Last summer I noticed a lot of leaf loss on a 5′ columnar boxwood I’ve had for several years. I didn’t know why, and didn’t have a chance to deal with it. This spring, I noticed leaf discolouration on a much smaller boxwood (2 years in my garden) and not far from the older one. I’ve included a picture of the 2nd showing underside and leaf tops. I think this fits the description of leaf miner, but would love your diagnosis and advice. I have a third, old and gorgeous pyramidal b.w. on the other side of the garden, and I don’t want it to get infected too!
Thanks for contacting the Toronto Master Gardeners about your boxwood problem. It’s a little difficult to see the leaf undersides clearly in your photo, but if the problem is boxwood leafminer the tell-tale signs are blisterlike, yellow/orange spots on the underside of the leaves. That’s where the larvae of the boxwood leafminer midge are living between the upper and lower coatings of the leaves.
You’ll find some steps to take in this answer to a previous question on the same topic:
In addition to suggestions about pruning in the link, if the infestation is small (or if you’re very patient) you can also squish individual leaves between thumb and finger to kill the larva inside.
Another link with information on this and other boxwood leaf problems is below. Keep in mind that any U.S. links might suggest pesticides (insecticides or herbicides) that are banned from cosmetic use in Ontario. I hope this helps. Good luck with your rescue operation.