This does not appear to be an infection of any serious consequence, based on the symptoms, as a number of leaf-spotting fungi and bacteria can cause minor damage to most trees, shrubs, and perennials late in the growing season. As the leaves fall in autumn, the infections will cease or lay dormant until the weather conditions become conducive enough later next year. You can collect and dispose of its fallen leaves, though avoid removing them from the plant prematurely since it still needs to re-absorb whatever carbohydrates and other resources it's going to store before shedding the leaves. Given the reliance on cooperative weather conditions, not all infections recur with equal severity from one year to the next. In this case, no fungicide or other pesticide treatment is needed (or even effective at this point). There's a very minor amount of insect feeding damage visible, but here too, it's inconsequential and does not require intervention.

As an aside, is this a volunteer seedling elm, or a cultivar selected for resistance to Dutch Elm Disease? We ask because the former might not be worth investing the time into growing given how prevalent and damaging that disease can be; while resistant cultivars aren't immune, they stand a much better chance of not contracting serious illness.

Miri