Hi, I have an Ash Tree that has generally been very healthy. My neighbor has the same Ash tree. They have noticed the leaves starting to curl into themselves and a white substance on them. It seems to have taken over their tree. I noticed this started on my tree just yesterday (I've looked every day). I have attached a picture. Can you tell me what this is and how to combat it? We love our tree and have worked over the past five years to keep it healthy and growing strong. 

Thanks!