How to Identify, Prevent, and Treat Sooty Mold in the Garden

When I moved into a new house in Tennessee, I fawned over the beautiful backyard covered with lush green grass and the patch of hellebores planted by previous owners. But as the hellebores stopped blooming and trees leafed out, I noticed the backyard was coated in a sticky, black substance.

This substance didn’t arise from the grass and flowers it covered but rather from the two large hackberry trees shading the backyard. As wind and rain occurred, the substance fell from the trees to the rest of the yard. With some research, I learned a fungus called sooty mold creates the dark color. Since then, I’ve seen this mold discolor countless trees and aggravate homeowners as their cars and houses become covered with this tar-like fungus.

In this article, I’ll explain why sooty mold occurs and how you can identify it. Then, I’ll share a few ways to prevent the fungus from occurring and deal with it when it arrives.

What Is Sooty Mold?

Sooty mold refers to a group of fungi that coat leaves and stems in a dark substance. Some genera of this fungi include Aethaloderma, Capnodium, Cladosporium, Scorias, and Trichomerium. All of these fungi grow on sticky substances like honeydew secreted by sap-sucking insects and sweet juices exuded by plants. Therefore, you’ll often see this mold covering plants previously attacked by aphids, scales, leafhoppers, and other small pests.

It attacks many different types of plants, but it commonly impacts trees and shrubs. Hackberries, elms, maples, hollies, laurels, and azaleas are common targets.

Although this mold doesn’t harm plants directly, it’s unsightly and can prevent plants from completing photosynthesis. Severe infestations cause the plants to lose energy and lead to leaf drop. In severe cases, plants can lose entire stems or branches.

Even if sooty mold doesn’t harm its host plant, it can turn otherwise beautiful landscapes into a sea of black. If plants are heavily infested with honeydew-producing pests, the sticky substance often falls onto cars, sidewalks, and other low-lying structures. Since it can form wherever there is honeydew, these objects also become covered in a layer of soot.

How Does It Occur?

Sooty mold doesn’t attack plants but rather feeds on sugary substances. The fungi’s primary food source is honeydew, a sweet liquid secreted by sap-sucking pests. However, the fungi may also feed on sugary excretions released by plants. Regardless of the food source, remember that it doesn’t feed on plant tissue!

The spores move throughout the environment by wind and water. When the spores land on honeydew or another sticky substance, they germinate and produce thin root-like structures known as mycelium. Sooty mold mycelium is black, which explains the fungi’s dark color. As the mycelium grows, plants become coated in a thicker black layer.

Symptoms

The main symptom is a dark gray or black substance coating leaves, twigs, and stems. The discoloration first appears as small dots, but it can eventually blacken large sections of leaves.

Along with this dark substance, you may also spot honeydew-producing pests like aphids, whiteflies, and scales. If you touch the leaves, they may be slightly sticky.

How to Prevent

Prevention lies in keeping plants free from the sugary substances the fungi feed on. Controlling sap-sucking pests and removing honeydew are the best ways to prevent this ugly fungus.

Prevent Sap-Sucking Pests

The best way to prevent sooty mold from attacking trees and shrubs is to keep them free of the pests that produce honeydew. And that means you need to know what pests to look out for! The following pests all suck plant juices (known as phloem) and secrete honeydew.

Encourage Natural Predators

So, how do you prevent these sap-sucking creatures from attacking your plants? While prevention strategies vary by pest type and species, encouraging natural predators is a good place to start.

Aim to plant a diverse habitat that includes an array of flowering plants that bloom throughout the season. Native plants are a great option since they provide nectar and pollen for predators and also serve as host plants for larval forms of predators that feed on honeydew-producing pests.

Avoid Applying Broad-Spectrum Pesticides

Next, avoid spraying broad-spectrum pesticides. While these chemicals may eliminate the pests attacking your plants, they’ll also kill the predators that keep them in check. When the pests reappear, they’ll quickly multiply and flourish due to the absence of predators.

If you opt to apply pesticides, use best practices. Choose natural options like neem oil and Bt, and follow product instructions. Apply them during a calm evening when pollinators are inactive, and make sure to spray only the areas infected with the noted pests.

Water Regularly

Drought-stressed plants are more susceptible to pests like aphids and scales. Therefore, proper irrigation is a key component of keeping these pests away. 

While you may think that shrubs and trees are happy without irrigation, remember that even large plants require supplemental water during hot and dry periods. If you go a few weeks without rain, grab a hose and give your plants a long drink. When it comes to watering trees and shrubs, one deep watering is better than a few shallow waterings.

If you struggle to stay on top of watering, automate your irrigation systems to keep your plants happy. Drip irrigation systems and soaker hoses both work well for shrubs and trees—once you set the system up, just attach it to a timer and let it go.

