Tree dirt on new roof

I read once that installing zinc strips of roof would prevent discoloration from overhanging trees. If this is so, how would I install and what store would carry such a product. My new roof, 2 weeks old, already has staining from tree dirt. Any comments or suggestions without cutting down tree?

The zinc strips have nothing to do with tree sap. The discoloration of shingles is from a fungus that comes from other roofs. They have cleaners to remove it and zink or copper strips will prevent it from growing. Check hardware stores. Some carry them, I'm not sure about bog box stores.