Lack of water?

Or possibly plum leaf curl. Not as serious as peach leaf curl but not nice. If the leaves are also curling up then, for now, strip the yellow curly ones off and burn them or dispose of them safely. DO NOT compost them. Then in early winter and again in very early spring spray with Bordeaux mixture solution. Although it is not pleasant stuff and is copper sulphate (toxic) based, the powers that be have ruled that it is organic(????). If that is what it is and you are prepared to forego the fruit this year (usually not many in the first year) then you could spray now as well but dispose of the fruit safely. Maybe something else but I can't see lack of water, except maybe in April, this year.