very sick bush

(Question)

Is there any hope for my bush? It has almost like a horrible tar which is thick on its leaves. The minimal new foliage also has this blight, just not as thick. The branches are deformed with scabs all over. I sent you pictures under separate cover (email) because I don’t know how to get a pic on this form.

Toronto-20150828-01761-2

Toronto-20150828-01761

(Answer)

Although it is difficult to tell with certainty from your photographs, it appears that your shrub has a scale disease, as well as sooty mould.   Of the two, the scale disease is the most serious, and is almost certainly the cause of the mould.

Sooty moulds will typically not kill plants: they grow only on the surface of the leaves and branches, and the gray or black coating is itself one of several species of fungi or moulds.  Sooty moulds are generally considered to be a cosmetic problem, and only in very severe cases will a sooty mould cover the leaves so completely as to block the sunlight so that photosynthesis cannot take place.

The primary cause of sooty mould growth is sucking insects, and these are what appears to be this shrub’s disease: a scale disease caused by one of the several varieties of scale insects.  Many plant sap-sucking insects feed on the leaves and stems of trees and shrubs, producing a sweet, sticky excretion called honeydew.  It is on this honeydew that the sooty mould fungi grow and flourish.

A heavy scale infestation can weaken a plant, causing symptoms of stress such as slow growth, water stress, and leaves turning brown and dropping prematurely.  Eventually a heavy infestation may result in the death of the plant.  Treating scale diseases is complex because different approaches are needed at different times in the insects’ life cycle. (For example, applications of horticultural oil are effective during the spring/early summer period when the insects are in the crawling phase of their life cycle).   If plants fail to thrive or are damaged repeatedly by these infestations, the best course of action may be to replace the affected plant with a pest-resistant species or cultivar.  Here is an excellent overview of scale diseases, their identification and management: https://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7408.html