Variegated dogwood when to prune???

I was told by a friend that dogwoods are subseptible to a fugus or disease if pruned now. He recommend the middle of the summer. I have on my dogwood a shoot perhaps below the graft ( not sure it has been grafted ) but right at the base ( almost like a water spout). I would like to cut this off. What is the best time for this?
Thanks.