How to Fix Rust Spots on Leaves During Flowering

Beginner Grow Guide is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. If you click on a link on this site that takes you to Amazon, I will earn a small commission and help keep the lights on at no extra cost to you ūüôā

If you’ve ever developed a sudden rash, figuring out the cause, let alone treating it, can be annoying. Is it an allergy from something you’ve been exposed to in the environment?

Is it caused by some kind of food or drink you consumed? An insect bite? You know something is wrong, so it mustn’t be ignored. But you can’t fix the problem if you don’t know what is causing it.

Brown Spots On Leaves During Flowering Of Cannabis

Brown or rust-colored spots on your cannabis plants are a bit like that unknown rash. You don’t want to ignore them, because if there’s one thing you know,¬†cannabis health problems tend to manifest in the leaves.

Brown Spots On Leaves During Flowering Of Cannabis

In most cases, brown or rust-colored spots indicate either a fungal infection, a deficiency in magnesium or calcium, or nutrient burn. Such spots could also be a pest problem.

Before you can treat the problem, however, you first need to figure out which one is the guilty party.

Rust Fungus

Sounds icky, doesn’t it? Trust me, if your plants get this fungal infection, they won’t like it either. Rust fungus is a disease-causing fungus that spreads by airborne spores, ones that spread quickly from plant to plant.

Leaves accosted by these spores will typically develop brown spots and eventually curl up and die. Fortunately, it’s easy enough to determine if rust fungus is the cause of the brown spots on your leaves.

How to Determine If Your Plants Have Rust Fungus

Gently rub a finger across a brown spot on one of your plant’s leaves. Does some color come off on your finger? If the answer is yes, your plant has rust fungus. If not, the problem is more likely due to a nutrient deficiency.

What Causes Rust Fungus

Rust fungus is a common problem with almost any green leafy plant, and it can develop under certain optimum conditions. This type of fungus is most likely to develop if you keep the temperature between 60 and 80 degrees F with high humidity.

If those conditions sound familiar, it’s because that’s exactly what you want for your cannabis crops to thrive. Well, THAT sucks.

How to Prevent Rust Fungus

If rust fungus requires moisture, then get rid of all the moisture, right? Nope. That’s not going to work.

Marijuana thrives in a slightly humid environment. You can reduce the likelihood of rust fungus developing, however, by putting your plants in a slightly humid but well-ventilated grow room with cooler nighttime temperatures.

The cool, dry air will help keep fungi-loving moisture at bay and prevent spores from latching onto plant leaves.

How to Determine If Your Plants Have Rust Fungus

Air should also be able to circulate easily between plants, so a good way to prevent rust fungus from developing is to remove some of your overcrowded leaves. In fact, periodically cutting away larger fan leaves during the flowering stage is a good practice as it not only allows leaves on lower branches to receive more light and therefore more growth, it also reduces the risk of mold growth caused by overcrowding.

Rust fungus spreads by wind and water, so try to avoid splashing water on leaves when watering and keep your leaves from touching the soil, even cutting away those that hang too close.

By the way, make sure you have an adequately-sized grow tent that provides plenty of space between plants. This will help prevent the spread of any spores.

How To Get Rid Of Rust Fungus

Once your plants have an infestation, it’s hard to get rid of it, so prevention is really the key.

That being said, if you want to try to rid your plants of rust fungus, first remove and dispose of any and all infected plants as well as any debris between them. It’s best if you burn them, but either way, be sure to get them out of your house right away.

Spores travel quickly and you don’t want to infect any of your other plants. Once that’s done, treat your remaining plants with a fungicide.

While there are an array of chemical sprays available online, spraying your plants once every ten days with a natural home-made fungicide from sulfur is one of the best cures. Baking soda mixes also make good remedies.

Mix one teaspoon with a quart of water. You can also try making a garlic spray by mixing a half cup of minced garlic with a quart of water. Just let it sit for a full 24 hours before you spray it on your plants.

Magnesium Or Calcium Deficiency (Or Lock Out)

If rust fungus is not the problem, the spots on your plant’s leaves may be due to a magnesium or calcium deficiency either because you are not feeding them enough of these minerals or because the plants are locked out and unable to absorb them.

The flowering period is when your plants will really start to use up magnesium and calcium, so if there is a deficiency, this is when it will start to show in the form of rust or brown-colored leaf spots. Left untended, these spots will spread to all your leaves and significantly reduce flower production.

