Q: My hawthorn tree has a lot of berries but most have rust colored, short, stringy things covering them. I recently sprayed the tree with water and plumes of orange dust came off it.
A: Your tree is afflicted with cedar-hawthorn rust. The name of this disease describes the life cycle of the fungus. It develops on a red cedar tree and then infects hawthorn trees growing close by. After growing on the leaves, orange, dusty spores are released that colonize nearby cedar trees.
You can interrupt the fungus life cycle by raking up and destroying all leaves that drop from the hawthorn.
Spray chlorothalonil (Daconil, etc) on the hawthorn three times in spring, beginning when the leaf buds open. Consider replacing your tree with ‘Winter King’ hawthorn, which is resistant to cedar-hawthorn rust.