White spots on mint leaves is something you should never take for granted or let them be. It is a sign that something is wrong.
The thing is there are many different reasons for mint leaves to develop white spots.
So, it is important to investigate and narrow down the potential cause. Focus most of your time and efforts on this since once you know what’s causing it, the solution is usually straightforward.
Why are there white spots on mint leaves? Fungal diseases including powdery mildew and white rust are usually the most likely causes. Similarly, pests can cause this as well.
But they’re not the only ones.
Nutritional problems, water quality dust and overwatering are other reasons for white spots on mint leaves.
Causes of White Spots on Mint Leaves
Below are the most common causes of white spots on mint leaves.
Since there are a few of them, it is very important to narrow down and identify which one is the cause. This way, you can properly treat the cause and not just guess which one is more likely than the others.
Fungal infections are the number thing to consider if you notice white spots on mint leaves.
When it comes to mint leaves with white spots, the two main culprits to consider are powdery mildew and white rust.
Both have very different looks and symptoms.
So, it is important to identify which one is actually causing the issue if it is in fact a fungal infection.
Powdery Mildew is a well-known problem.
And if you’ve been gardening for a little bit, you’ve likely come across it at least once.
Its look is very distinct as it will grow a layer of white powder-like substance over leaves. In some cases, only a small area of a leaf is affected.
In other cases, you’ll see clusters of these white patches on leaves.
And if things have gotten worse, entire leaves will be fully covered by this white dust like substance.
So, what causes powdery mildew in mint?
Warm weather, high humidity and lack of airflow are usually the reasons for this mold.
As such, make sure that you provide enough space between your herbs and other plants. This will allow sufficient air circulation to prevent powdery mildew.
Similarly, overwatering and keeping the soil moist makes the environment ideal for mold as well.
Also, avoid low light and damp conditions.
How to get rid of white spots on mint leaves caused by powdery mildew?
I like to use neem oil. However, be careful with neem oil as it is potent.
If you get the pre-mixed version, then you can apply it using the spray bottle the product comes in.
But if you get the concentrated version, make sure to dilute it enough.
Use two teaspoons of organic neem oil for every half-gallon of water. Mix thoroughly and put the solution into a sprayer or spray bottle.
Apply on the affected areas. Leave intervals of few days in between applications.
After a while, the powdery mildew will go away.
White Rust is another cause of white spots on mint leaves.
In this case, if you see round white spots that look like polka dots on leaves, white rust is likely your culprit.
This is how it begins.
But after a while, they will spread and become small groups of white patches.
For me, it looks like tapping a small paint brush with some white paint on the leaves.
In this case, it is usually overwatering, high humidity and lack of air circulation that causes white rust to occur on leaves.
As such, make sure there is enough space for air to flow between your herbs when planting.
Control humidity if you notice it is too high. And avoid watering the plant too frequently and wetting the leaves when you water the plants.
Additionally, warm days in combination of cool nights also increases the risk.
You can use fungicide to get rid of the white rust.
Other Related Posts
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Besides diseases, pests are the other main cause of white spots on mint leaves.
With pests, the challenge is to figure out what kind of pest is causing your mint leaves white spots.
In most cases, the likely causes include spider mites, scale, whiteflies and aphids.
Mealybugs are another potential one.
However, in this case, the white spots are more likely the mealybugs themselves rather than the damage they’ve done.
That’s because mealybugs look like white cotton-like creatures. The are very tiny and can move about individually or as a group.
So, when you see them, you may see a small white dot or a group of small white dots that make a somewhat bigger white patch.
Either way, it is important to immediately treat pests once you see them.
Ideally, it is best to catch them when there are only a few present. This makes it so much easier to deal with.
Try to avoid infestations since they cause more damage then. And it takes much longer to eradicate them pest problem.
Once you see any pests, immediately isolate the plant.
These bugs can quickly move from one plant to another. So, if your houseplants or herbs are bunched beside one another, it is important to check the others as well.
After you’ve done that, treat as soon as possible.
I like to just hose off the bugs using a stream of water. Try to be thorough and get them all.
Of course, do this somewhere you can make a bit of a mess.
You can use the garden hose or a showerhead.
The goal is to dislodge all the insects you can from the mint plant.
It may take a few hoses to get rid of them all. Leave a few days in between each hosing session.
When it comes to white spots on mint leaves, nutrient deficiency is something you should always suspect.
As with other plants and herbs, lack of certain minerals will cause unwanted and unpleasant changes in the leaves.
And the in the case of mint, there are a few specific nutrients, when the plant lacks them, that leads to white spots.
Note that the deficit in these nutrients or compounds not only affects the appearance of the leaves but also the health of the plant.
So, if you diagnose certain nutrient deficiencies, it is important to address them.
That’s because if these conditions persists they could eventually cause the mint to dry out and die.
That said, it is not only lack of certain minerals that can cause these issues. Too much fertilizer also has some side effects. As such, avoid overfeeding the plant.
Lack of Magnesium
Magnesium plays an important part in plant growth. While it has a role in many of the processes in plant life including activating specific enzyme systems, it is necessary for chlorophyll production.
Plants like mint need sufficient magnesium in order to function.
In this case, the mineral plays a big role in collecting light from the sun or artificial lighting to support photosynthesis and chlorophyll production.
