a particular nutrient, brown spots can be a telltale sign.
- Calcium (Ca) Deficiency
Calcium is essential for cell wall development and overall plant structure. A shortage of calcium can make plants more susceptible to diseases and pests. You may notice brown spots on your leaves, especially on newer growth.
These spots are usually accompanied by curling leaf tips and edges. A foliar spray of commercially available Cal-Mag is a first-step remedy you can try.
Common symptoms of a cannabis calcium deficiency include:
- Small brown spots appear on new growth.
- Leaves may become distorted and brittle.
- Magnesium (Mg) Deficiency
Magnesium is crucial for photosynthesis. A shortage can reduce photosynthetic efficiency by 25–50%, which will harm overall plant health and yield.
A deficiency typically shows up as yellowing between the leaf veins and brown spots on the older leaves. If your lower leaves develop brown spots, magnesium might be the issue.
Common symptoms of a magnesium deficiency include:
- Interveinal Chlorosis, which is yellowing between the veins of older leaves, leads to brown spots.
- Severely affected leaves may drop prematurely.
- Potassium (K) Deficiency
Lacking sufficient potassium can also cause weed leaves to turn brown. Potassium deficiency can decrease the quality of the buds and affect their potency and flavor.
Typically, potassium deficiency is caused by improper soil dynamics and root-soil interactions that incorrectly regulate potassium transporters.
Symptoms to look for:
- Brown Edges: You may notice brown or yellow leaf tips, which are occasionally crispy-like.
- Interveinal chlorosis: yellowing between the veins of older leaves, followed by brown spots.
Pests and Diseases
Pests and diseases are another common cause of brown spots on cannabis leaves. These unwanted guests can wreak havoc on your plants if not dealt with promptly.
Regardless of the type of crop, pests are a huge issue for many farmers. Read about an analysis of the most common cannabis pests done by the US Ministry of Environment and Climate Change Strategy.
Here are the most common pests to worry about:
- Spider Mites
Spider mites are tiny pests that can cause big problems. They suck the sap from the leaves, leaving behind small yellow, brown or bronze spots. If you notice webbing on your plants, spider mites are likely the culprit.
The spider mites have this ugly-looking cream-yellow oval body with 2 dark spots on either side of their abdomen, and their feeding habits will weaken your cannabis plant.
- Fungal Infections
Fungal infections like powdery mildew or cannabis leaf septoria can also lead to brown spots on cannabis leaves. These fungal infections compete for nutrients and stunt your plant growth, significantly affecting your yield.
Powdery mildew looks like white, powdery spots that can turn brown, while leaf septoria starts as small yellow spots that turn brown.
Environmental Stress
Cannabis growing is also affected by environmental factors which can stress your plants and cause brown spots, too. Depending on the strain, stage of growth, and size, cannabis plants like to be kept at a certain temperature and humidity, and unfavorable conditions could get your plant stressed.
In addition, overexposure to heat, wind, and light can also lead to brown spots and buds.
- Overwatering
Watering cannabis is essential for their growth. However, overwatering and underwatering are common mistakes, especially for new growers. When the soil is constantly wet, the roots can’t breathe, leading to nutrient deficiencies and brown spots may appear.
To make sure you’re watering your plants right, check out our complete cannabis watering guide, where we give you a week-by-week breakdown of how to water your plants the right way for the best yields and to avoid overwatering or underwatering.
- Heat Stress
Cannabis plants like it warm, but too much heat can cause problems. Depending on the strain, plants typically like the temperature at 65–80 °F. Overexposure to heat can lead to burned tips and edges, which can appear as brown spots.
How Do You Diagnose Brown Spots?
Before you can fix the problem, you need to figure out what’s causing it. Here are a few things to look out for when diagnosing brown spots on cannabis:
Check for Symptoms
Start by closely inspecting your plants. Look at the size, shape, and location of the brown spots. Are they on the edges, tips, or middle of the leaves? Are they dry and crispy or more like blotches? Noticing these various leaf symptoms can help narrow down the cause.
Test Soil and Water pH Levels
Nutrient absorption is heavily influenced by pH levels. If the pH of your soil or water is off or the pH is too low, your plants might not be able to take up the nutrients they need, even if those nutrients are present.
Use a pH meter to test your soil and water. The ideal pH range for cannabis is between 6.0 and 7.0 for soil and 5.5 and 6.5 for hydroponics.
Look For Patterns and Locations
Take note of where the brown spots are appearing. Are they on the older, lower leaves, or the newer, upper growth?
Different patterns can indicate different issues. For example, nutrient deficiencies often show up on older leaves first, septoria spores block phothosynthesis, while pests might target new growth.
Solutions for Brown Spots On Leaves
Now that you’ve identified the cause, here’s how to tackle the most common causes of brown spots.
- Nutrient Management
Treating brown spots due to nutrient deficiencies will require you to adjust your feeding regimen.
Calcium Deficiency
- For calcium deficiencies, add a calcium supplement to your nutrient mix. Products like Cal-Mag are specifically designed to address calcium and magnesium deficiencies and are crucial for healthy leaf development. You can also use dolomite lime or gypsum as a soil amendment.
