|white growth on leaves of althea bushes|
|Sounds like powdery mildew to me. It's early in the season so you may just want to prune away all of the affected stems and leaves and remove them from your garden. This will get rid of the disease spores and also open your shrub up to better sunlight penetration and air circulation. There are all purpose fungicides you can spray as preventative measures but they won't cure the disease already present so pruning it off is the best course of action.
Good luck with your plants!