Rust Spots On Leaves During Flowering

The flowering of plants can be a wonderful thing to behold; however, if you’ve noticed reddish-orange spots on your cannabis leaves recently, it’s possible that your plants are suffering from leaf rust or rust-colored spots — particularly rust spots on leaves during blooming.

It could confuse you as to how leaves can rust, as rust marks on cannabis leaves can’t be the same as rust spots on metal things. Once you begin to see rust-colored spots on your plant, these are produced by either a deadly rust fungus disease or a magnesium or calcium nutrient shortage.

Rust fungus appears on the leaves as a powdery rust-colored coating. The fungus can sometimes leave small brown spots or a dark brown border on the leaves. While rust spots don’t usually kill plants, they can have undesirable side effects.

Getting rid of rust fungus or brown spots on leaves during flowering might appear to be a challenge, yet there isn’t too much need to panic.

Brown spots on weed leaves

In our guide, you can learn more about brown spots on weed leaves and whether it is fungal growth or a deficiency. By the end, you’ll have enough information on how to introduce preventative measures to prevent rust fungus or prevent a dark brown edge from the nutrient burn. (Read Will Weed Killer Kill Flowers)

How Do You Treat Rust Spots On Leaves?

Before rushing to fix brown spots on fan leaves, it is better to have a rough understanding of what can lead to these diseases that can spread quickly.

Here is a breakdown of exactly what could be leading you to fix rust spots, be it a disease or a calcium or magnesium deficiency.

Rust Fungus

Rust fungus comes from a special fungal parasite that only affects plants. The scientific name is Pucciniales or Uredinales. Although, unlike some other fungi, which are decomposing living organisms, rust fungus will affect living plants rather than dead plant leaves.

The parasite enables the plant to live while causing damage as the plant cannot absorb nutrients properly.

Nutrient Deficiencies

Apart from a fungal infection, brown spots are caused by a magnesium or calcium deficiency and possible nutrient burn.

However, you’ll mostly discover rust-colored spots are because of fungus or rust mold.

Identifying Rust Fungus On Leaves

Plant rust happens when your growing environment is too moist. With a lengthy exposure to dampness, it leaves the door open for rust fungal attack.

Fungal spores propagate the infection, which means one diseased cannabis leaf can spread the disease to neighboring leaves or plants. Cannabis leaf septoria can be spread by bacteria, nematodes, animals, and other plant pathogens.

Diseased on cannabis leaf

How To Spot Rust Spots On Middle Aged Leaves

Here’s how to see if your plants have rust spots:

  • Are there white or yellow leaf spot on the plant’s upper or lower leaves and lower branches? In addition, affected leaves can have orange or yellowish color spots or streaks underneath.
  • Check if there are any pustules on the leaves’ underside, Pustules will look like small blisters and are usually orange to brownish red.
  • Don’t confuse rust fungus with leaf septoria as this causes leaf spotting yet is caused by Septoria lycopersici. It is a destructive disease that is severe in wet or high humidity areas.
  • You can see reddish-brown spots on the plant’s leaf surfaces during the advanced infection stages. The spots have a powdery complexion and will be accompanied by fungal reproductive spores.
  • Weed plants suffering from rust show sure signs and other symptoms during flowering stages.
  • The exact symptom is based on how far the infection is. In the latter stage of infection, you’ll discover reddish bumps infecting most of the leaf surface.
  • To figure out if these spots are infections or nutrient burn, rub one of the spots between your fingers.
  • If the rust rubs onto your fingers, it’s an infection. If not, your plants are suffering from you adding too many nutrients and thus nutrient burn.

With this, you may wonder if rust fungus on cannabis leaves is harmful? As we said, the fungus won’t kill a marijuana harvest. The parasite has to have a living organism to stay alive; thus, the fungal parasite has a food source.

Even with rust infection not killing your plants, you will need to purge plants of this nuisance. Plants with this can quickly infect your entire garden. Luckily, some plants are immune to some strains of plant rust parasite but can develop infections if exposed to other fungus strains.

To get rid of this, purge infected plants as soon as possible, or else it will spread to your remaining plants, and should this happen, it could mean growing new plants.

Despite the tough fungal spores, leaf rust fungi can be removed from cannabis crops using the right methods.

Here is reasons plant rust is harmful to marijuana plants:

  • Rust infections cause weak leaves, which turn yellow, and then the plants shed these leaves, but without the plant dying. However, when your cannabis plant loses leaves, it loses the chance to make energy through photosynthesis and respiration.
  • Once this happens, a plant will fall into survival mode. It stops growing new leaves, and plant growth is stunted.
  • Sometimes, plants need extra water in the flower production phase. However, if we water plants too often without sufficient sunlight exposure, they begin to rust. The reason is, rust thrives in damp conditions, and sadly, rusting causes flowers to die and fall off.

Why Are My Leaves Turning Brown During Flowering?

Marijuana growers need to deal with many things. First, they have the right kind of equipment for their plants. Besides, it legal obligations and financial matters. Plant protection is a critical area that can make all the other areas insignificant and should be a top priority.

