We have planted Jojo plum tree couple weeks ago. Some of tts leaves are curled and there is also something rot like in the fruit. I have not found any aphids in leaves. We have mostly sandy soil and water quite often. What is going on?
Two separate problems; the puckered, curled leaves are caused by plum aphids. most likely plum curling aphid rather than mealy plum aphid. They cause this sort of damage in mid to late spring for a few weeks, but newer growth after that time period is unaffected. Whilst the effects might be unsightly, it doesn't harm the tree, so plum aphids can be tolerated to avoid using insecticides. Info on plum aphid here https://www.rhs.org.uk/advice/profile?pid=681
The fruits seem to have been affected by the larvae of Plum moth - the larvae enters the fruit (you can sometimes see a depressed area on the outside which indicates its in there) then eats usually round the stone, leaving its frass (poo) behind. The fruit you show is only lightly affected; the interior of affected plums often looks much worse and you may even find a larvae/caterpillar inside some fruits - they look pinkish. You can hang a pheremone trap in the tree around May time - when you see its caught an increasing number of moths over a period of time, its time to spray. You don't say what part of the world you're in, but information on plum moth here https://www.rhs.org.uk/advice/profile?pid=554, though the recommendations for insecticide treatments apply to the UK - there may be other treatments in other parts of the world.