- For example, a product containing 5% calcium can be applied at a rate of 1 teaspoon per gallon of water.
- You can also apply lime or gypsum to the soil to increase calcium availability.
- Follow a nutrient-feeding schedule, keep an eye on your plants, and make adjustments to the nutrients that your cannabis needs as necessary.
Magnesium Deficiency
- Use a magnesium supplement, such as Epsom salts (magnesium sulfate). Apply at a rate of 1 teaspoon per gallon of water.
- Apply a foliar spray of magnesium sulfate for quicker uptake.
Potassium Deficiency
To combat a shortage of potassium:
- Use a potassium-rich fertilizer, such as one with an NPK ratio of 5–10.
- Use a ratio of 1-2 teaspoons per gallon of water, but always follow the specific instructions on the fertilizer package to avoid over-fertilization, which can cause nutrient burn.
- Pest and Disease Control
If you’ve figured out that pests or diseases are causing brown spots, here are some actions you can take to eliminate them.
For spider mites, try natural remedies like neem oil or insecticidal soap. These can be effective for taking them out without harming your plants.
For fungal infections, products like potassium bicarbonate can help control powdery mildew, while copper-based fungicides are effective against leaf septoria.
- Environmental Adjustments
Environmental factors can often be adjusted with a few simple changes.
Proper Watering Techniques
Avoid overwatering by allowing the top inch of soil to dry out between waterings. Ensure your pots have good drainage to prevent water from sitting at the bottom and causing root rot.
Temperature and Humidity Control
Keep your growing environment within the optimal temperature range of 65–80°F during the day and slightly cooler at night.
Use fans and ventilation to control humidity levels, keeping them around 40–60% during the vegetative stage and slightly lower during flowering to prevent mold.
How Do You Prevent Brown Spots in the Future?
Prevention is always better than cure, and the best way to deal with brown spots is to prevent them from happening in the first place.
- Monitor and Maintain
Inspect your plants for signs of stress or deficiencies as often as possible. Catching problems early can prevent them from becoming serious issues.
- Use Preventive Measures for Pests and Diseases
Just like our bodies, pests and diseases proliferate when the plant environment isn’t at its best hygiene. So, implement preventive measures like regular cleaning of your grow space, using pest-resistant strains, and applying preventive sprays such as neem oil.
- Search and Implement Optimal Growing Conditions
Maintain optimal growing conditions by monitoring and adjusting temperature, humidity, and pH levels.
Frequently Asked Questions (FAQ)
What are the most common nutrient deficiencies that cause brown spots on cannabis leaves?
Calcium Deficiency: Calcium is crucial for plant cell wall integrity. A deficiency results in brown spots, curled leaf tips, and edges, especially in new growth.
Magnesium Deficiency: Magnesium is vital for photosynthesis. A deficiency shows yellowing between leaf veins and brown spots on older leaves.
Use nutrient supplements like Cal-Mag and conduct regular soil and pH tests to ensure nutrients are available for uptake.
How can pests and diseases cause brown spots on my cannabis plants?
Spider Mites: These pests feed on leaf sap, leaving small yellow or brown specks that coalesce into larger spots. Look for fine webbing on leaf undersides.
Fungal Infections: Fungi-like powdery mildew and leaf septoria cause brown spots. Powdery mildew appears as white spots turning brown, while leaf septoria starts as yellow spots turning brown. Both thrive in humid conditions.
Combat pests with beneficial insects or organic pesticides. Control fungal infections with good air circulation, humidity control, and fungicides.
How does environmental stress contribute to the development of brown spots on cannabis leaves?
Overwatering: Excess water leads to root rot and nutrient deficiencies, causing drooping leaves, yellowing, and brown spots.
Heat Stress: Excessive heat burns leaf edges, causing brown, crispy spots. Maintain temperatures between 65 and 80 °F.
What steps should I take to diagnose the cause of brown spots on my cannabis leaves?
- Visual Inspection: Examine affected leaves for size, shape, and location of spots.
- Check for Pests: Look for signs of pests on leaf undersides.
- Test Soil and Water pH Levels: Ensure pH is in the ideal range ( 6.0–7.00 for soil, 5.5-6.5 for hydroponics).
- Identify Patterns and Locations: Note if spots are on older or newer leaves.
How can I prevent brown spots from appearing on my cannabis plants in the future?
- Regular Monitoring: Inspect plants regularly for stress signs or deficiencies.
- Proper Nutrient Management: Provide a balanced diet and adjust based on the growth stage.
- Environmental Control: Cannabis requires optimal temperature, humidity, and light levels. Ensure they are in moderation.
- Preventive Measures for Pests and Diseases: Clean the grow space regularly, use pest-resistant strains, and apply preventive sprays.
Final Thoughts
Brown spots on your cannabis leaves can be worrying but with the right approach, you can diagnose and fix the issue, ensuring your plants stay healthy and productive.
Visit Aussie Hemp Seeds and check out our collection of fast-growing seeds to Join our community of passionate growers to take your cannabis cultivation to the next level!