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What Causes Brown Spots on Begonia Leaves (And Solutions)

So, there is your colorful begonia sitting proudly at the corner of your home. You feel happy how this plant has made your life brighter and calmer especially on gloomy days.

But suddenly, you see those awful brown spots forming in its leaves. You get a little surprised and panicked.

Your next conquest would be to know what causes those brown spots to appear. Well, worry no more because this article will help you figure things out. 

The possible reasons why brown spots appear on the leaves of your begonia are the following: excess light, temperature stress, pest infestation, diseases. Whatever the cause is, you need to take a series of actions to cure your begonia.

Keep reading and I’ll explain point by point how each cause leads to the formation of those unwanted brown spots.

How to Identify Begonia Brown Spots?

The type of brown spots you see on the leaves of your begonia will most likely give you hints to know the possible causes. Here’s a simple guide on how to find out the causes.

First, locate where the brown spots appear. Are they on the mature leaves or the young ones? Or are they on the middle of the surface, at the tips, or all over the leaves?

Next, see how big and how many brown spots are. Are they large or small? Are there many of them or just a few? 

Third, investigate how they look. Are they water-soaked, dry, or brittle? What’s the color? Are there halos and rings around the spots?

These things will help you diagnose the causes of why brown spots appear in your begonia leaves. By mere observation of the symptoms, you can fairly suspect what the real problem is. 

Why Does My Begonia Have Brown Spots? 

Generally, growing begonia is easy. It thrives in a wide variety of climates and can withstand even unfavorable conditions.

However, there are some instances when your begonia gets susceptible and it becomes prone to stress. 

Those stressors make the plant weak. But how? Well, let’s dive into a more detailed discussion of that part. 

Here’s a list of the common reasons why brown spots appear in the leaves of your once beautiful begonia plant. 

Excess Light

The light requirement of begonias depends on which type of subgroups your on-hand plant belongs to.

It can be fibrous-rooted, rhizomatous, and tuberous begonias. They can thrive in places with bright, indirect light, semi-shade, or half–shade. 

Because they’re more acquainted with having shades, direct exposure to sunlight can be detrimental.

The strong light intensities can damage the leaves. As a result, it develops leaf scorches on its surface reflected as large brown spots. 

Scorches may appear not only at the center of the leaves but even in the margins and veins. At times, it can lead the entire leaf to wilt, die, and fall off. 

How to Treat:

Relocate your plant somewhere more shaded. If it’s located outside your home and the light intensity is stronger than usual, bring it back inside.

If it’s near a window and the heat radiates from the glass, put a sheer curtain in between. 

Another option is for you to move your begonia away from the window when the light gets too bright and strong.

You can just put it back once the light condition gets more tolerable like during the early morning and late afternoon.

That way, you can save it from absorbing excess light energy that can potentially damage the cells.

Temperature Stress

Begonia plants prefer a temperature that’s within the range of 59 to 79oF (15 to 26oC). It will not tolerate a very cold temperature because it’s not frost-tolerant.

If your plant gets exposed to sudden and drastic changes in temperature, chances are its leaves will crystallize. 

At night, when it’s too cold, begonia leaves can get frozen. When the day arrives and the temperature elevates again, the leaves will thaw. That process will cause severe damage to the plant tissues. 

How to Treat: 

Insulate your plant during times of extreme cold. Use cotton sheets to protect them from getting frozen.

Keep them away from drafty locations like near air conditioners, fireplaces, windy windows, etc. 

Pest Infestation

Although pests aren’t much of a problem for begonia, the presence of foreign organisms such as whiteflies, mealy bugs, aphids, spider mites, and weevils can potentially hurt the plant.

You’ll only notice them once the brown spots become visible on areas where they’ve made a feast. (Sources: Clemson University)

Since most of them feed on the plant tissues by sucking the sap, they usually leave tiny lesions on the surfaces.

That’s why you’ll see those small brown spots scattered in the leaves. 

How to Treat:

The simplest way to control pests is to manually remove them from the begonia plant.

Of course, you have to be vigilant when it comes to this since these organisms are quite tiny and are almost invisible to the naked eye.

Regularly checking your begonia would help you spot the pests at the onset of their infestation. 

