Fertilidust will restore all of the product to a tree the same way that petting/watering will. So if you come across a tree that has been completely stripped bare and that has been petted/watered too recently for you to do so then you can use fertilidust on it. Other than that it has no effect on adults.
Yeah, what Humbabella says! If I'm on a street where the trees haven't been petted/watered for a while I will fertilidust them before I harvest! I could just pet and water but hey, have dust will sniff..I mean use!!!!!