Decking problem

I built a new Chestnut deck about 3 months ago. I oiled it with 3 coats of good quality decking oil and was very happy with the way it came up.
Just recently there have been black dots starting to appear and they seem to be spreading like some sort of fungis or mould. In the worst patches the timber is now almost totally black.
The deck is partly under cover, but we have had lots of wild weather lately and it certainly gets wet at times.
Have tried to scrub off marks with warm soapy water and also tried to rub off with thinners, but neither worked.
Have sanded a small section and it has brought the timber back to look like new. I am now thinking that my only option is to sand the whole deck.
Would appreciate any tips on what I should oil it with or what steps can be taken to prevent this happening again.

Cheers JHA.

Jha,
I have never worked with chestnut so am not aware of its problems but it does sound like a fungal problem. As you have already sussed it is best to sand it back. I would then find an anti fungal wash of some kind. Chances are that the wood was a bit green in the first place. I would be in no hurry to re oil it. Leave it a month or so and over that time give it a few scrubs with fungal treatment. The aim is to let the wood dry out as fungus likes moisture. When you do re oil get one with anti fungal good stuff in it.
Regards
John

G'day JHA,

I would pop into Bunnings and look into a product called TSS by Feast Watson (or possibly Integrain). You'll need to use it alongside an oxalic acid based cleaner, but I think that should get you out of trouble.