Cannabis marijuana leaf on a farm in America

If you’re unsure if the spots are due to a deficiency or lockout, there are other symptoms that can tip you off.

Additional Symptoms Of a Magnesium or Calcium Deficiency

Too little magnesium or a lockout of this important mineral is often the cause of rust-colored spots on your plants. If your leaves are turning yellow, but the veins are remaining green, this is a sign that your plants are getting insufficient magnesium.

More and greater spots will begin to form and some leaves will curl and die. With a calcium deficiency, often the spots will appear more brown than rust-colored, and they’ll have a dark brown edge to them.

Lower leaves will begin to curl, the plant will become stunted, and root tips will wither.

Nutrient Lockout

Soil pH will impact the ability of your plants to absorb all the nutrients they need, and a pH level that is outside its ideal range can cause your plants to suffer from nutrient lockout. Too little acidity (a pH level well above 7) and your plants cannot absorb the all-important NPK trio of nutrients–nitrogen, phosphorous, and potassium.

Too much (a pH level well below 7) and they cannot absorb other important nutrients as well like the calcium and magnesium we’ve been discussing.

Nutrient lockout can also happen if your plant’s roots are exposed to an environment that is excessively damp, cold, or acidic, or if the plant roots are stunted. Additionally, nutrient lockout may happen if there is an excess of calcium, potassium, or ammonium in relation to magnesium in the soil.

How to Fix a Calcium or Magnesium Deficiency

Before you consider any fix, first make sure that deficiency rather than nutrient lockout is the problem. To find out, you’ll need to test the pH of your soil by testing the water runoff from your pot.

Cannabis marijuana plant detail

If you don’t have a pH meter, testing papers will do just fine. Note that calcium and magnesium require a pH of 6.5 to 9.1 to best be absorbed. If your soil pH is outside those limits on either end, you’ll know they are being locked out, and you’ll need to make adjustments.

If, however, your soil pH is within the range–and you’ll want it closer to 6.8–then your plants need to be fed more magnesium or calcium. This will require a little trial and error on your part as providing your plants with too much of one can lockout the other.

Take it incrementally, monitor the results, and be patient.

Nutrient Burn

Nutrient burn is basically the flip side of nutrient deficiency. This occurs when you’ve so overfed or over-fertilized your plants that they end up storing far more of a nutrient than they can possibly handle.

You’ll know when this is starting to happen as your plant’s leaves will begin to turn brown or reddish, and the leaves will begin to dry and curl up. Root damage will also occur.

You can fix nutrient burn by removing and tossing out the damaged leaves and flowers, then flushing your medium with clean, pH balanced water. Then, nurse your plants back to good health with a tonic combination of silver nitrate, humic, fulvic acids, vitamins, and minerals.

When they’re looking better, resume regular feeding but at 3/4 strength. They’ll need time to recover.

FAQ

Why Do These Spots Generally Tend to Appear During the Flowering Phase?

It’s not like rust spots don’t happen during other stages of growth, but it happens more frequently during the flower phase because that is the time your plants are most exposed. So much of their energy at this time is focused on growing and developing flowers, which leaves other parts of the plant susceptible to disease and deficiencies.

Who is at a Greater Risk? Indoor Growers or Outdoor Growers?

If you’re counting on the answer to this question helping you make a decision whether to grow indoors or outdoors, you’re out of luck. Indoor and outdoor growers are at equal risk. Rust fungus can thrive indoors where there is poor ventilation, but high humidity outdoors can be just as inviting to a fungal infection.

Close Up Flowering Marijuana Buds on Cannabis Plants with Flat Vintage Filter

The same goes for nutrient issues, be it providing too much or too little nutrition. It all comes down to paying close attention to your plants.

What is the ideal pH level for plants to absorb nutrients properly?

Marijuana grows best in slightly acidic soils with a pH range of 6.5 to 7. A 6.8 soil pH is ideal.

Rust Spots On Cannabis Leaves: Final Thoughts

Just as a mysterious rash can cause grief for the human body, knowing what to look for to determine the cause of mysterious rust spots will get you half way to treating and fixing your problem. Like most problems, the key is keeping a close eye on your plants and catching it early so less harm is done.

Follow that rule, and your crop should grow just fine.