Updated: 9/19/24

While you want your cannabis plant to grow as green and strong as possible, not everything might go according to your plan, and the appearance of rust spots on the leaves during your plant’s flowering phase is definitely something that can rain on your parade.

You might be wondering, “Why are my marihuana leaves turning brown?”

The brown/yellow/orange spots on cannabis leaves resembling rust are usually actual rust - however, not the same type as we know it from doors or windows. Those spots are an early sign of a rust infestation - an infection caused by a fungal parasite, Pucciniales, also known as Uredinales. It’s the most common reason for rust-looking spots appearing on your plant.

Another common cause of rust spots on weed leaves is either magnesium or calcium deficiency, with the former one usually being the case. How can a deficiency happen? Either your plant does not receive enough nutrients, or there’s something that is blocking it from properly absorbing them.

Nutrient burns, pest attacks, heat burns, and light burns are also possible, although these causes are less common.

So, how do you fix the rust spots on cannabis leaves? We’ll discuss that below, going into more detail about each of the causes of the brown spots on weed leaves and what to do in the specific situation.

Rust Fungus Cannabis - How to Save Your Plant?

First things first, remember not to confuse rust fungus with leaf septoria. Although they look similar, the main difference is that leaf septoria has yellow surrounding the spot, meanwhile, rust fungus spots have a fully rusty-looking appearance, with no specific shade located in one place.

Rust fungus usually attacks plants exposed to moist or damp conditions. Plants kept in sunny and warm conditions during the day and wet at night (for example, if you water your cannabis plants late in the evening) and those that get overwatered are most at risk.

How do you minimize the risk of your plant developing rust fungus? First of all, water it during the early hours of the day after checking if the top layer of the soil is dry. Also, make sure that you water the plant and not the foliage, as well as provide your plants with the right temperature and humidity conditions. If you’re growing several plants, keep them in different spots so that if one gets infected, it won’t spread so easily.

How do you treat a cannabis plant that developed rust fungus? If your plant has already developed rust fungus, here are a few things you can do to save it and prevent other plants from getting infected with it, too:

Prune the infected leaves so that the infection doesn’t spread further. Make sure to disinfect the tools you use - a mild bleach solution will do wonders. Also, ensure you dispose of them carefully - don’t keep them near your other plants or use them as compost. Ideally, you should burn them, but if you can’t do that, just keep them as far from your other plants as possible.
Use a copper fungicide on the plants, making sure that you don’t touch the buds with it. You can also use a homemade one, for example, one made of two aspirin tablets and a quart of water.
Make it routine to use either sulfur dusting or neem oil weekly to prevent rust fungus from reappearing.
If your plant is covered significantly, it might be better to consider throwing it away so that your other plants don’t get infected.

Nutrient Deficiency - What To Do?

As mentioned, another common reason for rust spots on cannabis leaves is nutrient deficiency, with the culprit typically being magnesium deficiency, although calcium deficiency might also sometimes cause rust spots to appear.

Nutrients are crucial for the cannabis growth process to go without any hiccups. However, sometimes, one of them is lacking for whatever reason, which negatively affects the whole plant.

Unfortunately, since nutrient deficiency can develop almost unnoticed (it always starts small), there’s not much you can do to prevent it aside from making sure that you feed your plant all the required nutrients. If you notice signs of either deficiency, you need to act immediately, as when left untreated, it can affect your plant’s ability to absorb other nutrients.

How to treat nutrient deficiencies? Well, the first step would be to determine which nutrient is missing. Although the symptoms of both of these are similar, there’s one major difference between them, and that is that when your plant lacks magnesium, the areas between the leaf’s veins will turn yellow, which doesn’t happen with calcium deficiency.

If that doesn’t help, chances are that your plant is actually experiencing a nutrient lockout, which, simply said, is when a plant is unable to absorb the nutrients it needs to survive. Here’s what to do in that case:

Check the pH level of the medium your plant is in. Ideally, it should be between 5.8 and 6.2, and if it falls outside of this range, it needs to be fixed.

First, you need to remove your plant and flush it with clean water. Make sure to clean the roots, as there might be some buildup there.
Use a pH Up or Down solution depending on which side your growing medium falls. This should help rectify the pH, but make sure to check if, after using the solution, the medium falls within that ideal range.

If the pH level is fine, chances are the problem lies in the environment - ensure your cannabis has ideal conditions for proper growth.
Check the magnesium and calcium balance as a precaution - they can both influence the other’s absorption rate, so you might find that there’s there the problem lies.

Nutrient Burn - How to Treat a Cannabis Plant Affected by It

Although not as common, another reason for rust spots can be nutrient burn. This is when you feed your cannabis plant too many nutrients - especially common if you use bottled nutrients, as those are usually very concentrated.

Rust spots are not the only symptom of nutrient burn - if that’s what causes your plant’s issue, you might also notice yellowing or browning dry-looking leaves that might circle at the end. However, in some cases, the opposite situation might happen, and your leaves will actually turn deep green, almost to the point of looking unnatural.

How do you fix a plant that experienced nutrient burn? First, flush the growing medium with clean water, making sure to get any buildup. Then, remove any damaged leaves and buds. Use a plant tonic to treat it and change the fertilizer to one with only ¾ strength of the previous one.

As for prevention, the best thing to do is to always read the manufacturer's instructions regarding any nutrient product you will be using on your plant. If you’re planning on changing your plant’s soil to one that is already filled with nutrients, don’t use any additional products for a few weeks after repotting.

Pest Infestation, Light and Heat Burns - How to Fix It

The final three causes of rust spots are pest infestation, light burns, and heat burns.

First - insects. If your plant experiences pest infestation that causes it to develop rust spots, one of the first things you should do is separate it from your other plants. Some insects are big enough to be visible to the naked eye - if you see any, pick them up. Then, treat your plant with an insecticide and clean the room the plant was in thoroughly.

On the other hand, if your plant experiences light or heat burns, your course of action should be to check the light source and fix it according to your specific strain’s light requirements. Following specific instructions can help you ensure you will be rewarded with pretty potent buds.

The Bottom Line

While rust spots might not be the most aesthetically pleasing sight on your cannabis plant, they are easily treatable as long as you know what has caused them to appear in the first place. Most times, it’s a process of trial and error - if one thing hasn’t worked out, another should.

Still, taking care of those rust spots as soon as they appear is essential for ensuring that at the end of the cannabis growth journey, you will be left with tons of beautiful and delicious buds just ready to be packaged and sent on their way to their new home.

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