Septoria Leaf Spot
Septoria leaf spot is a fungal disease with the potential to cause four different leaf reactions, depending on the conditions at the time of infection. Affected foliage could become peppered with small brown patches; develop white and silvery blemishes; get almost completely covered with discolored areas; and develop round brown spots with yellow to brown edges. While septoria leaf spot itself doesn't kill the poplar tree, it makes it susceptible to additional health problems that might. Fungicides sold under the name Daconil are used to control severe infections.
The insect poplar borer makes its home in the roots, root collar and trunk of balsam poplars, Eastern cottonwoods and trembling aspens. You usually see signs of infestation at the base of the tree, where you find very fine sawdust indicating the insect larva has been digging holes in the wood. The bark also shows swollen spots, and tree sap is visible flowing out of the tree. The adult insect also causes damage by feeding on the poplar's leaves. According to Canada's Natural Resources website, the poplar borer is prevalent in trees that grow under less than ideal conditions.
Marssonina Leaf Spot
The fungus Marssonina causes leaf spots on the poplar's foliage. Earlier infection appears as very small brown spots, but as the disease progresses, the patches merge to form large areas of dead tissue. This fungus also attacks leaf stems and young branches. The Marssonina remains alive, but inactive, through the winter. For this reason, removing the affected parts of the tree before spring works as a preventive. Otherwise, as soon as the weather warms up, the spores begin to grow again reinfecting the poplar tree. In severe cases, premature leaf drop occurs.
Poplar Bud Gall Mite
This mite is impossible to see with the naked eye, but you'll be able to recognize its damage. The poplar bud gall mite causes abnormal growth to develop from the leaf buds.The floret-shaped woody galls prevent leaf and stem development and serve as the mite's nests, which they also use to overwinter. A fresh gall looks green but turns red or brown as it ages through the growing season. Since the mites live in the galls, removing all of the abnormal growth prevents continuous attacks. Utah State University Cooperative Extension also recommends dormant oil to control a severe mite infestation.