UKC

I built a campus board outside on a wall on the lee side of my house last year. I have wooden (Ash i believe) rungs from Crusher Holds which have started howing a bit of black mould over the Winter.

I didn't treat the rungs at all (as per some advice I received) - was this a mistake? Any advice on how to shift the mould?

Scrub with a bleach solution.  If its easy to remove and re fit then get them in direct sunlight to dry them out as UV light is also good for getting rid of mould. 

If you have asthma or anything you may want a mask as mould spores aren't good to breathe in. 

Thanks, i might give that a go. Was afraid that any sort of chemical treatment might damage the wood. 

You can treated with a couple of coats of danish oil, diluted with a bit of white spirit. It will provide a bit more protection and shouldnt affect the friction on the rungs too much. Never done it myself but heard its not a bad solution. 

Wash them with Patio Magic (or the identical, but significantly cheaper Aldi Patio Cleaner when it's in stock).

It won't have the instantaneous effect of bleach, but it will work within a few days. But then the residue (even when it dries) will keep mould at bay for up to a year. 

I used to use oxalic acid to remove staining from teak decks on yachts. Some fabric conditioners have it as an active ingredient so you can use that. Rince well afterwards.

Or just use washing up liquid and a sponge...

Once they're clean, a nice gloss varnish should keep any more mould from taking hold...

The mould is already digesting the wood.