This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
For ashes, dirt, mould, mushroom, bacterian dirt, aged look, oxide, corrosion, grime aspects, liquen, etc, etc, etc.
age time machine filth past paint future dirness mud soil corrosion dust ash grime
This filter has earned its author one of our rewards. They are given to the authors of popular filters and include a free copy of Filter Forge.