You have aphids among other sucking insects. You need to use a horticultural oil or soapy water.

But they are attacking your tree because something is predisposing it to attack. Maybe too much water or too little?

Water is unlikely to be a problem. We are at field capacity here after a wet winter and there has been ample rain.

Certainly a lot of aphids in the picture. They are very common and not necessarily indicative of any particular problem other than themselves. They are valuable food for many of our garden birds and the ladybirds will also probably be along soon if you hold your nerve. Personally unless this is a prize fruit tree I'd ignore them or blast them off with the hose. If you must treat just go to the garden centre and find something for aphids.