The aphid excrement (honeydew) can fuel the eyesore growth of sooty mold, but that fungus doesn't infect plants or cause plant diseases, and will wear off on its own over time. Leaves shaded by a heavy layer of the dark sooty mold might be dropped early by the affected plants, but it generally doesn't cause them serious or long-term damage. Honeydew can indeed be annoying on materials like furniture, but hosing it down periodically may be enough to reduce it, since it's mostly sugar-water in composition.

Aphids on a mature tree like Tulip Poplar don't damage the tree to any significant degree, and don't need treatment. If you wanted to have a professional treat the tree with insecticide, be aware that such pesticide ingredients can impact much more than just aphids, including pollinators and beneficial insects that eat aphids, of which there are several. Aphid numbers usually decline without intervention due to being fed upon by those various beneficials and from infection by insect-specific fungi. There is no do-it-yourself treatment option for mature trees with aphids; any pesticide would need to be applied by a hired professional, either because of the spray or injection equipment needed, and/or because the chemicals involved would be regulated by Maryland law where only certified pesticide applicators are allowed to purchase and apply them.

Miri