A recent newsletter from STOKES Seed Co., here in Canada, says that Nematodes can damage roots so that the plants show deficiency signs of water or nutrient shortage. Ah Ha! Maybe this is the mysterious Dahlia "virus" that has been reported as apparently "cured" by extra fertilizer or a treatment with Epson's Salts. It is well established that virus cannot be cured in a mature plant, so the plants that can be stimulated to grow out of the disease symptom stage must be overcoming something that inhibits nutrient uptake by the roots. Nematodes are an excellent suspect.
What about a perfectly healthy looking plant that develops yellow leaf blotches and/or veins for no apparent reason? Could be Nematodes again!
Conclusion:
(1) If you have a sick, new Dahlia, from a suspect source - dig it and the surrounding soil up and throw it all far away. Virus or nematodes - you don't want it.
(2) If it is one of your own previously clean stock, then you could try to encourage growth with Epson's Salt (1 tsp per Gal) and /or some high N fertilizer with trace elements. Both could be used as soil drench or as a foliar spray or both. Meanwhile, in case it is virus, take out some insurance and keep the plant well sprayed with Cygon (Methioate or similar systemic), so that no aphid will be able to eat and move on.
I used this Cygon isolation technique to maintain a virused stock of Jean M for two years past the time I should have discarded it, in order to continue the breeding experiment that gave me Hy Clown. Since viruses are rarely transmitted by seed I was able to get healthy seedlings, even with Jean M as the pod parent. However I never cut flowers from it to avoid tissue fluids on the knife contamination - and I dug and divided it last so that shovel and tools would not carry virus.
Note: Epson's Salt is Magnesium Sulphate. Most Magnesium salts are quite soluble and therefore readily leached from soils. Plants use Magnesium to make chlorophyll, and cannot make healthy growth without a certain amount. Compost has plenty, but many soils are deficient.