Calathea: Diseases, Common Problems and Parasitic Pests

Common Diseases and Problems of Calathea

Root Rot

This disease is mainly caused by overwatering or poorly draining soil that prevents oxygen from reaching the roots. When roots are waterlogged, they eventually become necrotic and can no longer absorb essential nutrients for the plant.

The first symptoms often appear on the foliage: leaves turn yellow, become limp and droop. Upon inspecting the roots, they may appear dark brown or black, sometimes slimy and emitting an unpleasant odour.

To prevent this issue:

• It is essential to use well-draining soil suitable for Calatheas. Ideally, it should consist of a light indoor plant compost based on high-quality turf, supplemented with coconut coir and perlite.
• A pot with drainage holes is essential to remove excess water after each watering.
• Empty the saucer or outer pot of any residual water.

If rot is detected, act quickly by repotting the plant in fresh, dry soil after cutting away affected roots with a sterilised tool.

Leaf Spots

Leaf spots are typically caused by bacterial or fungal infections, encouraged by excessive humidity and poor air circulation around the plant. These spots may vary in colour depending on the pathogen involved:

• Brown to black spots, often surrounded by a yellow halo, indicate a bacterial infection.
• Grey to black spots, sometimes slightly powdery, suggest a fungal attack.

Excessive moisture, particularly when water lingers on leaves after misting, promotes the development of these diseases.

To limit spread, affected leaves should be cut off and discarded (do not leave them on the soil, as they may contaminate other parts of the plant).

Good air circulation is crucial to prevent infections: place the Calathea in a well-ventilated spot, but avoid cold draughts that could weaken the plant.

Curling Leaves

The Calathea is particularly sensitive to humidity fluctuations and growing conditions. When its leaves curl inwards, this often signals low humidity or water stress. This reaction is a natural defence mechanism to reduce water loss.

The main causes include:

• Overly dry air, due to heating in winter or exposure to air conditioning.
• Underwatering, preventing the plant from maintaining sufficient hydration.
• Thermal shock, such as abruptly moving the plant from a humid to a drier environment.

To remedy this, increase ambient humidity by regularly misting the foliage with non-calcareous water or placing a humidifier nearby.

Regarding watering, Calatheas prefer slightly moist soil at all times, but never waterlogged. Use filtered or rainwater, as tap water—too high in lime—can damage the plant over time.

Browning Leaf Edges

Another common issue with Calatheas is browning leaf tips. This can result from several environmental factors:

• Mineral-rich watering water: the accumulation of lime and chlorine in tap water may cause leaf burn.
• Low ambient humidity: as a tropical plant, Calatheas suffer in overly dry air, leading to desiccated leaf edges.
• Excessive light exposure: Calatheas thrive in moderate light—avoid direct sunlight entirely.

To prevent this, water the plant with filtered, demineralised, or rainwater. Increasing humidity with a humidifier is also beneficial.

Calatheas are quite prone to developing damaged leaves with brown edges

Calathea Pests

Parasites are one of the main problems for Calathea, especially when the environment is not optimal. Overly dry air, lack of maintenance or poorly drained soil can encourage their appearance. Several insects and mites attack this plant by feeding on its sap, leading to general weakening and damage to the foliage.

Red Spider Mites

These microscopic mites appear mainly when the air is too dry. They settle under the leaves, causing discolouration and gradual yellowing of the foliage. At an advanced stage, fine webs can be observed between the stems and the underside of the leaves.

Low humidity and poor ventilation encourage their proliferation, particularly in winter with heating.

Solutions:

• Start by showering the foliage with plenty of water directly in the bathroom
• Increase ambient humidity by misting the foliage or using a humidifier
• Clean the leaves regularly with a damp cloth to limit their development
• Apply a solution based on black soap and rosemary essential oil in case of infestation
• Introduce beneficial insects like Phytoseiulus persimilis, a predatory mite that feeds on red spider mites

→ Read our very detailed article on red spider mites to learn more about treatments.

Mealybugs

Mealybugs are small white insects covered in a waxy layer that attach themselves to stems and under leaves. They weaken the plant by sucking its sap, causing yellowing and slowed growth. Their presence often leads to honeydew, a sticky substance that can encourage sooty mould. They are visible to the naked eye as small white cottony clusters.

Solutions:

• Isolate the plant to prevent spread.
• Remove mealybugs with a cotton bud soaked in 70° alcohol.
• Apply a diluted solution of black soap with 1 teaspoon of the same alcohol and rapeseed oil for a natural treatment.

→ Read our article on mealybugs to learn more.

Thrips

Thrips are very small insects, often black or brown, that feed on plant sap. They leave silvery or translucent marks on leaves, which eventually become deformed. Their rapid reproductive cycle makes them a difficult pest to eradicate. They can cause significant damage and even kill the plant through prolonged weakening.

Dry, warm air encourages their development, and an already weakened plant is more vulnerable.

Solutions:

• Isolate the plant at the first sign of symptoms.
• Spray a neem oil-based mixture to limit their spread.
• Introduce natural predators like Orius laevigatus, which feed on thrips.
• Place blue sticky traps to monitor and reduce thrip populations.

→ Read our guide for everything you need to know about thrips.

Aphids

Aphids mainly attack young shoots of Calathea, where they gather in colonies. They weaken the plant by sucking its sap, causing deformed leaves and slowed growth. Their presence also leads to sticky deposits that encourage sooty mould.

Excessive nitrogen fertiliser stimulates plant growth, which can attract aphids. A weakened plant is also more susceptible to attacks.

Solutions:

• Wash the plant with a jet of lukewarm water to remove aphids manually.
• Spray a mixture of water and black soap for a natural treatment.
• In case of severe infestation, apply an organic insecticide based on pyrethrum.

→ Read our article on indoor aphids for our treatment advice.

Aphids can weaken Calathea, they are visible to the naked eye and settle on stems and the underside of leaves

How to Care for Your Calathea to Prevent Diseases

The Calathea is a tropical plant that thrives in a humid atmosphere. It is recommended to mist the foliage regularly with non-calcareous water or use a humidifier.

It is also important to avoid draughts and sudden temperature changes, which can stress the plant.

The Calathea needs a slightly moist substrate at all times, but without excess.

Tap water, often hard and chlorinated, can cause a build-up of minerals harmful to Calatheas.

The Calathea prefers filtered light. However, do not place the plant too far from a light source to avoid slowing its growth.