sounds like aphid leaf curl which my large plum has, unfortunately its said to be almost impossible to eridicate, but u can control it. damage for this season has been done and leaves will curl and not look too good. yes you can spray it if you want, but it won't affect the leaf curl process, but will control the aphids.

the most important thing to do is spray the tree at end of winter JUST before bud break. i made the mistake of spraying this year end of jan, which worked in 05, but due to the long cold winter i sprayed too early and the leaf curl has returned with avengance. i noticed yesterday after the heavy rain made the branches droop lower that some leaves were just covered.