I need help to identify a fungus that causes havoc to my linden tree. I just discovered this summer that my young linden tree suffers a fungal disease. Since I planted a few years ago, this fungal disease weakens the tree but does not kill it. The tree starts out strong every growing season but the leaves start to die off (all green part of the leaves die off except the veins). This summer the fungus effectively affected almost all leaves so removing the diseased leaves is not practical. Here are the picture of an affected leaf and magnified pictures of the fungus. The fungus is on the underside of the leaves.
What kind of fungicide can be used to treat this fungus? I sprayed a copper based fungicide yesterday but it does not seem to help. Thank you for your help.