Disease trees and ground cover plant
The white substance on the dead nettle is left by plant hoppers. It's harmless and the plant hoppers would not have caused the damage you are seeing. We suspect that the plants may have drought damage or a disease such as web blight: http://extension.umd.edu/hgic/rhizoctonia-web-blight-flowersgroundcovers
The cherry tree has lichen on it. This is harmless. Lichen uses trees for a support structure, but it is not a parasite. However, when a tree begins to have a lot of lichen, it can be a signal that the tree is aging or declining.
The crape myrtle has powdery mildew.https://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs If it is a yearly problem, it's best to remove the crape myrtle and replace it with a powdery mildew resistant variety.