I have a shady garden in Toronto. I first saw the yellow leaves with black dots on my holly this year. I have clay soil. I’ve removed the damaged leaves.
Yellowing leaves can point to several potential issues so it can be a bit difficult to pinpoint what might be happening with your holly. The good news is that it is entirely possible that this is just the natural process of your holly dropping its leaves. While hollies are considered “evergreen,” they will still drop older leaves in the spring. Typically, the interior leaves will yellow and drop. If this is normal leaf drop, it should stop now that we are entering into the summer. If the leaf drop continues into the summer months, it is likely another issue.
Overwatering or underwatering can also lead to yellowing leaves. Hollies prefer moist but not saturated soil. Let the soil under your holly dry out and only water if there has been less than about 2 cm of rain in the previous week. Saturated soil can also lead to root rot, one of several fungal diseases that attack the roots. Root rot will also be accompanied by dieback on twigs, so keep an eye for other signs of this disease. Unfortunately, root rot diseases are not readily treatable and removing the plant may be the only option in this case.
Hollies may also suffer from several types of leaf spots or blights which are fungal diseases that affect the leaves. Cylindrocladium leaf spot begins as tiny yellow spots that later enlarge to circular spots with a tan to brown center and a purple-black border. Holly leaf spot diseases are usually more severe after wet springs. Minimize leaf wetness by irrigating before midday so the leaves dry rapidly in the afternoon. Removal of infected fallen leaves, as you are already doing, reduces the amount of the inoculum present for new infections.
High soil pH can prevent the plant from absorbing iron resulting in yellowing of new leaves. However, it appears from your photo that it is the older leaves that are affected, and new leaf growth appears fine. So it is unlikely this is your issue. Chlorosis can be readily addressed by adding an acidifying fertilizer or iron chelate to the soil.
Finally, check on the underside of the leaves for scale insects. These appear as dark bumps that don’t move. Scale can be treated with a summer formula horticultural oil (not a dormant oil) sprayed on the undersides of the leaves as per the manufacturer’s instructions.