Our client has a large arboretum and gardens in Buckinghamshire and has reported extensive disease on the rhododendrons. I thought it would help others to post both the diagnosis and treatment process.
As you can see, the plants have a sooty mould on the upper surfaces (there is a clue in the question, this is “sooty mould”) and a series of scale insects on the undersides of the leaves- the white fibrous one looks like cushion scale and there are also a series of different types of scale on both sides of some of the leaves. Scale insects suck the sap and reduce the vigour of the plants, and often also secrete a sticky substance that coats the leaves- causing the sooty mould to stick firmly to the upper parts. We also see “bud blast” which is when the buds turn black and have black fungal projections over them- this latter is caused by the leafhopper. Control of these problems will be tricky in this case as the area of infection is large (the main avenue is at least 100 yards long, and there are rhody’s 10m deep either side affected).
The RHS advise a systemic insecticide as well as some more “green” solutions that would work in a less extensive area. We are working up a sraying regime that will take into account the life-cycles of the various critters involved but willprobably mean multiple doses over the coming months.
The good news is that there seems to be no sign of the more serious Phytophthora (sudden oak death) which can affect Rhododendrons.
Will keep you posted as to the outcome