Can cannabis plant leaves turn a rust color? Yes, they can! Rust on cannabis leaves mainly happens during the flowering stage.

Rust-colored spots on your cannabis plants could be an indication of a serious problem and you need to act fast to ensure your grow is healthy.

For starters, it could be due to magnesium and calcium deficiency. Nutrient burn or pest infestation can also cause the formation of rust spots.

To get the right treatment, you need to get to the root cause.

rust spots on marijuana leaves

Causes of Rust Spots on Cannabis Leaves

Here are some of the causes of rust spots on leaves:

  • Rust fungus
  • Magnesium and calcium deficiency
  • Nutrient burn
  • Pest infestation
  • Heat and light burns

1. Rust Fungus

Rust fungus results from a fungal parasite that affects plants. The airborne fungus spread quickly to different plants. Unlike other fungi, rust fungus affects only living plants.

It keeps the plant alive while causing damages such that the plant cannot have proper nutrient absorption. The leaves begin to develop brown spots and later die.

Rust fungus is common among indoor growers and closet grows because it thrives in places with high humidity and poor ventilation. It can also occur when the plants are close to each other.

While rust fungus doesn’t kill your plants, it can cause an array of grave side effects.

How To Know If Your Plants Have Rust Fungus

It is pretty easy to know if rust fungus is the main cause of brown spots in your cannabis plants. If you spot one leaf with a brown spot, rub your finger against the rust spot, if the color comes off from your finger, it’s an indication that your plants have rust fungus. If not, they could be suffering from nutrient deficiency.

If the leaves of a single plant are affected by the spores, they can spread to others easily.

Causes of Rust Fungus

Rust fungus in cannabis plants thrives under high relative humidity and long periods of wetness (roughly below 20 degrees C). This kind of environment can be a result of high soil moisture, overwatering, dense shading, and tall weeds surrounding the plant.

Can You Prevent Rust Fungus?

Since rust fungus requires moisture to develop, you could be probably thinking of eliminating moisture. However, that isn’t an appropriate solution. Cannabis requires a humid environment for healthy growth.

You can prevent the fungus from developing by reducing the humidity slightly and keeping the plants in a properly-ventilated room. The room should have cooler night temperatures.

Dry and cool air prevents the development of rust fungus. Proper air circulation between plants also prevents rust fungus. Removing overcrowded leaves is magical too.

Cutting larger leaves from time to time is recommended during the flowering stage. Leaves on the lower branches get more light for proper growth. Cutting the leaves prevents the growth of mold that results from overcrowding.

When watering your plants, do not splash the water on the leaves. Additionally, ensure the leaves do not touch the soil as rust fungus spreads through wind or water. Ensure you have ample space between the plants as it reduces the spread of the fungus spores.

Another great way you can prevent rust fungus is to use a fungi-resistant strain. There are plenty of mold-resistant strains you can buy for your cannabis plants.

Preventive fungicides with sulfur also work wonders. They work amazingly well when dealing with rust fungus, fungal spores, powdery mildew, and other plant-related diseases.

However, note that these fungicides only prevent the fungal spores from coming out. If there is an infection, the sulfur fungicides are not useful anymore.

How to Eliminate Rust Fungus

It is tough to remove rust fungus once your plants are infested. The good thing is that it’s possible.

The first thing you need to do is remove and throw away any infected leaf or plant. Take care when removing the leaves because the spores can infect other plants. Avoid contact as much as possible.

After removing and disposing of infected leaves, use a fungicide to treat the remaining plants. You can use chemical-based or homemade fungicides.

Mix a teaspoon of baking soda with one quart of water. Alternatively, mix half a cup of crushed garlic with one quart of water. Let it stay for about 24 hours before you spray it on the cannabis plants.

2. Magnesium and Calcium Deficiency

Apart from rust fungus, another possible cause of rust spots on leaves is a lack of magnesium and calcium. During the flowering stage, plants require calcium and magnesium.

Nutrient deficiency occurs when plants do not get enough minerals. Leaves begin to show rust spots that spread to the other leaves if left unattended. As a result, flower production goes down.

Signs of Calcium and Magnesium Deficiency

How can you tell your plants are mineral deficient? Some signs can help you know your cannabis plants are low on minerals

Lack of magnesium causes rust spots on the leaves. It could be a mineral deficiency or the plant doesn’t absorb the nutrients effectively. The leaves begin to turn yellow while veins turn green.

If unattended, the spots get bigger. Eventually, the leaf dies. Leaves with calcium deficiency have brown edges and curly tips.

Calcium deficiency problems aren’t curable. You can only limit or control the extent of the damages. If you ignore the problem, it only gets worse.

Other nutrient problems that can affect your plant’s leaves include a lack of zinc, iron, and potassium. However, these deficiencies do not cause rust spots but growers should be aware of them.

