UKC

I've got a wooden wall in my garage which I notice is starting to get black mould in one bottom corner near the door. Any advice for preventing mould? The floor is painted. Is there anything else I can do to prevent damp/mould?

Cursing England for being so damp. 

Seal the wood? 

If it wasn't so damp, you wouldn't need a woody.

Clean it, bleach it and seal with gloss paint.

Try to identify and prevent the source of the water, noting that it may make more sense to regard the board or section of as a consumable and replace periodically.

Sounds like it could be damp coming through the floor?

If this is the case, then a sheet of DPC covering the whole area can help. In our cellar (probably damper than most garages I'd imagine) this has reduced general damp / grossness considerably. 

Yep, or paint some Blackjack (bitumastic stuff) on the floor. Only a good idea if it is a limited area that you won't be walking on.

Is the wooden wall that is going mouldy actually touching the floor?  I notice you say that it is in one corner, nearer the door.  Is that bit potentially colder than the rest of the garage?  Is it the northern or eastern corner?  Is it furthest away from any heat source, such as an adjoining wall with your house?  This could also be a ventilation problem.  If so, it might not be rising damp but could just be damp in the air that is being condensed in that corner.  If any of this rings a bell, it might be that going to the considerable effort of sealing your floor may not actually improve things.  You could try a combination of the following:

A bit of ventilation

A small electrical radiator, especially near to that corner

Paint the affected area with a fungicide, thus stopping the mould from growing there.

I would also be careful of having sheets of plywood screwed to a wall with almost no gap between the wood and the wall - walls like to breathe and if the wood stops it, you could cause the formation of condensation between the wall and the wood which would also cause problems.

Had the same problem - ended up investing in a pretty heavy duty dehumidifier which clears it up pretty fast. 

Mould needs food, clean it with dilute bleach.