Sooty menace
Q
What is covering the leaves of my Rhyncospermum jasminoides? Jenny Storey (via email)
A
This black deposit is sooty mould. It is growing on sugary honeydew excreted by sap-sucking insects, such as aphids or scale insects. Rhyncospermum jasminoides is prone to infestations of cushion scale insects.
There is no treatment for sooty mould other than wiping it off, but the scale insects can be dealt with using Vitax Organic Pest & Disease Control Concentrate, Westland Resolva Bug Killer or Bug Clear Ultra. The best time to spray is during late June/early July.