How to Fix Rust Spots on Cannabis During Flowering?

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Added 14 November 2022

If you drive an old car, you must be used to seeing rust spots on it somewhere. Petrolheads adore those spots and even have a name for them — patina. But what if you start seeing similar spots on your cannabis plant?

Now, that's a different thing altogether! No, your plant is not getting rusty — it is going through a condition like nutrient deficiency or fungus. 

In any case, you must not ignore such rust spots on your cannabis plant. You need to fix the issue immediately, or your plant can suffer dire consequences like reduced yield, weak health, diseases, and a lot more. 

What are these rust spots, what causes them, and how you can fix and prevent them? Find out in this article below.

What are Rust Spots in Cannabis Plants?

What are Rust Spots in Cannabis Plants?

As the name suggests, rust spots are spots on your cannabis plant, most leaves, that appear to be rusty — reddish-brown in color and sometimes coarse to touch. There are a few causes for such rust spots, such as:

  • Rust fungus
  • Magnesium or calcium deficiency
  • Nutrient lockout or burn
  • Pests 

The first two — rust fungus and nutrient deficiency — are the most common causes of rust spots on cannabis leaves. 

Whatever the cause for the rust spots, the problem is potentially dangerous for your plant. In the earlier stages, it may appear to be just some spots here and there. However, if you ignore them, they can cause various problems such as withered leaves, stunted growth, an unhealthy plant that is susceptible to diseases, low yield, and severe cases, even death of the plant. 

Let’s take a look at each cause of the rust spot below.

Cannabis Rust Fungus

The most common cause of rust spots on cannabis leaves is the rust fungus — a fungal parasite, Pucciniales (aka Uredinales) that only attacks living plants. This fungal infection develops brown spots on cannabis leaves during the flowering stage and has the power to destroy the yield entirely. 

What distinguishes this fungus from other fungi types is that it only attacks and feeds off of living plants, unlike other fungi types that can also survive on dead plant matter. If the plant is moist for too long, rust fungus can attack by producing spores that spread via water and air — the reason why you may notice more rust spots after watering your plant. 

Apart from merely showing up as brown spots on the leaves, this rust fungus can also hamper your plant’s photosynthetic and nutrient absorption processes. 

What Causes Rust Fungus in Cannabis?

What Causes Rust Fungus in Cannabis?

As mentioned before, rust fungus appears when the plant is exposed to moist or damp conditions. This is because the fungus thrives in temperatures between 68°F to 86°F (20°C to 30°C), especially when the leaves are wet. And it proliferates during the warmer hours of the day.

So, if you tend to leave your plant wet during the night and the days are sunny, your plant is at a higher risk of rust fungus. It also becomes more at risk if you water your plant a lot more than required or do so during the late hours of the day. 

Ideally, it is recommended to water the plant early in the day when the top layer of the soil is dry so that the water can evaporate during the day. Plus, you should water the plant by pouring water on the soil as opposed to pouring it on the foliage, this keeps the canopy dry.

How to Identify Rust Fungus?

How to Identify Rust Fungus?

There are various causes of rust spots on cannabis leaves, which is why it is crucial that you know exactly what you are dealing with. In this case, let’s take a look at how to identify if your plant is suffering from rust fungus or other causes of rust spots. 

Rust fungus generally affects cannabis plants during the flowering phase, when the fungal spores travel to the plant. And these spots start appearing on nearly all green or leafy parts of the plants. 

Here are some signs that can help you identify rust fungus:

  • Yellow or white leaf spots on the lower tiers of the branch or on either side of the leaves
  • Yellowish or orange spots or stripes on the affected foliage’s underside 
  • Orange to brown blister-like bumps (or pustules) under the infected leaves 
  • Spots that appear to be powdery and reddish-brown in color

Remember, the symptoms of rust fungus depend on the severity and progression of the infection. So, orange spots only appear in the later stages of the infection.

You can even test the spots to identify if it is rust fungus. Simply rub your finger on the spot — if the spots disappear upon rubbing and stick to your finger, you are dealing with rusk fungus. It is basically a mildew problem. 

