Sooty moulds are surface contaminants – they do not attack the plant directly. They require a nutrient source on which to grow, and this is most commonly the honeydew excreted by a number of sap-sucking pests (e.g. aphids, mealybugs, scale insects, whiteflies). Droplets of honeydew are shed by these pests and fall onto surfaces below where they are feeding. This is frequently the upper surface of leaves, but can also be stems, branches, fruit, etc., and also any other objects situated below the infestation.
Honeydew contains high levels of sugars and a range of other nutrients. The sooty mould fungi use these for growth. Sooty mould growth is most prevalent where air circulation is poor and humidity high, providing periods of extended wetness (although heavy rain may sometimes wash the growth from the leaf surface).
Occasionally, sooty mould growth develops on sugary, sticky exudates produced by the leaves of the plant itself. Certain plants (e.g. some Cistus species) are more likely to produce such exudates.