Hi Lisa and thanks for your question about the leaf miners on your dahlias. They probably got stressed while you were gone... lack of water? This invites pests on any plant and you got the leaf miner. Here is what the University of California Integrated Pest Management site says:

"Leafminers rarely require treatment in gardens. Provide proper care, especially irrigation to keep plants vigorous. Clip off and remove older infested leaves. Plant resistant species or varieties. Small seedlings can be protected by protective cloth.
On plants such as cole crops, lettuce, and spinach, clip off and remove older infested leaves. Leafminers are often kept under good control by natural parasites. Insecticides are not very effective for leafminer control."

As leaf miners over winter in the soil and the damage is actually from the larvae laid as eggs on the leaves, there are several things you can do if you don't want to just wait for the predators to move in next year. This list is from the University of Utah:

• Check transplants for signs of leaf mines and white stippling before planting; destroy infested plants.
• Clip and destroy infested leaves to prevent larval development.
• Plant a diversity of crops in a location, as monoculture plantings can be more susceptible to infestations.
• Place a floating row cover (lightweight plant fabric) (Fig. 8) over plants to inhibit adult flies from entering and laying eggs.
• Secure edges and ends of fabric for
  better protection. Note that this method is only effective when planting
  into a site not previously infested with leafminer pupae.
• Avoid excessive levels of nitrogen fertilization, as this can increase leafminer infestations.
• Remove plants near vegetable gardens and fields that are known to be leafminer hosts. These include weeds such as nightshade (Solanum americanum) and dock (Rumex obtusifolius), and ornamentals such as chrysanthemum, marigold, dahlia, gerbera, and gypsophila.
• Immediately after the final harvest,
  remove plants and deeply plow crop residues to remove food sources and
  inhibit pupal development.
• Rotate crop plantings.
• Use adequate irrigation to keep plants healthy and reduce stress.

I also leave my dahlias in the ground to overwinter but do keep them watered over the summer. To dig them or not is always an issue with our winter freezes, but since the roots are not really affected, you probably don't need to (I can hear the digging fans screaming in the background!). Being proactive next year with using row covers early in the season and removing leaves with signs of damage would be the first step I would recommend and then wait for the predators to move in and help control this population.

Now I have to go check my dahlias...



Hi again Lisa and thanks for more dahlia pictures! It looks like you are seeing a dahlia virus, maybe even several different ones. It looks like there is not an easy control and if you were raising them for show you would need to remove the affected plants.
https://pnwhandbooks.org/plantdisease/host-disease/dahlia-virus-diseases

As there are thrips, aphids and nematodes that spread the viruses you might want to remove those plants anyway. Another option is to "let the strongest survive" as some of your plants probably have resistance to the viruses. There are no chemicals recommended for treatment at this time.
https://hortsense.cahnrs.wsu.edu/fact-sheet/dahlia-virus/

In my case, I just ignored the virus, hoped the plants would put out new leaves and let the ones that couldn't survive disappear. I have about half my dahlias left.