A customer would like to know what is killing her young sycamore. They are marginal at best in our area and this was planted. It looks to me like a canker issue rather than just a leaf fungus but not sure if it matches canker stain. Thank you, Ann Hruska, Dickinson Conservation District
That does look like a canker.
I'm wondering if the fungal canker causing agent entered the tree after sunscald split the tree last winter. When we have snow on the ground, the sun can reflect off of the snow and heat the trunk of think barked trees on days when temps are below freezing. This causes the bark to split, and could have opened the tree up to this fungal infection. I would recommend that the landowner prune off the branch with the brown leaves to look for the staining that is described in the linked article. If the canker is suspected at that point it's best to remove the tree and dispose of it, best way would be to burn it to avoid spreading the fungus to other trees.
I hope that helps. Feel free to reach out by replying to this string or by using the information below if you or the landowner has any follow up questions.