Depends on the cause, the most likely, (without seeing a picture of a leaf) is Plum aphid. Look on the back of the leaf along the veins for tiny grey aphids.
Not keen on spraying but one of the Horticultural soap insecticides would help without making the fruit cull of nasty chemicals.
As long as the tree is healthy otherwise the cropping should be ok. Mind this cold weather won't help.

THANKS
Theres no sign of aphids ill try & send a photo