This disease, caused by a bacterium (Xanthomonas campestris pv. hederae), occurs frequently on English ivy growing in humid areas. Leaves kept constantly moist are particularly susceptible to invasion by these bacteria. The bacteria spread from plant to plant in splashing rain, on insects such as ants, bees, and flies, and on contaminated equipment. People also spread the bacteria when they work around wet, infected plants. Other organisms often enter the dying tissue and stem lesions, causing further decay.

Several different fungi cause leaf spotting on ivy. Some of these fungi may kill tender stems. Others merely spot the leaves and are unsightly. The fungi are spread from plant to plant by wind, splashing water, insects, and contaminated tools. They spend the winter on diseased plant debris left in the garden. Leaf spot affects a wide variety of plants, including flowers, vegetables, and groundcovers. Most leaf spot fungi do their greatest damage in temperatures between 50° and 85°F.

Several species of this common insect feed on ivy. Mealybugs damage plants by sucking sap, causing leaf distortion and death. The adult female mealybug may produce live young or may lay eggs in a white, fluffy mass of wax. The immature mealybugs, called nymphs, crawl all over the plant and onto nearby plants. Soon after they begin to feed, they produce white, waxy filaments that cover their bodies, giving them a cottony appearance. As they mature, they become less mobile. Mealybugs cannot digest all the sugar in the sap, and they excrete the excess in a fluid called honeydew, which coats the leaves and may drop onto surfaces below the plant.

Several different types of scale insects attack ivy. Some types can infest many different plants. Scales hatch from eggs. The young, called crawlers, are small (about 1/10 inch) and soft bodied and move about on the plant and onto other plants. After moving about for a short time, they insert their mouthparts into the plant, feeding on the sap. The legs atrophy, and the scales remain in the same place for the rest of their lives. Some species of scales are unable to digest fully all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew, which may cover the leaves or drop onto surfaces below.

This common black mold is one of several species of fungi. These fungi grow on the sugary material left on plants by aphids, mealybugs, scales, whiteflies, and other insects that suck sap from the plant. The insects are unable to digest all the sugar in the sap, and they excrete the excess in a fluid called honeydew, which coats the leaves. The honeydew may also drip from infested overhanging trees and shrubs onto ivy growing beneath them. Sooty mold is unsightly but is fairly harmless because it does not infect the leaf tissue. Extremely heavy infestations prevent light from reaching the leaf and may cause the leaf to turn yellow. The presence of sooty mold indicates that the ivy or another plant above it is infested with insects.

These mites, related to spiders, are major pests of ivy. They cause damage by sucking sap from the undersides of leaves. As a result of their feeding, chlorophyll disappears, producing the stippled appearance. Spider mite webbing traps cast-off skins and debris, making the plant messy. Under warm, dry conditions, mites can build up to tremendous numbers.