The leaf is the first thing infected by the windborne spores, often it is the youngest shoots, it is nearly always those on the lowest branches. It attacks from the bottom up of a poplar tree, because the main source of the infection (the dead leaves) are lying on the ground. The higher branch shoots have a longer window of resistance simply because they are so far away from the disease. The leaves begin to develop PycnidiumPycnidium which are the fruiting bodies that spread the disease to other hosts. The infection begins to spread up the host to the top branches, creating leaf spots and cankers where ever it can. After the disease has used all the resources of the leaves, they have become black and shriveled. The leaves fall like normal in the fall, and overwinter on dead leaves on the ground. In the spring the wind picks up the Pycnidium S. musiva spores and it carries them to find a new host to start the infection all over again. It is worth noting that the cankers can spread the disease if they happen to break, they will release spores of their own, however, this is not the main source of infection as S. musiva cankers can live in the branch of the tree far longer than the S. musiva on leaves.