Spraying them off with water will also help get rid of unwanted visitors. Some use horticultural oils and insecticidal soaps to kill the colonies and eggs. In most cases, pesticides are no longer necessary. 

Diseases

Diseases are caused by pathogens either fungi or bacteria. If your plant gets infected with a certain fungus, it will show changes in its appearance.

One visible symptom is the appearance of brown spots on the leaves. 

The Penn State University enumerates seven diseases that can endanger the begonia plant. These are the following: 

Bacterial Leaf Spot and Blight

This is caused by Xanthomonas campestris pv. Begoniae. Brown spots appear water-soaked with yellow rings around them. Infected leaves die one by one. 

Botrytis Blight

Botrytis cinerea is the culprit behind this disease. The leaves of your begonia develop a tan color. It also induces the rotting of the crown. 

Foliar Nematode

Foliar nematode disease shows symptoms like the appearance of reddish, bronze to brown spots. It is caused by the pathogen Aphelenchoides fragariae.

Powdery Mildew

This disease is caused by Oidium. The infected parts look white and powdery due to the growth of fungus. It can infect the leaves, stems, and flowers. 

Pythium Rot

The stems and petioles of the plant die and rot due to Pythium. Leaves also develop browning. Eventually, the whole plant dies. 

Rhizoctonia Crown Rot

Caused by the pathogen Rhizoctonia, this disease also leads to the rotting of the stem. The foliage develops browning and the entire plant dies.

The presence of fungus is also observable as a white, powdery coating appears on the soil. 

Viruses

Viruses cause severe chlorosis on the leaves of your begonia leaves. Infected leaves become deformed and the plant remains stunted. 

How to Treat:

Immediately remove infected portions of the begonia plant. It’s also necessary to isolate the plant so as to prevent infection. Place the begonia in a well-ventilated area and lower humidity.

Keep the plant from getting wet foliage to avoid the growth of fungi. If the disease intensity is high then consider using fungicides to save your begonia. 

Rust Spots on Begonia

Another reason why brown spots appear in your begonia plant is the infection of the fungi called rust.

There are different types of rust namely: canker rust, gall rust, and leaf rust. They infect several parts of the plant including leaves, twigs, and branches.

Rust spots initially appear as light discoloration on the surface of the upper part of the leaves. 

Later, it develops into a rusty color. These are the spores that spread all throughout the lower portions of the leaves. 

How to Treat: 

Remove the infected leaves of your begonia. Lower humidity and keep the area where it’s placed dry. This will prevent fungi from growing and spreading.

Fertilizer Problem

One of the perils of overfertilization is damage to the roots. Excess soluble salts in soil raise the salinity of the soil and lower its pH. It makes the water and nutrients less available to the plant.

With such a condition, the roots are forced to exert so much effort in drawing in these necessary elements. 

As a result, they incur damage often in the form of root rot and shriveling. The leaves of the plant turn brown because of a lack of water and nutrients. 

How to Treat

Leach off excess salts using water. Drain the soil and let it rest for an hour. After that, pour water again to remove the remaining salts. 

Poor Air Circulation

Proper ventilation is necessary for plants to thrive. Moving air will constantly supply the much-needed carbon dioxide for plants to create their own food. It will also remove the moist air around preventing the growth of fungi. 

When there’s enough ventilation during hot days, it will help reduce the temperature in your home.

It will help lessen the stress caused by the increase in temperature.

Begonia plants that are placed in a room where there’s no good airflow will tend to develop diseases caused by pathogens. It will also suffer from temperature stress.

Solution:

Open the windows of your home to let the airflow in and out. You may also opt to move your begonia outside for a few hours to let it breathe in the fresh air.

If you have an exhaust fan, turn it in to suck out the intact air inside the room. 

Humidity

Begonias love high humidity. Around 70 to 90% moisture in the air is needed so that the plant won’t dry up and die.

Low humidity will cause dehydration because the plant will easily lose the moisture it contains. 

On the other hand, humidity that’s too high will favor the growth of pathogens. This will increase the chance of developing diseases in plants. 

There are two possible reasons why plants develop brown spots when humidity is not ideal. One is the leaves curl and wilt out of dehydration. The other is it acquires diseases.

Solution: 

If your begonia is suffering from a lack of humidity, a daily supplement of misting will help.

