caused by the fungus Nectria cinnabarina these almost illuminous orange pustules [about 1mm diameter] take over the surface of the bark a little almost like having the measles…. [if you know what I mean] except for plants.
How they get there is quiet simple… affecting living or dead material the spores enter through damaged or necrotic wood and are usually spread by rain splash or/ and also from cutting/ pruning tools that haven’t been cleaned properly.
There is no real/ chemical control for this. But good garden hygiene is generally the best place to start. In it does infect, prune back to well below the last piece of infected material.