What is wrong with my hydrangea?
Barbara Foster, by email
Stefan says: Your lovely plant is affected by an insect pest called the cottony cushion scale, which seems fairly common on hydrangeas this year as other readers have written to me about it, although it is more usually a problem on evergreen shrubs such as rhododendrons and camellias. It is one of many species of related sap-sucking scale insects.
Although extensive infestations can weaken plants through their feeding activity, a more general problem is that, like other sapsucking pests, they secrete a sticky sweet substance called honeydew. This in turn attracts dark-coloured mould fungi called sooty moulds and when these spread, they can limit the amount of light reaching the leaves, which then turn yellow.
Repeated spraying with an approved contact insecticide should limit the spread of the pests by killing them before they settle down to start sap sucking.