help with plant diagnosis
This looks like two issues on the dahlia.
Left photo looks like planthoppers on the stem. See our website for more information and photos. No control is necessary. They are not a reason for decline. http://extension.umd.edu/hgic/planthoppers-flowers
Foliage - this looks like powdery mildew, a fungal disease. Make sure the plants are in full sun with good air circulation. All you can do is control the best you can. Fungicides will only protect new or uninfected foliage. They are protectants, not curatives. It is up to you if you want to spray.
Next season monitor and spray as soon as you notice the mildew. Follow label directions. http://extension.umd.edu/hgic/powdery-mildew-annuals-bulbs-groundcover-perennials-and-vines