This I think is a very common condition on Poplars (this was on a Black Poplar (Populus nigra) at Dalton. Generally known as foliage rusts but caused by the genus of fungi called Melampsora and not particularly destructive since it seems to affect the leaf quite late in the year and not long before the leaf is shed anyway. When considering the pros and cons of the deciduous strategy of trees, infestations like this illustrate what trees would be like if they didn’t get an annual chance to shed all their parasites, fungi, viruses etc. Unfortunately it will happen all over again next year as spores are released from the leaf litter on the groud into the air and infect Larches. The fungis quickly produces more spores on the Larch needles which then infect the Poplars. Both types of tree are needed for the fungus to perpetuate itself.