However, if the rust spots remain intact when you rub them, it is likely not due to rust fungus but a nutrient issue. 

How to Treat Rust Fungus in Cannabis?

How to Treat Rust Fungus in Cannabis?

If you have identified rust fungus on your cannabis plant, time is of the essence — you must treat the problem immediately as it can spread quickly to other plants in your garden. Here are some techniques that can help you get rid of rust fungus:

  • Prune infected leaves and dispose of them carefully so the spores don’t travel to other plants 
  • Disinfect your pruning tools and the entire grow room to ensure no spores are left behind — a mild bleach solution would come in handy 
  • Then, apply a copper fungicide to the plant to remove the fungus entirely from the plant but ensure you don’t apply the fungicide to the buds as it can leave a nasty aftertaste
  • Every week, apply a sulfur dusting (or neem oil) to the cannabis plant to prevent rust fungus from reappearing. You can also use baking soda with mild horticultural oils
  • If the plant is covered in fungus, it is advisable to toss the plant to protect your other plants 

Remember, when you prune the infected leaves, you must discard them properly. Avoid using the leaves in compost, either, and do not leave them around other plants — the spores can travel easily via air or water. The best way to deal with infected leaves is to burn them. 

If you don’t want to use a commercial fungicide, you can even make a fungicide at home. Here are a few ideas:

  • Baking soda mixture, where you need to mix 4 teaspoons of baking soda with a gallon of water and an ounce of horticultural oil
  • Aspirin mixture, where you mix two aspirin tablets to a quart of water
  • Bordeaux mixture, where you mix copper sulfate with lime water

How to Prevent Rust Fungus in Cannabis?

How to prevent rust fungus in cannabis

While it is not impossible to prevent rust fungus, you can still prevent it to a large extent with a few precautions. Here are some tips to prevent rust fungus from reappearing on your cannabis plant:

  • Make sure your grow area has the right temperature and humidity, good ventilation and airflow, and is clean
  • Spread out the plants if you're growing multiple plants in one grow room
  • Choose fungi-resistant seeds from reputable seed banks — seed genetics play a role in resisting fungi
  • Ensure your plant gets adequate light throughout the day so the excess water can evaporate 
  • Apply sulfur fungicide. It was used by Ancient Greeks over 2000 years ago and it is still one of the best ways to prevent rust fungus; another alternative is neem oil as it is organic and just as effective 

1. Magnesium or Calcium Deficiency

Rust fungus vs. Magnesium deficiency

The second most common cause of rust spots on cannabis leaves is a nutrient deficiency of either magnesium or calcium. Here, magnesium deficiency is more common than calcium one. 

This happens when the plant experiences a shortage of either compound in its nutrient routine, and since it needs these nutrients during the flowering stage, the rust spots become more common during bloom. 

The two most common reasons why your plant may be experiencing this nutrient deficiency are, one, you are not feeding it enough nutrients, or two, it is experiencing a nutrient lockout where it can’t absorb the nutrients properly from the medium.

Let’s take a look at each type of deficiency in detail.

Magnesium Deficiency

Magnesium deficiency is one of the most common deficiencies your plant may experience, and it is identifiable by rust spots and leaves turning yellow. And this deficiency may occur for the following reasons:

How to Identify Magnesium Deficiency in Cannabis?

In most cases, magnesium deficiency starts showing itself on the plants after 4 to 6 weeks of its start, and the first sign is usually interveinal chlorosis, where the areas between the leaf’s veins start yellowing, followed by the development of rust spots. 

As the issue progresses, the rust spots grow in number and size and start overtaking the tips of the foliage. In some severe cases, the plant may also start appearing droopy and leaves may start to curl up or wither. 

Calcium Deficiency

Rust fungus vs. Calcium deficiency

On the other hand, calcium deficiency is another reason why your plant may experience rust spots on the leaves. With calcium deficiency, the rust spots appear with a dark brown outline and the leaf tips also start curling. 

