Have you read things like THIS?

tj

You cannot cure cercospora yet so, it can come back.

Local end of Summer/Fall weather -if it is very dry- may minimize the number of lesions and help make itself invisible.

Have had it visible for 25+ straight years here. Call it old dependable, hu?

Fungicides only help control it, not cure it, so they are useful when you usually have heavy infestations. Of course, they work best if applied starting around the end of June ish.

Wait for 2 or 3 seasons to get an idea if it becomes visible or not where you live. Use clean sanitation practices to help minimize its re-occurrence.

And is really only applicable in areas wth high summer heat and humidity. Here, cercospora is pretty much a non-issue. Even overhead watering seems not to encourage it. Rarely does one see it outside of a nursery setting and it will generally disappear once the plant is in the ground

Like with any fungal disease, good garden sanitation will help a lot. Typically spotting will start at the base/oldest foliage first so removal of these affected leaves can help to slow the spread. Pluck off and destroy all the leaves at the end of the season and rake up any fallen ones. Keep a good layer of mulch surrounding the plants to prevent any spores from splashing up. Use drip irrigation or a soaker hose when possible and water infrequently but deeply when you do.

Thanks all for the answers, but I'm not sure I have the answer I was looking for. So is a cercospora outbreak independent from year to year, where it will only appear if the heat/humidity conditions cause it? Say it appears one year, and either I treat it or not (doesn't really matter for the question), and the leaves fall off for the winter. It won't automatically appear no matter what the following year because the plant "has the disease" will it? It will only appear again if the heat/humidity conditions are again favorable for it, is that right?

It is always there because the plant has it. If the climatic conditions where the plant is located are optimal, the leaf spots may be minimized in some years such that you may not notice it. If climatic conditions are not optimal, you may always notice leaf spots. If you want to know whether you have a chance of seeing it, try asking local plant nurseries if customers have complained about cercospora in hydrangeas or ask your agriculture extension service. Or set up an experiment with a hydrangea that has it and see if it gets leaf spots in July-October in the next few years. You may also be able to use a magnifier on leaves after chlorophyll production goes down and see if leaves show the leaf spot. I can "see" the spots sometimes... unaided... in May-July ish or so.... Looks like a green leaf with a spot that has a very slightly different shade of green (darker).

OK, hey thanks for all the info. Guess I need to watch and hit with fungicide at first signs of appearance.

Or make a note in a wall calendar of when you noticed leaf spots and begin applying fungicides a month before that.

I try to control it only using drip irrigation (so no overhead watering of leaves) and clean sanitation practices. Fungicides were more expensive 25 years ago. That is when I moved to my current home and ran into the problem with one hydrangea.

https://ugaurbanag.com/cercospora-leaf-spot-on-hydrangea/

OK, thanks Luis. I'll be careful with the watering and everything, and see if I can avoid it next year!