Plants suffer from the presence of aphids in a few ways. Depletion of xylem and phloem (water and liquid plant food) weakens and starves it. Many leaves of the plant will dry out and die, making photosynthesis and the creation of glucose difficult. Once leaves shrivel up and die, fruit is exposed to more sunlight than usual which can scald it, making it visually less appealing if not damaged. Aphids also secrete a sweet liquid substance called 'honeydew' which attracts other insects like ants to the plant. The honeydew can grow mold/mildew on it creating black ashy spots on the plant. One large, healthy population of aphids can do all this damage despite their individual size. This is why the Potato Aphid is the bane of many a gardener.