What visible consequence results from the aphids excreting excess honeydew on leaves?

Growth of sooty mold, making leaves look black and dirty

Aphids feed continuously by draining sugary sap from the plant's phloem tissue. As they consume large volumes of fluid, they excrete the excess water and sugar as a sticky substance known as honeydew, coating the foliage and surfaces below the infestation. This sticky layer of honeydew creates an ideal growing environment for a specific type of dark fungus known as sooty mold. The proliferation of this mold results in the plant appearing discolored—specifically black and dirty—which is often the most aesthetically visible sign of infestation. Furthermore, this mold layer can impede the leaf's capacity for efficient photosynthesis, adding further stress to the plant already experiencing sap depletion.