Ask an Arborist: What is Causing the Black Mildew Sticky Substance on My Patio, Plant Materials, Vehicle and Driveway?

Introduction

As a property owner, noticing a mysterious sticky, black substance on your patio, plants, vehicle or driveway can be quite concerning. This substance, commonly referred to as “sooty mold,” is often a result of scale insect infestations. But what exactly are scale insects, and how do they lead to this black mildew?

In this post, our certified arborists at Townsend Arborcare explain the factors behind this phenomenon and provide you with the knowledge to manage and prevent future occurrences.

Understanding Scale Insects

Scale insects are small sap-sucking pests classified in the order Hemiptera. These insects can be divided into three main groups: armored scales, soft scales and mealybugs.

• Armored scales: Armored scale insects, such as oystershell scale and euonymus scale, have a hard, protective coating. They feed on plant fluids, which can weaken and even kill plants over time.
• Soft scales: Unlike armored scales, soft scales, such as magnolia scale, do not have a separate waxy coating. They produce a sticky excrement called honeydew, which can attract ants and promote the growth of sooty mold.
• Mealybugs: Mealybugs are covered in a white, waxy substance and also produce honeydew. They are often found in plant crevices, on stems and under leaves.

How Scale Insects Cause Sooty Mold

1. Honeydew production: Soft scales and mealybugs excrete honeydew as they feed on plant sap. Honeydew is a sticky, sugary substance that can coat leaves, stems and surrounding surfaces.
2. Fungal growth: The sweet nature of honeydew attracts fungi, leading to the growth of sooty mold. This mold gives affected areas a characteristic black, sooty appearance.
3. Impact on plants: Besides causing unsightly black mildew, scale insect infestations can lead to premature leaf drop, stunted growth, branch dieback and overall plant weakness.

Addressing the Problem

If you notice the signs of scale insects, it’s crucial to take action promptly.

Identify the type of scale:

Knowing whether you are dealing with armored scales, soft scales or mealybugs can help in selecting the appropriate treatment. Different scales respond differently to treatment options.

• Armored scales: Primarily treated by introducing natural predators or applying oil-based insecticides.
• Soft scales and Mealybugs: These can often be managed by systemic insecticides or insect growth regulators that impede their development.

Physical Removal:

For lighter infestations, you may be able to manage the problem manually.

• Brush off insects: Use a soft brush or cloth to gently remove scale insects from plant surfaces. This can be effective particularly in the early stages of infestation.
• High-pressure water spray: Spraying affected plants with a high-pressure water hose can help to dislodge the pests without harming the plants. This method also helps wash away some of the honeydew and sooty mold.

Use of insecticides:

In cases where the infestation is severe or persistent, chemical control may be necessary.

• Horticultural oils: Horticultural oils are effective against many scale insects by smothering them. Apply these oils during the growing season when scales are not in their protected armored state.
• Systemic insecticides: These are absorbed by the plant and poison the pests as they feed. Make sure to follow the instructions for the safe application in order to avoid harm to beneficial insects, pets and people.
• Contact insecticides: These kill on contact and are suitable for visible pest populations. Make sure to select products that target scale insects specifically.

Professional help:

When the problem is extensive and challenging to manage on your own.

• Consulting an arborist: Arborists have the expertise to diagnose pest issues accurately and recommend the most effective treatment plan. They can apply specialized treatments that might not be available over the counter.
• Ongoing care: Consider entering a care plan with an arborist for regular monitoring and maintenance to prevent future infestations.

Preventing Future Infestations

Preventing scale insect infestations involves ongoing care and attention. Here are steps that can help.

Regular monitoring:

Regularly check your plants for early signs of scale insects or sooty mold.

• Frequent inspections: Inspect your plants at least once a week, especially during the growing season. Look closely at the undersides of leaves, stems, and crevices where scales tend to hide.
• Spotting early signs: Early detection can involve spotting honeydew accumulation, ants or discoloration on leaves.

Maintaining plant health:

Healthy plants are generally less susceptible to pest problems.

• Proper watering: Ensure your plants are neither overwatered nor underwatered. Consistent and adequate watering promotes strong, healthy plants.
• Balanced fertilization: Over-fertilizing can make plants more attractive to pests. Use fertilizers suited to your plants’ needs, and follow recommended guidelines.
• Pruning: Regular pruning removes weak or infested branches, improving air circulation and reducing the likelihood of scale infestations.

Use of beneficial insects:

Introducing natural predators can help maintain balance in your garden.

• Ladybugs: These are voracious consumers of soft scales and mealybugs and can significantly reduce their population.
• Predator mites and lacewings: These insects also feast on various scales and can be beneficial.
• Attracting predators: Planting a variety of plants that provide nectar and pollen can attract and sustain these natural predators.

Cultural practices:

Implementing good gardening habits can also play a role.

• Cleaning debris: Regularly remove fallen leaves, twigs and other plant debris where scale insects might overwinter.
• Quarantine new plants: Before planting new plants in your garden, inspect them for scales and other pests and, if necessary, keep them isolated for a period of time.

Conclusion

Understanding the cause of the black mildew sticky substance on your property is the first step in effectively managing it. Scale insects are a common issue, but with the right approach, you can protect your outdoor areas and keep them looking beautiful.

For any uncertainties or persistent problems, don’t hesitate to reach out to the experts at Townsend Arborcare. Our knowledgeable and reliable arborists are here to help you maintain healthy plants and a pristine property. Contact us today.