To control moss, use iron sulfate or ferrous ammonium sulfate, after spiking or power raking to break up the thick moss layer. The best tact of course, is to not allow compaction, fertility, or especially water management get out of hand. Controlling these relatively simple factors will keep these problems from ever developing . Many aphids cause leaf discoloration, disfigurement, and occasionally premature leaf drop. Honeydew covered surfaces will often turn black as sooty mold fungi move in and colonize the sugary substance.
Some aphids cause galls.
The poplar gall aphids cause marble-sized galls at the junction between leaf blades and petioles on cottonwoods and other poplars. High numbers of aphids can weaken trees and shrubs, but rarely kill them. The sweet honeydew is attractive to ants, flies and wasps. The attraction of undesirable insects along with the honeydew mess and damage to painted surfaces caused by honeydew are factors which often motivate people to control aphids on shade trees. If compelled to spray these plants, use an alternative product such as insecticidal soap to reduce the impact on beneficial insects.