After the flowers fell the green plums started growing. As soon as they were 1/4" across, They began developing these little sunken black marks. I have been consistently spraying with fruit tree spray. How can I control this?
My best guess is plum curculio (see Figure 2, looks like yours).
"The Organic Gardener's Handbook of Natural Pest and Disease Control" (p170) suggests spreading a cloth on the ground and tapping the tree -- twice a day -- with a padded mallet. Gather up the bugs that fall and destroy. Or pasture your chickens in the orchard...
Or they suggest spraying "with kaolin clay beginning at petal fall and continuing for up to 8 weeks".
Or as a last resort, monitor carefully for scars and spray with pyrethrin as soon as they occur: twice, 7-10 days apart. Just be careful not to do this before pollination can occur, as pyrethrin will kill pollinators too.