This may be late in coming but I just saw your question, part of the problem is Poplars, the often have black leaf spot, but that will not kill your trees. The yellowing may be a symptom of something else. How much are you watering and what types of soils do you have? If you are watering to much, which can be why the leaves are yellowing. They can be over watered and die from that. Another point, if I may be so bold as to ask Why Poplars?