Hello, Sally. You can rule out "too much sun" if some of the affected leaves are protected from the sun by leaves above it. You should also check to see if someone has used herbicides in the vicinity and make sure there are no mineral defficiencies or toxic levels with a soil test.

Just from the look of the affected areas, I suspect a virus has invaded the shrub and is causing the blotchy yellow areas. A sample sent to your Agric Extension Service can help rule out mineral issues and -sometimes- viruses.

Viruses are common and they are behind some of the famous white-green leaves in such variegated hydrangeas like H. macrophylla Mariesii Variegata. In other words, they do not necessarily harm the shrub.

But.... there are some viruses like Hydrangea CMV & ringspot that cause abnormal growth and some yellowing. In such cases, the shrub needs to be discarded. CMV produces some mottling and ring spots on the leaves but yours does not appear to be like that.

Without proper identification, it is difficult to tell you what to do (other than send samples to your Agric Extension Service). Personally, if the plant looks as great as yours does, I would assume there is no need to remove it except for aesthetic reasons. Coincidence: I have seen yellow leaf blotches on camellias too, at a local nursery two/three years ago.

Keep monitoring it just in case,
Luis

Thanks, Luis, for your input, very helpful. I have been at a loss as to the cause of the problem. If it is a virus, will the disease die during the winter? I'm going to attach a picture of the one on the opposite side of our walk. They are both Cityline Venice. I've had them about 4 years now. Just so strange how some of the leaves & actually the shrub itself appear very healthy, while other leaves look really bad. Do you think there is a possibility of a fungus?
Hoping the winter kills whatever it is.

Not likely. Viruses spread all over the shrub so winter kill would not kill them. It might kill leaves and stems with virus but you would get more when Spring arrives. If it were like that -and that would be great as far as I am concerned- a shrub like the variegated hydrangea with its white-green leaves would have that variegated look for just one season in cold zones.

I do not think it would be fungi either as it does not tend to turn leaves yellow like that. You can tell by getting a magnifying glass and looking for fuzzy or hairy-like structures only in the yellow areas of the leaves. The most common fungal disease in hydrangeas is powdery mildew and the leaves turn a whiteish or grayish color at first instead.

Luis

To rule in or out variegation, you could take a cutting and grow it in a different area. Would be a fun experiment!

Well, sure hope it's not a virus. I LOVE these hydrangeas. I checked the leaves more closely, Luis, and you are right - it's not a fungus. I've been looking at some pictures on line. Do either of you think this could be iron chlorosis? I do think, as Luis suggested earlier, I need to get the soil tested. Take a look at how healthy the blooms are!