Calathea brown spots on leaves is very easy to notice because it quickly messes up the beautiful patterns and colors on the plant’s leaves.
That said, it is very important to pay attention to these spots because they mean something wrong is happening.
In some cases, it can lead to more serious problems.
Calathea brown spots on leaves are usually caused by disease including bacterial and fungal infections. Leaf spot will often be your primary culprit here.
Other possible reasons for brown spots on calathea leaves include humidity issues, too much direct sunlight, edema, overwatering, underwatering and nutrient deficiency.
Calathea Brown Spots on Leaves Causes
Below, I’ll go through a light of why your calathea has brown spots. There are many reasons for this.
But not all of them are relevant or applicable to your current problem.
Therefore, it is important to narrow down the different causes below and eliminating the others before you determine what’s happening to your beloved plant.
Leaf Spot Disease
The most common cause of calathea leaves with brown spots is leaf spot disease.
There are many different kinds of leaf spot diseases including those bacterial and fungal leaf spot infections.
More importantly, each of these varieties will produce different signs and symptoms.
For example, Septoria leaf spot causes leaves to turn yellow then brown later on. It also tends to affect the older, bottom leaves more.
Similarly, bacterial leaf spot will usually present spots that are about an inch wide. These are water-soaked lesions that have brown to tan colors.
Fungal leaf spot diseases will also vary quite a bit with varieties like the fusarium and Alternaria infections.
Leaf spot infections will cause different kinds of brown spots on calathea leaves. The shapes of the spots, their colors, where the dots appear and their patterns will help you identify the issue.
If you do notice this happening, the first thing you should do is isolate the plant.
Keep it away from your other houseplants to avoid spreading the infection.
Make sure to keep the plant in a dry, warm location that has good non-direct sunlight and ventilation.
Additionally, remove all the affected leaves. You want to prune all the leaves with brown spots on your calathea. This will prevent the disease from spreading.
Make sure to properly discard the leaves in the trash.
Because excess watering, more specifically, wetting the leaves while watering is the cause, make sure not to water from overhead.
Do not wet the leaves when you water. Instead, water directly onto the soil.
Wet, damp environments encourage the growth and spread of these microorganisms.
If the cause of the fungal infection is fungal, you can use a fungicide as well.
Unfortunately, even if you follow these steps and do things properly to try and treat your calathea leaves with brown spots, there’s a chance that things will keep getting worse.
In case the plant keeps deteriorating and the infection keeps spreading, the last resort is to throw away the infected plant.
In this case, makes sure to discard the plant properly to avoid any potentially spreading the disease.
While we’re on the topic of infections and diseases, one thing I’d like to mention is powdery mildew.
Powdery mildew is fairly easy to spot because of its very unique appearance.
In this case, you’ll see you calathea leaves slowly get covered by a powder white texture. This starts out as small patches.
But if not treated, you’ll see entire leaves covered with a snow-like white layer.
That said, there is a species of powdery mildew that produces a different kind of symptom. The oidium variety will turn the leaves into a paper-like texture and cause it to turn brown.
With powdery mildew, you can a fungicide if you already have the product at home.
However, from experience, using a DIY homemade treatment is more effective. And many gardeners will have their own formulations and cures.
I like to combine one tablespoon of baking soda with one-half teaspoon of liquid soap. Mix this in a gallon of water.
Then place in a spray bottle and shake to mix thoroughly.
Spray the solution on the affected areas of the plant.
I’ve also seen some growers use mouthwash to kill the mildew spores that cause the powdery mildew to spread.
As with all plants, calathea need proper nutrition. And if you haven’t applied fertilizer for years or never given your plant fertilizer, you may notice leaf discoloration.
In most cases, nutrient deficiency appears as yellow leaves.
However, it can also result in brown edges for calathea leaves.
The lack of the main nutrients including Nitrogen (N), Potassium (K) and Phosphorus (P) are usually the culprits. Although, magnesium is another very important element.
If your calathea lacks any of or a combination of these nutrients, you’ll see the lower, older leaves get affected most.
On the other hand, deficiencies in the micronutrients can also result in these problems.
But this time, the younger leaves are where the symptoms will likely appear.
If you’ve never applied fertilizer to your calathea since you bought it from the store or haven’t done so in years, then it is a good idea to use an all-purpose houseplant fertilizer.
Use a balanced fertilizer to get good amounts of N, P and K.
Similarly, make sure the plant food you get contains micronutrients.
On the other hand, if you’re already applying fertilizer, diagnosing the cause of the exact nutrient that your calathea is deficient is in a bit trickier.
The best option here is to a take a home soil test.
There are a few types of this.
You’ll want to do a soil pH test to see if the soil you’re using is ideal for your calathea. Soil with too high or too low pH will prevent nutrient absorption and availability despite the minerals being present in the soil.
