Sooty Leaves

Sooty Mould on an Ivy (Hedra helix) Leaf

Staying clean for a plant is important. Dirt, be it biological or the plain old dust variety, covering leaves can lead to a reduction in the levels of photosynthesis in a plant (1). The slippery, waxy cuticle of plant leaves should stop any particles from sticking and accumulating, however often a sooty deposit can be observed on certain plants. This is a sooty mould (any of a variety of Ascomycete fungi) that has begun to colonise the plant leaf. Often this is just a cosmetic problem, however in extreme cases inhibition of photosynthetic activity can lead to a reduction in the size of new fruits, shoots and leaves. Respiration may also be affected by the physical closure of stomata by the fungus' vegetative growth. Inspection of the leaves soon reveals that the mould easily flakes away with just the slightest abrasion. So how does it stick in the first place?
 

A Variety of Scale Insects on the underside of an Ivy Leaf

The answer lies with a superfamily of bugs, the Coccoidea, commonly known as scale insects. Coccoids are highly specialised insects, with a high number of morphological adaptations to allow them to feed which makes them an economically important pest of many crops (e.g. the cotton cusion scale, Icerya purchasi (2)). Sexual dimorphism is very pronounced, with ephemeral, winged adult males lacking developed mouth parts and large, sedentary females (3). It is the female form that is most commonly seen, and encounters with them, often seen underneath leaves, immediately explain the origin of their common name, scale insects. They are covered with a scale or waxy material that protects them as they feed on plant sap. Appendages are very mush reduced in the females, especially legs which combined with the coverings reduces mobility (3). One of the problems with the reduction of mobility is the build up of honey-dew waste. Coccoids have evolved a number of anal adaptations to counteract this, such as the ability to create droplets or expel the honey-dew by squirting it off, often a considerable distance for what are generally small insects. While ants will collect the honey-dew from some species, it often ends up covering surrounding plants parts. It is this sticky covering that allows sooty mould to establish and proliferate.

A Variety of Scale Insects on the underside of an Ivy Leaf

As stated previously, this is often just a cosmetic problem. However in a garden situation it is cosmetic problems that are often the most vexing – problems that I have seen most recently in Camelia bushes. Control of Coccoids in such situations is tricky. Pesticides may often be ineffective against established females, but will control juveniles. The females may be suffocated using oils. Once the insects are controlled however, the sooty moulds are easily removed using water sprayed on the leaves (1).

References:

1. Nameth et al., Ohio State University Extension Factsheet "Sooty molds on trees and shrubs" HYG-3046-96
2. Miller et al., 2005. Proceedings of the Entomological Society of Washington 107 pp. 123-158
3. Gullan and Krosztarab, 1997. Annual Review of Entomology 42 pp. 23-50