It’s worrying when you find spots on your orchid leaves. The spots may be brown or black or in transition between the two colours, with a few more thrown in. Here are three of the primary diseases – Botrytis, Black rot and Bacterial brown spot –  that cause spots on orchid leaves, how to recognise them and what to do about them.

Botrytis on orchid leaves

What is botrytis?

Botrytis is a fungal disease that affects many plants. If your orchid flowers and leaves have ugly brown spots on them, it could be this disease. Botrytis especially likes the Phalaenopsis species or orchids, and that’s the most popular orchid kept in homes. This disease likes cool, damp weather with increasing humidity.

How to spot botrytis?

Look for irregular brown spots on your orchid blooms and leaves. Older flowers are more vulnerable to this disease than more recently bloomed ones. As the disease progresses, the spots may grow bigger and there may be more of them. Sometimes a pink halo appears around the brown colour. In the later stages of the disease, the spots may join together and a gray web growth appears.

What to do?

To prevent botrytis, water your orchid plant carefully. Make sure that no excess water remains in the pot, and on the leaves or flowers. Remove any dead or dying plant matter from the orchid plant itself or from nearby plants. Do this frequently if needed and immediately after a plant (or parts of it) dies. Remove any infected part of your orchid once it becomes sick.

Stagnant air encourages botrytis to grow so ensure that there’s good air circulation within and between your plants. This is especially key when the orchid plant is in bloom. Decreasing the humidity during cool, damp weather (especially in the spring) will remove any excess moisture on the plant. This reduces the chance of the disease moving in.

Once botrytis has moved into your plant, you can’t get rid of it. You can use a fungicide to prevent further outbreaks. But I don’t recommend this as you’d have to spray the flowers. Just remove the affected flowers and change the plant’s growing conditions.

Black rot

What is black rot?

Black rot is another fungal disease that thrives in wet environments. The spores that cause black rot are capable of movement by themselves. They swim through water and attach themselves to whatever the water comes against, in this case orchid leaves. The spores spread quickly through the plant tissue. If you don’t treat the black rot, it may affect other plants and kill the one it first reached.

How to spot black rot?

You may miss the first signs of black rot on your orchid leaves. It starts out as small, translucent spots on the orchid leaves that look a little watery. It’s when the spots grow bigger and then turn brown and black that they probably come to your attention. The spots are of an irregular shape and may take over the whole leaf.

Black rot moves quickly so you have to as well. Keep an eye on the orchid plant leaves, especially on new leaves or those in the early stage of growth. It moves downward through the plant until it reaches the root.

If you don’t notice the damage caused, black rot can kill your orchid if it reaches the crown if your plant has a single stem.

To be sure that the black spots are indeed the result of black rot, check around the affected area. It should be yellow. and if you squeeze the black spot, it’s soft and water comes out.

What to do?

Be meticulous about watering your orchid plant. Read When and how to water my orchid for detailed information about this essential task. Dry your orchid flowers and leaves after the watering session if needed.

Good air circulation helps the roots to stay moist but not wet. Make sure the roots are spaced apart and not crowded. If there’s hardly any room in the pot for the roots, consider repotting the orchid at the appropriate time to make space.

At first signs of infection, isolate the affected plant and remove the affected leaves carefully. Make sure to remove some of the healthy tissue around the black rot so you’re sure you got all the infected part.

You can apply a fungicide to protect the healthy tissue that remains. Applying the fungicide as a poultice to the exposed area of healthy tissue protects it from new infection.

Bacterial Brown Spot

What is bacterial brown spot?

Bacterial brown spot is not a fungal infection. It’s a bacterial problem that infects the leaves of an orchid plant. Phalaenopsis orchids are the species of orchid most likely to become infected. It’s often confused with black rot as some of the symptoms are the same. and in its early stages, it can look like you’re just overwatering your plant.

However, like the other problems identified in this article, it’s caused by an excess of water. Bacteria love mist air and high humidity along with the high temperatures that orchids grow in. 

If you grow your orchids outside or in a greenhouse you may reach the temperature that bacterial brown spot loves. But chances are your growing conditions in your home won’t encourage this disease.

How to spot bacterial brown spot?

The leaves of your orchid swell up as they can’t eliminate the water fast enough. Small blisters form on the leaf as brown smudges and appear to be waterlogged – all signs similar to overwatering. The colour goes through brown to an ugly green. A yellow halo appears around the spot that slowly fades to black. This is the final stage when the blister falls down into the leaf leaving a hole.

Bacterial brown spot vs black rot vs other diseases

As I mentioned, sometimes these diseases are mistaken for each other. But it’s important to tell them apart as some are fungal and the brown spot is bacterial.

If you press an infected leaf and it exudes water, that’s not enough to tell the diseases apart.

If the spot on the leaf is small then you have bacterial brown spot. If it’s larger and takes over, say 1/3 of the leaf, then it’s black rot.

If the tips of the leaves are brown and then turn black, that’s not brown spot.

Bacterial brown spot occurs only on leaves and not on the root (see Root rot article) or the pseudobulbs.

What to do?

Bacterial brown spot spreads through water. So keep the water for each orchid plant separate. Don’t soak multiple orchids in the same bucketof water , for example. Ensure that the excess water drains way down the sink after you water your plant, and wipe any water droplets from the leaves and flowers. (See when and how to water my orchid for more information.) You may be interested in my article How to know if you’re overwatering your orchid? – Seven signs.

Ensure there’s good air circulation between the roots of your orchids and between any plants you have together. At the first sign of bacterial brown spot, isolate the affected orchid from any other plant.

Soak a cotton swab in 3% hydrogen peroxide and gently wipe the infected areas, But don’t do this on the underside of the leaves. Or for something stronger, purchase a bactericide at a nursery and follow the directions on the bottle.

If a leaf has many black spots on it, sterilise a cutting tool and remove the entire leaf.

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