Fertilize Properly

Without the proper nutrients, plants have a difficult time fighting off pests. Regular fertilizer applications give plants the nutrients they need to remain healthy and fend off invaders. However, be careful not to apply too much fertilizer.

Over-applying nitrogen can lead plants to develop extra tender leaves that are extra enticing to sap-sucking pests. Therefore, choose a fertilizer that matches your plant’s nutrient needs and apply it following dosing instructions.

Remove Sap-Sucking Pests When They Appear

If your plants become covered with aphids or mealybugs despite your best prevention attempts, honeydew will soon follow. Removing these pests shortly after they appear prevents the formation of honeydew and, therefore, sooty mold. Even if it does begin growing on a bit of sticky honeydew, it won’t spread to new areas if you remove the honeydew-producing pests.

If you can access your plants from the ground, spray the pests off with a steady stream of water. Since the pests may reappear within a few days, respray the infected area each day.

Another option is to spray the pests with neem oil or insecticidal soap. Both of these are natural products that kill the pests they come into contact with. Make sure to spray your plants during the evening or on a cloudy day since the combination of oil and direct sun can burn the plant.

Since it’s difficult to reach the leaves of tall trees, it’s hard to remove the pests by spraying them. If you have a healthy environment, natural predators may appear and take care of the pests. However, releasing insects like ladybugs and parasitic wasps boosts predator populations and helps with pest control.

Wash Off Honeydew

Since sooty mold requires sugary substances to thrive, removing these substances from your plants prevents the fungus from occurring. While it’s difficult to rinse honeydew off maple and elm trees that tower overhead, use a hose to rinse honeydew off of azaleas, camellias, and other smaller shrubs. Since this substance is so sticky, make sure to give the plants a good shower, then check the leaves once they dry. If they’re still sticky, repeat the process.

While removing the honeydew will prevent sooty mold from occurring, it’s only a temporary prevention strategy if you don’t get rid of the pests that released the sticky substance. Fortunately, a steady stream of water will often remove both the honeydew and pests.

How to Treat

Before we dive into treatment methods, remember that this is largely an aesthetic problem. While severe cases can harm plants, a few splotchy leaves are more of an eyesore than a serious issue. So, if you spot a few branches with spotted black leaves, don’t assume you need to climb a ladder and spray the aphids attacking your tree. In the majority of cases, your tree won’t face serious damage.

However, if you can’t bear the sight of black leaves or you notice your plant’s overall health declining, treatment is warranted. Choose one or more of the following treatment options.

Wash Off Sooty Mold

Unfortunately, the sticky sooty mold is difficult to remove once it occurs. If you can reach the infected leaves, stems, and branches, you can wash the mold off with a high-pressure stream of water or wipe it off with a soapy rag.

However, even if you remove it, it can reappear if the sucking insects remain. Therefore, the best way to remove the fungus for good is to treat the insects that produce honeydew.

Remove Honeydew-Producing Pests

If you remove the pests that produce honeydew, you’ll remove the sooty mold’s food source. When the fungus doesn’t have anything else to eat, it will stop spreading. And when heavy rains arrive, and the remaining mold washes off, your plants will look as good as new.

Since many different pests produce honeydew, the best removal method depends on the pest. However, the following removal methods work well for a variety of these pests.

Spray the Pests with Insecticidal Soap

You can control sucking pests like aphids, whiteflies, and leafhoppers by spraying them with insecticidal soap. Since this product works on contact, make sure to thoroughly coat pests for best control. Avoid spraying during sunny days and before heavy rainfall.

Spray the Pests with Neem Oil

If you find insecticidal soap isn’t doing its job, another option is to spray the pests with neem oil. Not only does this oil suffocate soft-bodied pests, but it also contains compounds that stop the pests from feeding and disrupt normal growth. Since the oil is effective against all stages of pests like whiteflies, scale, and aphids, it’s an excellent way to control sap-sucking pests.

Since neem oil can also harm beneficial insects, only spray the areas of the plant infested with the targeted pests. Spraying during a still evening is the best way to prevent the neem oil from traveling to other plants and harming pollinators. A light mist is enough to do what you need it to do.

Encourage Natural Predators

Another way to control sucking pests is to encourage insect predators that feed on these pests with their host plants. These predators will seek out their preferred plants and get to work. Some natural predators eat the pests, quickly killing them. Other predators lay their eggs in the pests, and the resulting larvae feed on the pests.

Natural predators often exist in the environment, especially in diverse environments. However, you can also purchase and release these predators. In the case of ladybugs, it’s better to attract them than to purchase. Purchased ladybugs are often harvested from wild populations.

Some effective natural predators include green lacewings, minute pirate buds, assassin bugs, and parasitic wasps. Each of these pests performs best at certain times of the year, so do some research before settling on a specific predator. They also rely on certain plants. Find out which of those you need to plant to bring them in.

Frequently Asked Questions