In your zone plums are usually pruned after risk of below zero weather is likely passed. I do not know about specific diseases in your area, but here I've never seen silver leaf. Black knot is the only specific disease routinely encountered here with most plums that doesn't attack the fruit.

In my nursery, I don't prune E. plums the first year at all, beyond removing broken and oversized branches- those more than a third the diameter of the trunk at point of their attachment to the trunk. I generally prune as little as possible until trees come into fruiting except for peaches.

Upright E. plums do benefit from early branch spreading for a more open shape and early fruiting.