explosion alpha

Unity Engine
1

I have recently upgraded my project from 2.5 to 2.6 which turned up a few anomalies but the one I don’t seem to be able to solve is the particle explosion now isnt transparent. I now get a black square around it.

This was a standard explosion taken from a unity example.

How would I solve this to regain the alpha transparency?

2

I had to set the alpha value of the material back to 0 as this had been reset during import