Are you sure that is white fungus and not mealy bugs? Mealy bugs will suck the juice from under the leaves and also gather in the little crevices near the leaf and stem. They really like mandevilleas. If I am growing my mandevillea in a pot I will tip the whole pot over and spray it down with neem. It makes it easier to get to all the under leaves and crooks and crannies that the mealy bugs hide in the vine.

Thank you for the reply. No, I am not sure it is a fungus and it could be mealy bugs. This "white stuff" is only on a couple of leaves and then the underneath of some of the lower leaves have black spots on them.
This is planted and not potted so I don't know what I should try - the stem looks good.

I think they are probably mealy bugs. Try squishing one lol. Just spray with NEEM or another soap and oil based product. The oil will smother them.
Get in the crooks and crannies. Look at the ties if you have anything tying up the vine. I had some green plastic tape holding my vine to bamboo poles and found the mealy bugs were on the inside of the tape too. If its in a pot tip the pot on its side and rotate it to get underneath.
If its in the ground. You can use a long handled broom to pull the leaves, vines up and then spray underneath the leaves good.