Q: Our two-year-old willow tree has developed yellow leaves and black spots on the leaves. We can't see any insects on the underside of the leaves and the trunk looks ok.
What do we do to help this new tree survive! S. Ulrich (South Dakota)
Leaf fungi usually infect the leaves in the spring, so treatments must be timed for then. Treating now will unlikely do you any good, so you'll have to wait until next year, but begin planning now. In the meantime, clean up and dispose of all fallen plant parts, particularly leaves as leaf fungi tend to overwinter there.
Being in South Dakota, you may want to contact a local arborist to investigate your willow. He or she will be more familiar with what fungus problems are prevalent in your area than I.