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Rust Colored Spots on Cannabis leaves-Preventing And Fixing

Rust Fungus Cured! Cannabis

Rust Colored Spots on Cannabis Leaves

Identifying, Preventing and Curing

Rust spots or specks on your cannabis leaves are most likely caused by four situations that are the following

  • Rust Fungus
  • Calcium Deficiency
  • Magnesium Deficiency
  • Nutrient Burn

Of the above, rust fungus is the most common reason for these unsightly, specks appearing on cannabis leaves.

I will go over each of these above situations and then go over preventative measures and ways to cure this issue.

First Things First

The first step for you to take before you begin to address this issue is discovering exactly what type of problem you are having.

How To Identify It

Rust fungus, which can appear on almost all green, leafy plants, is the leading cause for these dots to appear on your plants. However, there are other reasons for these spots. So, never assume that it is a fungus problem, nutrient deficiency, or other problems.


A mold or fungus infestation be detected visually and also by touching. It can be visually recognized as it causes plants to have multiple rust-colored spots on the leaves. There can be hundreds of these specs or spots on a single leave.

Once you see any spots on your plants, rub your finger on any affected areas on the plant. If, after rubbing, the spots are removed, and they go onto your finger or elsewhere, you have a fungus or mold issue. However, if the spots remain there after being rubbed, it is most likely a nutrient deficiency issue that needs to be addressed.

Rust Fungus

Why Should I Care?

When you have an out of control problem, your plant leaves will curl, with and the leaves may eventually fall off, resulting in your plants’ growth being stunted. What is worse, though, is that it will spread and ultimately affect your other plants if it is not taken care of.

What are the Causes?

It will grow on all, green leafy, plants given the right conditions, which are the following:

  • Between 60 to 80 degrees, and high overall humidity. (This is the ideal
  • During the Flowering Growth Stage

Unfortunately, the above is also considered to be the best conditions for growing marijuana.

Prevention And Cure?

Once your plants are affected, you cannot get rid of the fungus or mold on your plants. As a result, any affected plants will need to be carefully discarded. So, this should go without saying, prevention is the key to dealing with rust fungus. Fortunately, there are several ways for you to prevent fungus.

Grow Room Environment

The first step you should take is to ensure that your grow area is at the ideal temperature, adequately ventilated, clean grow room, and that the air circulates properly. Proper air circulation can be done by making sure your plants are not placed to close together. So, be sure to buy the correct size grow tent.


You should supply a sulfur-based fungicide. Some examples that are rated highly are the following:

Commercial fungicides, like Neem Oil, is one recommended fungicide. That will help prevent it from even showing up.

Organic Neem Bliss 100% Pure Cold Pressed Neem Seed Oil - (16 oz) High Azadirachtin Content - OMRI Listed for Organic Use


Fixing It

Rust fungus spreads easily. So, as stated, preventing this from happening is critical. But, if you do have it, you need to do the following:

  • Remove-You must first remove and dispose of any leaves that are affected by rust fungus. Rust fungus can spread quickly. So, you need to be sure that you do this quickly and thoroughly.
  • Apply Fungicide- After clearing and disposing of affected plants you need to apply a fungicide to the remaining plants that do not have any problem. The following is highly recommended on Amazon and is specifically designed to deal with fungicide.

Bonide Chemical Copper 811 4E Fungicide, 16 fl oz & (BND210) - All Seasons Horticultural and Dormant Spray Oil, Insecticide Concentrate (16 oz.)

 Proper Disposal Is Essential-You need to either burn or So, you need to make sure that the infected plants are either burned or thrown away. So, do not place any of the leaves in your mulch pile.

Choose A Fungi Resistant Strain

Fungi Resistant Strains

One way to reduce the chance of having these spots on your plant leaves is by purchasing mold-resistant cannabis strains. If you are growing outdoors in a wet environment, you should especially consider buying these types of strains. The following is are the five best, mold-resistant strains that are recommended Royal Queen Seeds.

  • Stress Killer Automatic-(Hybrid)-Sativa Dominant
  • Shinning Silver Haze-(Hybrid)-Sativa Dominant
  • Power Flower-(Hybrid-Sativa Dominant
  • Royal Moby-Sativa Dominant
  • Northern Light-100% Indica

Click Here for more information on these above strains.

There are many more mold-resistant strains. For further research, I would look at the following websites:

Also, just Google “Mold Resistant Strains,” and you’ll find hundreds of other strains that will better resist rust fungus.

Magnesium Deficiency

Rust spots, along with leaves turning yellow are a few ways to identify magnesium deficiency.

A magnesium deficiency is the most common form of nutrient deficiency that you will see in cannabis. Of course, it can be caused by not enough magnesium. However, it also can occur when the plant may have the correct levels of magnesium. But it is not correctly absorbing this element.

Some of the reasons why it would not absorb correctly are below.

  • Too much calcium, potassium or ammonium, in proportion to the magnesium
  • Very damp, cold or an acidic roots
  • Plant roots are stunted
  • Improper, EC, hydroponics ratings.

How Can It Be Prevented?

To prevent it, apply a fertilizer that has magnesium.

How Can It Be Fixed?

To fix it, you need to apply a spray that uses Epsom salts. The following, from Monterey, will work well for this purpose. Monterey LG7220 Epsom Salt for Plants Magnesium Sulfate for Gardening, 4 lb

Calcium Deficiencies

Calcium deficiencies also can result in these spots appearing with a dark brown edge or outline. A deficiency can also be recognized by curly-leaf tips with the aforementioned spots. For a calcium deficiency, you’ll be more likely to see brown spots instead of rust spots. However, it can sometimes be tough to differentiate between the two.

Calcium deficiency starts out small and is barely noticeable. But, once it gets going, it can come about quickly. You cannot cure a calcium deficiency problem, you can only control and limit the damage caused by calcium deficiency. And delaying or ignoring this problem will only make it worse.

Prevention and Treatment

Soil that has a 6.4 pH level or less can cause a deficiency. So, regularly check on your soil PH levels to prevent this from happening. If you do have this problem, apply measures to increase the soil PH levels. Also, you should consider using gardening soil that contains peat.

Bloom City Professional pH Up + Down Control Kit (Two 1 Gallon Bottles) 256 Total oz

Nutrient Burn

Just like people, you can overfeed your plants. Overfeeding or over-fertilizing your plant causes nutrient burn. Nutrient burn results in your leaves browning (sometimes the color is reddish) leaf’s edges are drying up, marijuana leaf curling up, and root damage.

You can’t repair any damage that has been done by nutrient burn. However, it can be easily recognized, which means that if you pay attention, you should stop any further damage from occurring.

How to Fix It

Nutrient burn can be fixed by doing the following:

  1. Discard-Remove all damaged leaves and flowers.
  2. Clean It Up-Flush out what you are growing your marijuana in (”medium”) with pure water.
  3. Treat It-Treat your remaining plants with a plant tonic composed of humic, fulvic acid and humic and other assorted vitamins and minerals.
  4. Fertilizing– To stop it from reoccurring again, only use a fertilizer that has 3/4 strength.

The following is a summary of how to fix “nutrient burn.” Click Here for a more thorough explanation.


Rust colored spots on your cannabis plants are one of the many problems you need to address. Rust fungus is the most likely reason for these rusty specks on your plants. All of the above issues can be prevented, and if needed, they can be fixed. So, practice proper gardening procedures, recognized any problem signs as early as you can, and you should be fine.


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