This deficiency generally starts small — easy to miss — but over time, it can grow significantly. And the worst part? You can’t really do anything about it except control the damage caused by this deficiency. 

So, you must treat calcium deficiency immediately or the rust spots may cover the entire plant, wither the leaves, and stunt your plant’s growth. The end result would be subpar yields with low bud quality. 

How to Identify Calcium Deficiency in Cannabis?

Here are some ways to identify if your cannabis plant is suffering from calcium deficiency:

  • The leaves are curling and developing brownish-yellow spots with a brown outline 
  • The root tips are wilting 

Yes, the symptoms do appear similar to those of magnesium deficiency but the only difference here is, with calcium deficiency, the leaves don’t turn yellow and the veins remain green. 

How to Treat Magnesium or Calcium Deficiency in Cannabis?

Once you notice any signs of magnesium or calcium deficiency, you need to act quickly so that your plant’s ability to absorb other nutrients does not get affected — that can lead to other deficiencies. 

Figure out which nutrient your cannabis plant is lacking and give more of it. If giving more of that nutrient does not solve the problem, your plant may be experiencing a nutrient lockout. 

In that case, begin by checking the growing medium’s pH as it can affect the plant’s ability to absorb nutrients. Ideally, the pH should be between 5.8 to 6.2; if the pH is outside this range, you need to rectify the pH.

For this, first, flush the plant and grow medium with clean water. Then, balance the pH of the nutrient solution by using pH Up or Down solutions and recheck the pH. Ideally, you should also check the pH of the runoff water and compare it to that of the nutrient solution — both readings should be largely similar. 

On the other hand, if the pH is within the ideal range, you need to check the environment — ensure the grow room is neither too cold nor too damp. 

You should also check the balance of magnesium and calcium — both can affect each other’s absorption rate. 

To prevent this type of deficiency from occurring again, you can use CalMag solutions in your plant’s nutrient routine. These products contain a balanced amount of calcium and magnesium, so you don’t have to worry much. 

2. Nutrient Burn

Rust fungus vs. Nutrient burn

If you have ruled out rust fungus and nutrient deficiency for rust spots, the next most common cause could be nutrient burn, which can also cause rust spots to appear on the leaves during the flowering stage.

Essentially, nutrient burn occurs when you feed excess nutrients to the plant. Remember, more isn’t always better when it comes to feeding cannabis plants. 

Nutrient burn is caused due to overfeeding your plants, and it can also happen if you only use natural or organic products on your plant. However, you need not worry about compost or bat guano — they aren’t strong enough to cause nutrient burns in most cases. 

But you need to worry about bottled nutrients that you may have purchased from your local gardening stores — sometimes, they are notoriously concentrated. Additionally, excess nitrogen, potassium, phosphorus, and other nutrients in the nutrient solution can also cause a nutrient burn.

How to Identify Nutrient Burn in Cannabis?

Identifying nutrient burn in cannabis is easy because rust spots aren’t the only symptom of this problem. Along with rust spots, you may also notice the tips of the leaves turning brown or yellow; the leaves may also look dry or curl at the tips. 

At the same time, the leaves may also turn deep green, as if they are oversaturated. This is another common sign of over-fertilization. 

How to Treat a Nutrient Burn in Cannabis?

Treating nutrient burn is easy — simply turn down the number of nutrients you give your plant. But it does require some planning (and a few additional steps), such as the following:

  • Flush the medium with clean tap water to flush out any nutrient buildup in the root zone 
  • Prune any damaged leaves and buds
  • Treat the plant with a plant tonic (humic or fulvic acid, or other vitamins and minerals)
  • Only use a fertilizer that has ¾ less strength than you previously used and check for the symptoms again

Most likely, the nutrient solution you are using is too strong. So, toning it down after flushing the medium should fix the issue. 

How to Prevent Nutrient Burn in Cannabis?