As such, lack of magnesium means the plant won’t have enough energy to push out new growth as well as promote leaf development.
Additionally, it will also be deficient in chlorophyll.
And this is what causes the white spots on mint leaves.
Chlorophyll is what makes plant leaves green. So, lack of it causes older leaves to develop white spots. You’ll also see the plant wilt more often than not.
To fix this problem, make sure to check your fertilizer.
Choose a fertilizer product that contains magnesium. In most cases, gardeners will choose plant food solely based on NPK. While that works, some fertilizers focus more on the main nutrients and either don’t add or add very little micronutrients.
As a result, your plants end up with magnesium deficiency.
Lack of Iron
Iron is another reason for white spots on mint leaves.
And it is the main mineral that’s needed for chlorophyll production.
Thus, lack of iron is often related to chlorosis, which is the yellowing of green leaves due to the lack of chlorophyll.
Lack of iron will usually present itself as the mid vein or the main vein of the leaf will stay green. But the leaves will lack the green color or turn pale (white in most cases).
Most of the time, iron deficiencies in plants are caused by problems with the soil.
In this case, overwatering the soil and too much fertilizer are usually the main reasons for this. Additionally, if the soil you’re using also happens to have an excess of manganese, it will also prevent efficient uptake of iron by the plant.
Therefore, it is important to figure out what’s causing the issue.
With overwatering, it is important to learn when to water your mint plant. Similarly, avoid using too much fertilizer.
More specifically, fertilizers that are high in phosphorus will decrease how well the plant is able to take in iron.
So, if you see white spots on mint leaves, you can either go with a fertilizer that has lower phosphorus. That’s the P in NPK.
Thus, choose a lower middle number.
Alternatively, you can pick a fertilizer that has sufficient iron as well.
As mentioned in the previous section, overwatering affects iron uptake from the soil.
This leads to nutrient deficiencies including lack of iron.
And because iron is the main nutrient needed for chlorophyll production, you end up with white spots on mint leaves.
Why does this happen?
When a plant is overwatered, the roots end up swimming or drowning in lots of water.
Unfortunately, excess moisture will push out oxygen from the air pockets between the soil particles. And in the place of air, in goes water.
As a result, the roots don’t get enough oxygen. And they suffocate.
Even if the roots do not suffocate to death (which leads to root rot), the time they are deprived of oxygen (or have lack of oxygen) also prevents them from absorbing enough iron and other minerals from the soil.
This leads to iron deficiency in plants which causes white spots on mint leaves.
As such, don’t overwater the plant, avoid waterlogging and use soil that gets compacted. Also makes sure to keep your mint plant in a pot with sufficient drainage holes.
In addition to overwatering, hard water or highly mineralized water also causes white spots on mint leaves.
Again, causes white spots on mint leaves by messing with the plant’s ability to absorb enough nutrients.
With plants, water quality usually refers to the mineralization of the water.
In short, hard water or tap water with excess amounts of certain minerals will affect how plants grow.
In many cases, they will cause brown or yellow leaves.
Hard water contains higher amounts of calcium and magnesium. While plants need both minerals, there’s a limit to this.
As a result, it causes stains and spots in plants.
With the case of mint, it causes white spots on the leaves.
On the other hand, the amount of minerals in tap water varies from city to city. Each locality adds these minerals to make the tap water safe to drink.
Unfortunately, many of them add more than others.
As such, if you happen to live somewhere with highly mineralized water, there will be more salts, fluoride, chlorine and other minerals in your tap.
The imbalance caused by adding these excess minerals affect how well the plant is able to absorb other nutrients.
Similarly, excess salts prevent water uptake by the roots or limit how much water they take in.
This means the roots cannot absorb as much nutrients as they normally do since fertilizer is absorbed by plants through water.
Therefore, if this is the case, it is a good idea to use rainwater, distilled water or let the tap water sit overnight so the minerals can evaporate.
In short, stop using hard water or highly mineralized tap water.
Instead, use water that does not contain excess salts or other minerals.
Dust Accumulation on Foliage
This is less likely. Nevertheless, dust can also cause white spots on mint leaves.
But in most cases, it is just dust on the surface of the leaves that make you think that there’s something bad happening.
Once you get rid of the dust, all is well.
While that sounds simple enough, it is important to note that cleaning your plants, including mint is important.
You don’t have to do them regularly. Nor do you need a schedule.
But once you see some dust particles collecting on the surface of the leaves, it is time to clean them.
That’s because dust build up on leaves affects your plant’s health.
The most obvious is dust will block sunlight absorption.
As such, this limits the raw material that the plant has for photosynthesis. And as a result, it reduced the energy that it gets from the process.
So, you end up with a smaller, weaker plant that isn’t producing as many leaves.
Additionally, the leaves it does produce are smaller than normal as well.
The other thing that dust accumulation on leaves does is clog the pores.
Leaves have tiny pores where the moisture passes to allow transpiration to happen. This helps the plant regulate water as well as internal temperature among other things.
If there’s a lot of dust clogging these pores, you end up with a weak plant with dull looking foliage.
So, if you see dust start accumulate in your mint’s leaves, clean them.
Since mint has lots of leaves, it is not practical to wipe the leaves one by one. Instead, give the plant a quick shower to wash off the dust.
Don’t forget to dry the leaves and let any excess moisture drain after.