Wilting can occur for several reasons such as overwatering, underwatering, root issues, voles http://extension.umd.edu/hgic/voles, etc.
In general, liatris likes a well draining soil particularly over the winter. It is tolerant to drought. The plants may need division after the fourth or fifth year and are not that long lived.
Deutzia - Wilting can be due to poor plant material, overwatering, underwatering,. poorly drained soil, voles (feeds on the roots of trees/shrubs), rootbound container plants, etc. These are cultural/environmental issues and not insect or disease related. Take a look at the planting process for trees and shrubs http://extension.umd.edu/hgic/trees-and-shrubs/planting-process
Deutzia likes a well drained soil in full sun or light shade.
Thanks. Since I planted the Deutzia near the wilted Liatris, do you think the wilting of the Deutzia is related? Do you think it is a soil problem?
Does the area get good drainage or does it stay very wet after it rains? Did you water the Deutzia regularly after you planted it? We don't know exactly what caused the problem. Most likely it was poor establishment of the plant, rather than a pathogen. It could have been poor plant material, wrong type of soil conditions for that particular plant, and/or lack of water or too much water.