All plants in a nursery have a hard life, living in constricting pots that heat up and dry up way more then they like. Add to that the need to water every day, when foliage often gets wet in the process (perfect for disease infection), it is surprising how resilient plants are. Just as you are more likely to get a cold when you feel run down, all these negative factors predispose nursery plants to disease problems that usually go away once all these factors disappear, and assuming the ensuing watering and planting are correct. But once a leaf is infected, it will usually continue to go, even if the tree is healthy. This is not a reason to panic. If new infections continue to crop up, then I might think again. Plants (including trees) are living things, and as such, not a one (even the most healthy) is blemish free.
I planted an Autumn Brilliance serviceberry in my back yard one fall. The following spring every leaf had rust (disease) pustules on them. I knew the tree was going through an adaption period and thus would not be as healthy as it would be later, so I just waited. After that first season, I have never seen even one rust spot evident.
So don't be alarmed. Pick the offending leaves off if it will make you feel better. Even if you picked half the leaves off, the tree will be fine.
