My Boxwood are affected by a disease causing cupping of the leaves and finally they die. I can see a white powdery stuff on them too.
Please let me know what remedy I can use to cure them.
Thank you for contacting the Toronto Master Gardeners.
From your description, I believe your plants have been infested by Boxwood Psyllids.
We are attaching a link, please see below, from the Missouri Botanical Garden on this pest. It will help you make a definitive on-site diagnosis, inform you of the insect life cycle, has pictures and will direct you to treatment options (just be aware that the pesticide regulations differ from Missouri to Ontario).
Hope this information is helpful.