What Causes Brown Spots on Avocado Leaves?

Discovering brown spots on avocado leaves is a common experience. These spots can signal a range of underlying issues, from simple environmental adjustments to more complex health problems. Understanding the visual cues your plant provides is the first step toward diagnosing the specific cause and determining the appropriate action.

Watering and Environmental Stress

A frequent source of brown spots on the tips and edges of leaves is an accumulation of salts in the soil, which is common when using tap water. As the avocado plant transpires, it releases water vapor, but minerals like chloride are left behind. When the concentration of these salts becomes too high in the leaf tissue, it causes a toxic “burn,” resulting in dry, brown, and brittle leaf tips and margins.

Overwatering is a frequent issue that leads to yellowing leaves, soft brown spots, and a wilted appearance. Persistently soggy soil can cause root rot, which impairs the plant’s ability to absorb nutrients and causes leaves to become mushy and brown. Conversely, underwatering causes leaves to feel dry and crispy, often curling inward before turning brown.

Direct and intense sunlight can cause a condition known as sunburn or sun scorch. This damage appears as bleached, yellowish, or brown patches on the parts of the leaves most directly exposed to the sun. Unlike salt burn which starts at the tips, sunburn spots can appear anywhere on the leaf’s surface that faces the intense light.

Avocado plants originate from humid, tropical environments and can struggle in the dry air common indoors. Low humidity can cause the edges of the leaves to dry out and turn brown and crispy. This is a response to the air drawing moisture from the foliage faster than the roots can replenish it.

Fungal and Bacterial Diseases

Beyond environmental factors, various pathogens can cause brown spots. Anthracnose, a fungal disease caused by Colletotrichum gloeosporioides, is common in humid conditions. It manifests as irregularly shaped, dark, and sunken spots that may be surrounded by a yellow halo and can merge into larger dead patches.

Algal leaf spot, caused by the parasitic alga Cephaleuros virescens, is characterized by distinctive raised, circular spots that can be grayish-green, orange, or brown. The spots have a velvety texture and appear on leaves where air circulation is poor. This disease thrives in high humidity and can cause premature leaf drop.

Cercospora spot, caused by the fungus Pseudocercospora purpurea, presents as small, angular spots on the leaves. Initially light brown or tan, these spots can darken to a deeper brown or purple as the disease progresses. They are more visible on the underside of the leaf and may be accompanied by a yellow halo. This fungus can affect indoor plants, especially when plant debris is left around the base.

Managing and Preventing Brown Spots

If salt buildup is the suspected culprit, flushing the soil is the solution. This involves slowly pouring a generous amount of distilled or filtered water through the pot, allowing it to drain completely. This process leaches the excess mineral salts out of the soil, preventing further accumulation.

Adjust your watering routine by always checking the soil moisture first. A good method is to insert your finger about two inches deep. If the soil feels dry, water thoroughly until it runs from the drainage holes. For overwatered plants, allow the soil to dry out more between waterings and ensure the pot has adequate drainage.

For light and humidity issues, move the plant to a location with bright, indirect sunlight to prevent sunburn. A sheer curtain can help diffuse unavoidable direct sun. To increase humidity, you can mist the leaves, place the pot on a tray of pebbles and water, or use a humidifier. Improving air circulation also reduces the risk of fungal diseases.

When leaves are damaged by disease or severe environmental stress, it is best to prune them. Use clean, sterilized scissors to remove the affected leaves, which improves the plant’s appearance and can prevent the spread of fungal spores. If a fungal disease persists, applying a copper-based fungicide according to the product’s instructions can help manage the infection.