It's a type of Ash(Fraxinus sps.) tree. Your "leaf" is only 1 leaflet of a larger compound leaf. The winged seed is typical for Ash trees. With more photos and info perhaps we could give you the exact species.

"...it won't help anything " and I'll add that adding salt to the soil can be damaging. And please....do not use any pesticide until you have carefully read and understood all of the directions. Also, home remedies shouldn't be used unless there is a good understanding of how they work, possible problems, and potential damage. Repeating jean again....don't use anything unless you know just what the problem is.

Yep, Macadamia, but since they're grown from seed, it could be any of several different species. I claim no expertise. But only a couple of species are cultivated for commercial nut production and they're either grafted or grown from cuttings. They like free-draining soils (your sand provides that), rich in organic matter (your sand precludes that, so at least mulch them well with straw or similar and give them regular weak liquid fertiliser and/or compost as top-dressing), and deep water at least weekly, probably more often in your very hot area with summer here now. The sick one (and probably both) appears to be lacking trace elements, so depending upon how serious you want to get, a dose of those wouldn't go astray. I've made it sound complicated, but basically rich soil, thick mulch and plenty of water in summer. You'll have to wait up to 10 years for nuts, and even then, possibly not many and of poor quality. If you want a food crop, I'd suggest buying known grafted varieties from a nursery, but if it's just for curiosity's sake, go for it! They can grow to 15 metres tall in ideal situations. Good luck!

.... not a fungus - likely oedema from over-watering. Need pictures of the whole plant and more info. Do you water before the soil dries out - when you can still see or detect with your finger that the soil is still moist? What is the general condition/appearance of the plant? Fertilizing? with what (NPK %s please)? how often? How big is the pot in relation to the plant? Repotted or potted up recently? What are you using for soil? Where do you live? What kind of light does the tree get? Al