Sorry I don't know the answer, but I have plants with similar damage. I think it is some type of insect. So far I haven't done anything to treat for it such as insectidal soap, but it is rather ugly. Besides eupatorium, my verbena stricta exhibits this type of leaf damage.

hello
i'm no expert,but if i had to guess...i'd say a type of mold from being to damp....if it is only the bottom leafs...i would just snap them off....if it is all the leafs...i might move the plant to a drier area...
again,i'm not an expert,but that is what i would think....
good luck

Thank you ontnative,for your input.
In my garden this is only one plant effected by such kind of disease.I do not treat it with anything yet.
Hala.

Thank you diggy for your response.I also think this can be a mold,fungi,but I would like to be sure in order to treat it properly.
Hala.

The circles around the spots tell me this is a fungal disease called leaf spot.

Pick off all infected leaves and distroy. Do not compost.
The spores spread like with most fungal diseases by water splashing on leaves and carrying the spores with it to a new leaf, so try to avoid getting the leaves wet when you water.

Spray with a fungicide that is good for leaf spot--neem oil is one. You may have to repeat because the fungus is in the soil too.

You have had lots of rain amd dampness in Toronto and that's probably why it started. It only lives in the top layer of the soil and if you get the hot weather they are predicting in August that will probably take care of it in the soil, but you want to get rid of it on the leaves, so pick and spray.

good luck

Hello Oilpainter,
I greatly appreciate you took the time to share your knowledge and tips.
thank you very much for your very informative and instructive response to my post.
regards.
Hala.

I should have said to clean up any debris around the plant.

The infected leaves fall, rot,and infect the soil the spores carried by water get on the leaves and infect one then it spreads to the next one. The leaves fall and it starts all over. That is why you get it year after year. Persistance in not letting the soil become infected, or hot dry weather that kills the soil spore, is the only way to get rid of it

Oilpainter,you make me more understand all those Scientific things.Prevention is the key to growing healthy plants I guess, and I will treat it more seriously now.I must clean my flower beds and I believe,I will get rid of the problem.
Thank you very,very much.
Hala.