Make sure to do this in the morning so the droplets have enough time to evaporate by the day. Using a humidifier will also give instant solutions. 

Improper Watering

Overwatered Begonia

Watering your begonia requires caution. This plant is prone to overwatering which is often a common reason for death. When there’s so much water, the root rots, leading to impaired function. 

We know that plants with rotten roots suffer from a lack of nutrients and water supply.

It’s therefore not a surprise to see the leaves developing brown spots. Those spots have a slimy characteristic and often smell awful. 

How to Treat:

If you suspect that overwatering is the problem, check the roots of your begonia to see the extent of the damage. 

If the rotten portion is only minimal, no repotting is needed. Just withdraw the water application until the roots fully recover. 

Allow the soil to dry completely before watering again. Remove the dead portions of the foliage.

For more information, read my articles on how to fix an overwatered begonia.

Frost Damage

The last possible cause of brown spots in begonia is frost damage. Remember that begonias are not frosting tolerant so when the temperature gets extremely low, they’re most likely to sustain injuries. 

The water inside the plant tends to form ice crystals. It disrupts the movement of fluids leading water and other metabolites to be unavailable to the plant cells. 

The damaged portions show water-soaked spots that are brown to black in color.

How to Treat:

Cover your begonia plant with cotton sheets to protect it from frost damage.

This is especially needed during the winter season. Provide light above the plant so that there’s an additional source of warmth. 

How Do You Treat Brown Spots on Begonia? 

Here are some tips on how to properly treat your begonia plant. This way, you’ll minimize if not entirely eliminate the chance of having brown spots on its leaves.

  • Avoid Direct Exposure to Light

Bright light is necessary for begonia but make sure to avoid direct contact. Remember to shade your plant during summer when the sun is releasing strong lights.

  • Watch Out for Drastic Temperature Changes

Slight changes in temperature are tolerable to plants but an extremely high or low is detrimental. It creates stress for plants leaving them in shock and unable to adjust to the abrupt changes in the environment.

  • Treat Pests as Real Enemies

Pests may seem harmless especially when they’re only a few in numbers. However, don’t treat them lightly as they have the capacity to ruin the beauty of your begonia. Don’t wait for them to get out of hand before you apply appropriate treatments.

In most cases, pesticides won’t be necessary. Just make use of the mechanical control measures to get rid of the pests. If using chemicals, just use the DIY solutions made out of horticultural oils and soap or dishwashing liquid.

  • Be Prepared for Diseases 

Although your plant may seem healthy and flourishing, don’t be complacent. Pathogens may suddenly appear once they get the chance. This happens when the environment becomes favorable for their growth and reproduction. 

Always use sterilized soil to ensure that pathogens are killed. Choose healthy plants for a start. And, remember to apply fungicide as a preventive measure. 

  • Use Slow-Release, Organic Fertilizers

Salt build-up easily happens when you’re using readily available fertilizers. Although they make nutrients instantly consumable for your plant’s uptake, they affect the soil’s pH and salinity. 

Instead of regularly applying fast-release fertilizers, opt for organic choices like compost. It will improve the soil’s texture while ensuring the supply of essential nutrients to the plant.  

  • Ensure Good Ventilation and High Humidity

Proper airflow and humidity help the plant remain healthy. High humidity will help the plant remain hydrated. Mist around your plant when the humidity gets low. 

Your plant needs fresh air too so make it a habit to open the windows of your home. There’s a low chance that diseases will develop with good ventilation.

  • Regulate the Watering of Your Begonia 

Overwatering is a common problem in begonia so make sure to water as needed. The soil must be moist but not too wet. Remove stagnant water in pots and coasters then ensure good drainage.  

Use good quality water as well. Be wary of tap water because it might contain high amounts of chlorine and fluorine. These will also damage the plant. 

  • Prepare for the Winter Season

Bring out the cotton sheets and blankets to protect your plants. Add extra light to create a warmer condition inside your home. Minimize watering during the cold seasons. 

There can be one or more reasons behind the brown spots on your begonia. Most of the time, it is a combination of the factors mentioned above.

When the environment becomes unfavorable, the plant experiences stress that weakens its ‘immune system’ as humans do. 

To prevent such problems, do your best to create the ideal growing conditions. It will help your plant gain vigor and sturdiness against diseases and other stresses.