Dogwood fungusOur 3 Cornus Alba planted last autumn all have fungus attacks on the leaves

Possibly a fungal disease called Cornus anthracnose.
the RHS has an article about it but I cant get it to load on my little lap top.

Have a look and see if it matches.

It does look like fungus, and may require immediate drastic action on your part if you want to rescue these trees. Fungus waits for nobody! lol. You should treat the trees and everything around them with copper sulfate fungicide you can pick up at the garden center. You can also make a homemade fungal treatment with sodium bicarbonate until you can get the good stuff. Now you have to figure out why the fungus is there in the first place. The trees can only withstand so many of these attacks. Something about the aspect is off. Is it too wet? Is it too shaded? No Sun? etc. Research the proper conditions these trees need to thrive, then compare it to the current aspect. Through process of elimination, the reason will become obvious. Then you'll know what corrective measures to take.

I wouldn't think that it's too bad. There looks as though most of the new growth of leaves are healthy and it's just some of the old leaves that are dropping. A dose of Fungus Fighter would help. They do thrive on wet soil so regular watering, a mulch and a feed could relieve the problem.

https://www.rhs.org.uk/Advice/Profile?PID=137

this is the link to the RHS site re cornus anthracnose.
but that suggests that C alba isn't susceptible to it.

they do have an advisory service where for a small fee you can have plant tissue examined and the disease diagnosed.
and to be honest diagnosis from a photo is difficult.

Thanks for all the helpful information...