It can bother outdoor growers more, although indoor growers need to take similar precautions. Marijuana plants have rust spots on leaves during flowering as the main symptom. New growers may not know exactly what to do, although it doesn’t take too long to learn what’s affecting your plant’s leaves and how to improve your plant growth.

What Do Rust Spots Look Like?

Rust spots generally tend to show you have an issue, and if not treated, they might dry and crumble, thus causing small punctures in the middle of your leaf. The spots are easy to see on healthy leaves as dark patches on the affected plant leaf.

The problem with rust spots found on leaves during flowering is you may not know the exact reason by looking, and you’ll need to investigate further. (Read House Plants With White Flowers Guide)

Here are all the reasons in more detail that can cause such issues.

Pest on Cannabis


A common reason for rust spots is pests, and you can find pests are responsible for unexpected or unwanted spots on the body of your plant. You’ll find this is more common for outdoor growers, but indoor growers can become infected by bringing infected plants indoors, where pests can spread from plant to plant.

Aphids can be one such pest and cause lots of damage. However, pests can be easy to spot with close inspection, and you can street your plants with soap solutions and neem oil.

Rust Fungus

Rust fungus causes brown spots to appear on plants and is common in some regions, especially if there’s a rust fungus outbreak in the area. The best way to tell about rust fungus infection spots is to rub your fingers on the brown spots and see if it sticks to your fingers.

Rust mold is common in some areas during the flowering phase and is a probable cause of rust spots appearing on leaves during flowering. This can be the case in high humidity areas where the temperature ranges between 60-80F.

You can discover this is more an indoor problem as the fungus thrives with high humidity and poor ventilation and air supply.

Proper ventilation means the fungi can’t latch onto leaves. With this, ensure you have sufficient space between your plants and your temperatures are on the cooler side. It is advised to remove your infected leaves to avoid spread, and you can treat these with sulfur spray.

Deficiencies in Magnesium and or Calcium

The next likely cause will be a nutrient deficiency or too many nutrients in your solution. Soil-grown plants don’t tend to suffer from such deficiencies, and it is indoor plants that are grown with hydroponic solutions such as those from Advanced Nutrients.

Excess or lack of nutrients results in rusty brown spots and are more common in the flowering phase. Most often, it is because of a magnesium deficiency, although it isn’t always insufficient magnesium causing this nutrient deficiency. A calcium deficiency can cause the same problems.

Should it be a magnesium or calcium deficiency, you’ll see the problem worsen as flowering progresses.

In your plants, half the problem is a deficiency, although too many nutrients can; lead to the same problems. Nutrient lockout occurs when plants can’t absorb nutrients as required.

A nutrient lockout can be fixed by flushing soil or a system if plants are grown in water. Do this, and you wash off all the nutrients from the soil or the plant roots. When adding nutrients back into a water solution, you can add half strength as there are fewer issues from a weaker solution than one too strong.

Soil pH can also lead to these problems and prevent your plants from getting all the nutrients they need. You can fix nutrient lockout by first correcting your soil pH levels or that of your indoor system.

Nutrient Burn on Cannabis

Nutrient Burn

Nutrient burn is when your plant stores too many nutrients and can’t use them anymore. The rust spots here are not a nutrient deficiency, rather too many nutrients that are making your plants sick.

Nutrient burns we have seen can be fixed by flushing the plant where excess nutrients are washed from the plant. (Learn How To Keep Squirrels From Digging In Flower Pots)

Here, you will use plain water to ensure your plant uses the stored nutrients. To avoid this, you can use a half cup of nutrients rather than a full cup until your plants recover from this nutrient lockout.

Do Indoor Growers or Outdoor Growers face the larger risk?

For rust spots, indoor and outdoor growers are at equal risk. If you have a pest-related issue, outdoor growers will face a more significant risk.

However, indoor growers can have an infested plant find its way into their grow room. Should indoor growers lack proper ventilation or have a room, they will face rust fungus problems. Outdoor growers can face similar issues if there isn’t proper airflow and plants face high humidity and hot temperatures.

How Do You Get Rid Of Rust Fungus?

If your cannabis plants have leaf rust, don’t panic, as you can use a few techniques to purge the plant from this harmful disease.

There isn’t one straightforward method to cure a cannabis plant, yet you can control the existing disease and stop it from spreading to other plants in your grow room.

  • Remove infected leaves and dispose of them. Leaving infected leaves in the open means, you can infect your other plants in your grow room.
  • Always disinfect tools using a mild bleach solution, or you can use hydrogen peroxide to kill or remove fungal spores from your gardening tools.
  • Avoid splashing water on plant leaves as rust fungi love damp surfaces.
  • Offer enough light exposure to your crops.
  • Keep enough distance between crops to stop excess moisture retention.
  • If the rust is from a calcium deficiency, add more nutrients as instructed.
  • Some plant and garden centers stock rust fungicides. You can try these through natural remedies like natural sprays such as minced garlic spray, sulfur spray, or copper sprays are as effective in treating your plant’s leaves.

Rust Spots On Leaves During Flowering

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