Nutrient burn is easy to occur and quick to damage — you need to prevent it from occurring again as it can stress out your plant and affect the yield. Follow these steps to prevent nutrient burn in cannabis:

  • Always read the manufacturer’s recommendations when using any kind of nutrient product on cannabis — more isn’t better
  • Stick to the nutrient schedule according to the plant’s strain and growing conditions 
  • If you are using new potting soil, it already has nutrients in it — refrain from adding nutrients to the soil for the first few weeks
  • If you notice any symptoms of a nutrient burn, immediately stop applying any more nutrients to the plant and flush the growing medium with clean water

3. Pests, Heat Burn, and Light Burn

Apart from the instances listed above, your plant may also start showing rust spots if it is suffering from a pest attack, or a heat or light burn. Although, these causes are largely uncommon. 

An uncommon yet possible cause of rust spots on cannabis leaves is pests. Insects like spider mites or hemp reset mites can cause similar spots to appear on the leaves, but instead of red, these spots tend to be more yellow or brown in color.  

Dealing with these insects can be difficult, but with the right steps, you can save your plant. Start by quarantining your plant from other plants, pick off any insects you see, and treat your plant with an insecticide. Also, clean your grow room to get rid of any leftover pests and their eggs.

To check if your plant is experiencing heat or light burn, check the light source. Usually, rust spots due to light burn only appear on the top tier of the plant that is closer to the light source. If you only notice rust spots in the top tier, it is likely due to the intensity of the light. 

To fix a light or heat burn, invest in a thermometer and a light meter. Learn more about your strain’s light requirement and set the light accordingly. Again, too much light is not a good thing — so don’t bombard your plant with too much light, which will burn the plant. 

Is Rust Fungus the same as Leaf Septoria in Cannabis?

Rust fungus vs. Leaf septoria

Do note that rust fungus is easy to confuse with leaf septoria, another fungal disease that plagues many cannabis gardens. The latter is an equally harmful disease caused due to Septoria lycopersici. 

You can differentiate between the two by looking closely at the spots — leaf septoria will develop yellow spots, not rust spots, and these yellow spots will appear first on the lower tiers of the plant. Some people also confuse this with calcium deficiency, but you can rub the spots to check if it sticks to your hand. 

It is crucial that you differentiate between the two before implementing any treatment to your plant. 

Rust Spots on Vegetative Cannabis Plants

We mentioned earlier that rust spots appear on flowering cannabis plants, but occasionally, rust spots may also occur during the plant’s vegetative stage. It is entirely possible that you may notice rust spots on your cannabis plants when it is in their vegetative stage.

However, the causes largely don’t change. Rust spots on vegetative cannabis plants also tend to occur for the same reasons: rust fungus, too much nutrition, or not enough calcium and magnesium.

The reason why rust spots tend to appear more on flowering plants is that they require a lot more nutrients like magnesium and calcium. But if the soil of vegetating cannabis is not pH balanced and the conditions are too cold or moist, then a nutrient deficiency can occur regardless of the plant’s growth stage. 

Similarly, a nutrient burn can also occur at all stages of the plant if the nutrient solution is excessively concentrated for the plant, especially if the plants are potted in new potting soil that is already loaded with nutrients. 

As for rust fungus, it tends to attack flowering cannabis plants more than vegetating ones, but if your plant is not lucky enough, it can still suffer a rust fungus infection when it is still growing. 

Follow the advice listed above to identify the problem and fix it. 

Summary: How to Fix Rust Spots on Cannabis During Flowering?

Rust spots on cannabis are scary — rightfully so — which is why you need to fix the issue urgently. But before that, you must identify the problem. Are the spots caused due to fungus, nutrient deficiency, or nutrient burn?

Once you have figured out the cause, simply follow this guide to fix the issue. And your yield will be just as good as you’d expect. When treated in time, rust spots don’t leave any lasting effect on the plant.

Although, in some cases, you may not be able to treat the plant completely. For instance, if rust fungus has overtaken your cannabis plant, treating it won’t be so effective. In that case, it is better to be rational and toss the plant. Be realistic and treat the condition accordingly. 


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