The other soil test is on that checks nutrients in your soil.
Both of these tests are fairly simple to do. And they home soil test kits come with instructions.
Another option is to send samples to the lab.
This is more accurate. And you’ll get a very thorough report. But it costs much more money and takes much longer.
If you have a horticulturist fiend or a plant expert who knows how to identify nutrient deficiencies, then you can ask them as well.
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Calathea leaves turning brown and crispy are a common sign of underwatering.
This happens if the plant is left without watering for weeks at a time. Sometimes, the change in the weather especially when summer arrives can catch you off guard as well.
Calathea brown leaves due to overwatering usually starts on the edges of the leaves. But it will keep spreading until you see entire leaves turn brown.
A telltale sign of lack of water is when you feel the leaves.
These will feel dry and crispy. The edges can also become very brittle.
As a final check, feel the soil as well.
If the soil is very dry, even at the lower part, then the plant is underwatered. Sometimes, it may even be dehydrated already.
In addition to calathea leaves turning brown, you’ll see the leaves curl and droop as well.
Water makes up 90% of the plants. So, less water causes them to droop.
Similarly, their leaves will curl to reduce the surface area. This slows down the rate of water loss due to transpiration.
Underwatering is usually caused by lack of water or not watering the plant enough.
When you water your calathea is very important as the plant needs moisture to survive, stay healthy, grow and thrive.
Calathea plants like evenly moist soil.
They do not like it when the soil dries out. Therefore, try to avoid letting the entire root ball go dry.
In most cases, once a week watering works really well. Although, you’ll need to adjust this based on the time of the year.
When it comes to watering, once the top 2 inches of soil has dried, you can water the plant.
The key is not to let the soil go completely dry.
The other potential cause of underwatering is how you water the plant.
To ensure that the roots get enough to drink, it is important to water thoroughly.
This means soaking the root ball until you start seeing the moisture drip from the bottom of the pot. Doing this allows the water the reach the roots.
In contrast, if you use shallow watering or add a little water each time, only the top part of the soil becomes moist.
This prevents the liquid from reaching the roots.
Overwatering is most commonly associated with yellow leaves. And this is what you’ll see your calathea develop initially.
However, as things get worse, you’ll see your calathea leaves turn brown.
This is usually a bad sign.
It means that some of the roots are now getting damaged.
As a result, the plant is not getting as much water or nutrients from the soil since there are fewer healthy roots doing the work.
Thus, the lack of moisture will start causing leaves to turn brown.
Like underwatering, the key to avoiding overwatering is to water consistently and to wait until part of the soil has dried.
You want to wait until the top few inches of soil has dried out between waterings.
This lets you prevent overwatering.
If you water while the soil is still moist or wet, this will eventually cause the plant to get overwatered.
Another important thing when watering your calathea is to always let to completely drain after you drench it with water.
This will prevent waterlogging and overwatering which can lead to root rot.
One common mistake it notice is watering in the evenings or late in the day. I do not recommend this as it increases the risk of the moisture not drying up.
Instead, water in the morning when there is plenty of light. This helps the moisture evaporate to prevent overwatering.
Sunburn or Scorched Leaves
Calathea leaves with brown spots can also be caused by excess light.
In general, calathea plants need plenty of light. However, in the wild, they live under the shade of the larger trees and plants.
As such, while they grow faster with good lighting, direct sunlight or exposure to very intense light is too much for the plant.
Note that lack of light slows down its growth. And if there is insufficient lighting, you’ll see your calathea look weak, become leggy and its leaves turn pale.
That’s a no-no.
But excess light, usually direct sunlight for extended hours on a daily basis will damage the plant’s tissues as well.
This causes scorching and you’ll see brown spots on calathea leaves develop. In some cases, these markings will look like burn marks as well.
Calathea do best with medium light. However, it is important to properly define what this means to avoid confusion.
Indoors, the plant will thrive in medium to bright, indirect light.
The key here is to avoid too little light. But at the same time, keep the plant away from direct sunlight.
Outdoors, your calathea will grow best in partial shade.
Again, keep the plant away from full sun as it cannot tolerate that much intensity.
As long as your calathea receives 2 to 6 hours of natural light daily, it will do very well.
While it can tolerate direct sunlight or intense light, its limit is about 1-3 hours a day. Anything more on a consistent basis will eventually damage its leaves.
If you do not get a enough natural sunshine into your home, you can use artificial lights to supplement the sunlight. Or you can use artificial lights on its own as well.
In this case, keep the bulbs 12 inches away from the plant. These bulbs emit heat. And over the span of 12 or more hours per day, they will eventually burn the leaves and cause brown spots on calathea if you keep them too close.