Affiliate Disclosure-Keep in mind that we may receive commissions when you click our links and make purchase. However, this does not impact our reviews and comparisons. We try our best to keep things fair and balanced, in order to help you make the best choice for you.

Rust Colored Spots on Cannabis leaves-Preventing And Fixing

Rust Colored Spots on Cannabis Leaves

Rust Fungus Cured! Cannabis

Identifying, Preventing, and Curing

Rust spots on cannabis leaves are most likely caused by four situations, which are the following.

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

Of the above, rust fungus is the most common reason for these unsightly specks appearing on a cannabis plant. 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 is causing the spots to appear.

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 affected leaves. (There can be hundreds of these specs or spots on a single leave) Also, the spots

Once you see any spots on your plants, rub your finger on an infected leaf. If, after rubbing, the spots are removed, and they go onto your finger or elsewhere, you have a fungus or powdery mildew. However, if the spots remain there after being touched, 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 rust fungal disease, your plant leaves will curl and eventually fall off, resulting in stunted plant growth. What is worse, though, is that it will spread and ultimately affect your other plants. So, you need to make sure that all rust fungi-affected leaves, especially includes fallen leaves, are removed and disposed of properly.

How Is It Different From Cannabis Leaf Septoria?

You may have heard of leaf septoria, which is another affliction that affects plants. It is different, however, from rust fungus, as a plant affected with septoria will have a yellow leaf spot and not rust-colored spots. These yellow spots will first appear on the lower leaf of a plant.

What are the Causes?

Rust fungi can thrive on all, green leafy, plants given the right conditions, which are the following:

  • Between 60 to 80 degrees
  • High overall humidity.
  • Rust spots on cannabis leaves will happen most likely during flowering.

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

Preventing Cannabis Rust Fungus

After a plant is affected it cannot be cured. As a result, any affected plants will need to be carefully discarded. So, prevention is the key to dealing with rust fungus. Fortunately, there are several methods of prevention.

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 too 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:

Bonide (BND1428) - Sulfur Plant Fungicide, Organically Controls Rust, Leaf Spot and Powdery Mildew (4 lb.)

Neem Oil is also recommended.

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

Fixing It

It 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 affected leaves. Rust fungal spores 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.
  • Proper Dispose Of All Damaged Leaves-Make sure that any 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. The following are the five best mold-resistant strains that Royal Queen Seeds recommend.

  • 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.

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 happen when cannabis is not able to absorb magnesium efficiently.

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 acidic roots
  • Stunted plant roots
  • Improper, EC, hydroponics ratings.

How Can It Be Prevented?

To prevent it, apply a fertilizer that has magnesium. A spray that uses Epsom salts, like the one shown below will work. 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. Curly-leaf tips with the aforementioned spots can also signal a deficiency. 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 small and is barely noticeable. But, once it gets going, it will come on like gangbusters. 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.

Nutrient Burn

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

Damage done by a nutrient burn cannot be repaired. However, it can be easily recognized, which means that you should stop any further damage from occurring if you pay attention.

How to Fix It

It is fixed by doing the following:

  • Discard– Remove all damaged leaves and flowers.
  • Clean It Up -Flush out what you are growing your marijuana in (“medium”) with pure water.
  • Treat It -Treat your remaining plants with a plant tonic composed of humic, fulvic acid, and humic, and other assorted vitamins and minerals.
  • 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.

Pests Infestation

A hemp russet mite or spider mite is a pest that can cause spots on leaves. However, instead of red, the spots will most likely be a distinctive brown or yellow color. Check out this article for information on getting rid of spider mites during flowering.

microscopic spider mite
Micro View of Spider Mite


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 or controlled. So, practice proper gardening procedures, recognized any problem signs as early as you can, and you should be fine.

Written by ganjashark

Does Smoking Weed Cause Bags Under Eyes and Wrinkles?

White Spots on Marijuana Leaves