What Causes Tiny Black Spots Underneath Bonsai Leaves?

As with human beings, plants are susceptible to unwelcome pests and diseases that will harm or kill them. Diagnosing these problems, like little black spots on the underside of leaves, can be somewhat tricky. Thorough examinations are often called for to find the correct answers before you can administer the appropriate treatment.

What Causes Tiny Black Spots Underneath Bonsai Leaves

Possible Causes of Tiny Black Spots on Underside of Leaves

Disease, insects, or overwatering could cause these tiny black spots or marks on your plant leaves. The cause will depend on the type of plant you’re attending to and, likewise, the treatment.

Sooty Mold

If your plant has begun to look like it’s been spending time sitting alongside fire and is covered in black soot, the chances are that it’s suffering from sooty mold. Getting rid of sooty mold can seem baffling, but it’s a fixable problem.

Sooty mold is a plant mold that grows in the honeydew or secretion of many common plant pests, like scale or aphids. These pests will cover your plant’s leaves in their secretions, in which the sooty mold spore will land and begin to reproduce.

Symptoms of sooty mold growth look a lot like the name implies. Your plant’s branches, twigs, or leaves become covered in grimy, black soot, making many believe that ashes might have been dumped there or that the plant somehow caught fire.

Most plants affected by this mold growth will have an accompanying pest problem. Some plants like roses and gardenias, prone to pests, will be more susceptible to sooty mold growth.

Getting rid of sooty mold is best done by treating the problem at its source—the pests that excrete the honeydew on which the mold needs to live.

Spider Mites

In some cases, dark speckles may appear on the underside of the leaves. These specks will be slightly blistered and are caused by the feeding activities of spider mites. Just as their name suggests, spider mites are small arachnids, so they end up making tiny webs.

Black Spot Fungus

Black spot fungus is one of more than 65 other fungi that attack plants. Once this fungus has become active, the only way to deal with it is by using an effective fungicide to prevent its spread.

While the damage caused is usually only cosmetic, severe cases can be detrimental to a plants’ health.

Mint Rust (​Puccinia menthae​)

In late summer, mint rust usually appears yellow and, by fall, your mint plants may have black spots on the leaves. These spots result from the spores of the mint rust fungus (​Puccinia menthae​) and can cause the leaves to die and fall off your mint plant.

It can then spread to other mint plants in the area, and the disease worsens in warm, wet weather. This fungus produces spores that get released in wet conditions and then usually spread by rain-splash. The disease can also be passed from plant to plant via hands, clothing, or tools.

To treat mint leaves, add a little horticultural soap or oil, and you have a method of treating that works by changing the pH on the leaf surface to a pH that the fungus can’t survive. The oil or soap causes the solution to stick, and its cost is just around four cents per gallon.

Lace Bugs

The tiny black spots on the underside of plant leaves could also be lace bugs, aphids, or some other kind of tiny black insect. While wearing gloves, check if you can slide them off the leaves. If they do come off, you most likely have an insect problem.


One can treat many plant insects like mites, aphids, and mealybugs with insecticidal soaps, which are safer than chemical pesticides and are favored by organic and natural gardeners.

They are made from soap salts produced when fatty acids from natural oils are mixed with alkali. This soap should be applied regularly and won’t harm hard-bodied insects like beetles and ladybugs.

What Causes Tiny Black Spots Underneath Bonsai Leaves

Other Spot Problems

Too much sun, insufficient humidity, and improper fertilizing can also lead to brown or black spotting. Spots could also be caused by fungal or bacterial diseases, which typically have yellow rings around them and can be uniform or different in size. There are some actions you can take towards proper bonsai tree care.

We have a detailed bonsai tree care guide with all the relevant information. Check the guide out here!

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First, cut off any affected leaves and check nearby plants to see if the problem has spread because those plants will also need to be treated. You can make up a mix of baking soda, dish soap, and water to treat the plants indoors and use a fungicide for those outdoors.

Pick up and destroy all fallen leaves. While pruning, cut away any stems with evidence of black spots. During late winter, spread a thick layer of mulch around the bottom of an affected plant to prevent rain from splashing soil-borne spores onto new spring growth.

Final Thoughts

You have hopefully gleaned some positive expertise and tips from this writing. Please keep an eye out for other informative gardening and bonsai articles that will soon start sprouting.

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I have always been a keen cricketer and enjoyed the outdoors, including gardening from time to time. At the age of 60, I often sit and marvel at the awesome wonders of creation. Cooking, reading, and writing are my main hobbies.

Brian Limerick

Brian Limerick


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