Powdery mildew causes a white powdery growth. This problem can be controlled with sprays of benomyl or sulfur.
Leaf spot caused by Septoria poseyi may spot the leaves. Infected leaves can be picked off and disposed of. No chemical control is listed.
Stem rot may rot the stems. The disease is caused by Sclerotinia sclerotiorum and may be controlled with drenches of benomyl.
Rust diseases may disfigure the leaves of mertensia. Rust can be controlled with sprays of sulfur.