Nutrient Lockout

The pH in the soil affects the absorption of nutrients. If it is not within the ideal range, your cannabis plants will suffer from nutrient lockout. When the pH level is more than 7, your plants cannot absorb essential nutrients like potassium, phosphorus, and nitrogen.

When the levels are below 7, the plants cannot absorb magnesium and calcium. Moreover, the nutrient lockout can happen if the plant roots are subjected to acidic, cold, or damp environments.

Lastly, too much ammonium, potassium, or calcium in comparison to magnesium can also cause a nutrient lockout.

Fixing Calcium and Magnesium Deficiency

The first thing you should consider before fixing the problem is to ensure its deficiency and not nutrient lockout. You can do this by checking the soil pH with a pH meter or testing papers.

For effective absorption of magnesium and calcium, the pH levels should be between 6.5-9.1. If the soil pH is outside these limits, your plants are suffering from nutrient lockout.

If the pH is within the required range and would like it up to 6.8, feed your cannabis plants with more calcium and magnesium. Exercise caution, as giving them too much can lead to a lockout.

3. Nutrient Burn

Nutrient burn is the opposite of nutrient deficiency. It occurs when you use too much fertilizer.

How can you tell your cannabis plants are suffering from a nutrient burn? The leaves turn reddish or brown. In addition to the leaves drying and curling up, damage to roots also occurs.

You can fix nutrient burn by removing damaged flowers and leaves. After that, flush the medium using clean and well-balanced pH water. Doing this flushes out excess nutrients from the soil.

Your cannabis plants begin to droop once you remove nutrients. When this happens, stop flushing and add some nutrients back. You can add half the nutrients you were giving them before and monitor their progress.

Depending on the outcome, you can either increase or reduce the nutrients. Give your cannabis plants a combination of minerals, humic, vitamins, silver nitrate, and fulvic acid.

4. Pest Infestation

Another common reason for rust spots on leaves is pest infestation. Pests are responsible for rust spots and a host of other types of spots.

This problem is common with many outdoor growers. However, indoor growers can also experience this problem when they bring infected plants indoors. Pests like spider mites and russet mites cause yellow or brown spots on leaves.

Other pests like aphids not only cause spots but also damage the leaves by leaving tiny punctures. They then move to different branches and breed in huge numbers.

You need to treat pest infestation as soon as you can to prevent breeding. The best way to eliminate pests on your plants is using neem oil or soap solutions.

5. Heat and Light Burns

Burns from light and heat do not appear as spots. They are easily identified when the leave’s edges begin to curl and get darker. If you see such signs on the leaves, this is an indication of heat and light burns. You can prevent this by moving the lights a little far away from the plants.

Indoor Growers and Outdoor Growers: Who’s at High Risk?

Both indoor growers and outdoor growers can experience rust spots on their plants. However, certain causes can affect indoor growers more compared to outdoor growers. For instance, pest infestation is more common with outdoor growers.

Rust fungus, on the other hand, is common among indoor growers because of lack of proper ventilation or too humid conditions. There are cases of outdoor growers that experience rust fungus problems. This is especially true with hot temperatures, high humidity, and improper airflow.

Both indoor and outdoor plants can suffer from nutrient-related problems. It can be either excess or lack of nutrients. You should be careful when watering your plants to provide them with the desired levels of nutrition.

Frequently Asked Questions

Q: What is the right pH for plants to absorb nutrients effectively?

A: You should maintain a stable soil pH of 6.8 for optimal absorption of nutrients.

Q: Why are rust spots common at the flowering stage?

A: Rust spots can take place even during other growth stages. However, they are more common during the flowering stage. At this time, the plants are more exposed. More energy goes into the development of flowers, leaving leaves more prone to deficiencies and diseases.

Q: Who is more susceptible to rust spots, indoor growers or outdoor growers?

A: Both indoor and outdoor growers are at risk of rust spots. All the aforementioned causes can affect both outdoor and indoor growers. The most important thing is to check for early signs and fix them immediately.

Q: Can rust fungus kill my cannabis plants?

A: Rust fungus only harms but it doesn’t kill your cannabis plants. Fungus is just a parasite that requires an organism to survive. Even though it doesn’t kill your plants, you should try your best to prevent it.

Final Thoughts On Rust Spots On Cannabis Plants During Flowering

If you want to be a successful indoor or outdoor cannabis grower, you should be keen on early signs of rust spots on your plants. Different causes of rust spots exist. The same is true of how you identify them and prevention methods.

The most common cause for rust spots is rust fungus. When your plants are rust-infected, you cannot cure them. That is why growers should seek to find rust disease signs early enough and how to prevent it from spreading.

Dealing with rust spots on your cannabis leaves during the flowering stage should be effortless with the above information. Taking early precautions saves your plants making you end up with a bumper harvest.