Is dieback a real disease? As in; a living pathogenic microorganism (i.e. germ, virus, bacteria)? Or is it just a term used to describe a common correlation (of symptoms) or syndrome?
Dieback is a term to describe the death of a woody plant with signs first appearing at the tips of leaves and roots. The signs then progress inwards so that the plant dies inwards from its extremities. But is also used as the name of the disease that is causing the process that is seen.
It can also be prefixed with a noun to describe what the causative organism is causing the plant to die, or the plant that is dying, or even the region. So, we have as examples:
A general description of the process affecting woody plants and shrubs affected by various/miscellaneous forms of stress.
Seemingly natural phenomenon, in which a stand of trees dies for no apparent reason.
The death of native plants in Western Australia, caused by Phytophthora cinnamomi.
The death of Kauri trees in New Zealand, caused by Phytophthora agathidicida.
I'll just clarify that dieback isn't a specific disease, nor does every case of dieback have the same cause - in general, the term refers to death of the newest growth, which shows as tip dieback, usually followed by dieback spreading downwards to more mature parts of the plant. Causal elements for this phenomenon may be fungal attack, a disease, usually at the root but not always, repeated insect infestations, unfavourable growing conditions or a combination of these. The term 'dieback' is more a description of the manner in which a plant is being affected rather than a specific infection.