and sprayed religiously with Daconil, every other day or so.

==>>> wow ... is that what the label recommended???

ken

Nope. The label said once a week. But that didn't work, so I tried more frequently. Either way it didn't help.

I think it somewhat risky to overdose any kind of garden chemical, in this case by factor 7.

No effect showing leads to several possible conclusions:

1) wrong stuff for the fungus/ disease
2) the Hydrangea were so stressed out it couldn't help anyway

If a disease keeps popping up every year there is something wrong with the plant and / or the site.

You sure your Hydrangea have the right conditions to grow?

IME if there is something wrong with the soil there is zip you can do- and I mean heavy soil if your favourit plant would need sandy soil etc

Bye, Lin

Here is a link that might be useful: Technical bulletin

I have a few hydrangeas that had leaf problems and it turned out the pH was too alkaline in that area. The soil was also compacted.
Just a thought concerning soil.

Well, up til this past year the hydrangea was happy and growing well. It's in decent soil that's a bit acidic. I don't think the soil is the issue.

My concern is that now it has spread and I want to contain it. I can't replace all of the soil on my property. I considered removing the hydrangea, but that's not going to help all of the other plants that have been infected and I'd like to try to save it since it has some sentimental value. To linaria, I've come to conclusion 2.

I need to know if the horticultural oil is going to be effective like I was told at the nursery.

First, it is extremely unusual for the same disease problems to affect a whole range of different plants - they are typically very plant-specific (the exception being powdery mildew). Therefore, it is highly unlikely that the same problem is affecting all your plants and so the same blanket treatment will not help.

It is of primary importance to get an accurate diagnosis of the specific problem for each plant in question and then utilize the appropriate remedy. Hopefully someone on staff at a better garden center can help or check out a Master Gardener clinic or your local extension service.

Often, the incidence of a lot of plant problems is more culturally related than an actual disease issue - fix the growing condition and the problem disappears. Fungal issues like leaf spot are often related to weather, overwatering, watering at the wrong time of day (leaving water on foliage overnight), poor drainage, lack of air circulation or even proper garden clean-up and sanitation. And mulching in winter can help a lot - a good layer of mulch smothers fungal spores that can overwinter on the soil surface and prevent them from splashing up with rains and reinfecting.

Your hydrangea issue is most likely cercospora leaf spot, a very common problem with a number of hydrangea types. If you read the attached info sheet, you'll notice that the condition is primarily due to cultural issues. Correct those and you will correct the problem.

Here is a link that might be useful: Cercospora leaf spot.