Identifying dahlia viruses
We've got a patch of dahlias in our yard inherited from the previous owner. We're now seeing an issue with a few plants, and after doing some cursory research, are not sure whether we're seeing a virus, or just weak plants. Basically, there are several smaller / struggling bushes with some mottling or marks on the leaves. We also have several other bushes doing great, but which also show some leaf discoloration. What we're wondering is if this discoloration always means there is a virus, and that the plants should be removed to prevent further spread, or if there are multiple possible causes.
Multnomah County Oregon dahlias
Dahlias are susceptible to viruses, like mosaic viruses. However, you would see the damage associated with the veins in the leaves. Leafminer insects (Leafminers http://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/vegleafminers.html) are probably the cause of the problems in your dahlia leaves. The larvae tunnel between the layers of the leaf and their tracks can be seen in random areas of the leaves. The leaf surfaces protect the larvae, and give them access to nutritional leaf tissue while they grow and mature, so it's very hard to control them except by squishing them in the leaf. The good news is that they usually don't really harm the plant - only a small percentage of the leaf's photosynthetic area is affected. Several insects, including flies, small moths, beetles and sawflies, use leaf mining. Most leafminer activity peaks at this time of the year. You can paint the leaves with horticultural oils, like neem, or use something like spinosad to discourage the adult fly from laying the eggs into the leaves, but the larvae will be safe.
Dahlias do best with several hours of sun, planted in loose, well-draining soil with good amounts of organic matter. Dahlia roots are quite large and need loose soil to spread out. Mulching with compost would help them, as mulch, by covering the soil, retards soil moisture evaporation. Mulch also helps moderate root temperature, keeps down weeds by blocking out sunlight to seedlings, and provides a slow, consistent source of organic matter as it breaks down. This article has additional information, Dahlias https://catalog.extension.oregonstate.edu/fs95/html.