Since aphids feed on phloem they excrete honeydew, which is a sticky waste product that collects onto lower lying leaves. Once deposited, the honeydew is a nice food source for sooty mold which may grow on the underlying foliage. Sooty mold will inhibit photosynthesis, so its growth can potentially cause severe harm to the plants.
Some Control Options
Some Non-Chemical Control Options: Conserve beneficial insects, such as spiders, praying mantids, assassin bugs, lacewings, ladybird beetle larvae and adults and parasitic wasps in outdoor landscapes. Also spraying water streams is effective to dislodge aphids feeding on plants.
Some Chemical Control Options: Insecticidal soaps and oils can be used to control aphids and are considered low impact insecticides. Other foliar insecticides containing such active ingredients as permethrin, cyfluthrin, carbaryl, deltamethrin, pyrethrins and tebufenozide or systemic insecticides such as those containing imidacloprid or acephate can also be used.

Yellow sugarcane aphid, Sipha flava (Forbes) (Homoptera: Aphididae). Photo by Bart Drees, Professor and Extension Entomologist, Texas A&M University.