Part of the lumber on the edge has rotten away. The rest of the lumber is okay.
I could use wood dough. But how can you get a nice edge. Fold and glue a piece
Of cardboard. Wrap it around the edge before for good definition then apply the wood dough. Or does someone know a better way?

Cut the bad part off and use it for something else. And buy a new piece because what you want to do won't be structurally sound.

If its in direct contact with concrete or the ground, you HAVE to use pressure treated wood or this will happen again soon. Moisture does permiate and travel through concrete. You need a durable waterproof barrier or a pressure treated wood that's resistant to rot. There are a few kinds that have a natural resistance... but they tend to be too expensive for framing use. But are suitible for things like outdoor furniture.

Wood putty is intended to fill small holes or defects... not to build a section of lumber with.

I agree with Smoothy. If the wood is structural, then wood dough will NEVER hold up the load. You can be in store for even bigger problems if you fail to address the situation correctly.

I agree with Smoothy because he's right. You probably need pressure treated wood to repair the rot. That way you still have the structural support AND you won't have rotten wood again in a few years.

Good luck.

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It is only a fascia board. Trim around the house

I'd till replace it... its not going to bond as well as you hope it will... with the humidity swings its going to pop off as the wood expands and contracts (because the patch won't). And all it takes is a crack and you have your entry point for more water to start the cycle again. If it was a small hole I would feel diffently, but there is nothing to contain it on the end of a piece.

I have heard of certain epoxy products engineered for such a thing... but wood putty isn't one... and they are proprietary and haven't seen them sold in a store. Only on TV on one of the various Home repair shows a few years ago.