How can honeydew lead to sooty mold on apples and other plants?

Homopterans, such as aphids and scale insects, ingest large quantities of plant sap to obtain necessary nutrients. Because they consume more sap than they require, they excrete the surplus as a sugary, liquid waste known as honeydew from the tip of their abdomen.12

When this honeydew falls onto the surfaces of plants, such as apple foliage or fruit, it provides a nutrient-rich medium for the growth of sooty mold. Sooty mold is not a single fungus but a group of dark-colored ascomycetes that grow epiphytically on the honeydew deposits.314

While these fungi do not directly parasitize the plant, the resulting black, splotchy coating can be dense enough to block sunlight, which reduces the plant's ability to perform photosynthesis. Furthermore, when honeydew and subsequent mold accumulate on fruit like apples, the produce becomes black-spotted and loses its marketability. Control of this condition typically involves managing the populations of the sucking insects responsible for the honeydew or washing the sticky residue from the plant surfaces.341