One half of our peach-plum is dying suddenly. On closer inspection today we saw all parts of the trunk (yes) and branches covered in green aphids (yes) on the move. A few of the leaves on the healthy part of the tree have aphids and there are a few ladybugs around. I sprayed it hard with water to wash most of them off. Could the aphids have killed the damaged half of the tree? Why only half the tree? I've never seen aphids moving like that and I know they generally stay put unless they've become winged. Has anyone seen anything like this or have any explanation?