I have the same issue except the whole leaf is turning yellow! But it's only on the new plants I bought about 3 weeks ago. The 3 I stored that are coming back are all dark green. I have only fed them twice with the Schultz liquid indoor/outdoor plant food. My snapdragons loved it and went nuts with blooms.

Thanks. I also use the Shultz. These are tubers I had planted in April. The dahlias I did this year are not going to be like the ones last year and I don't know why.

I believe could be whiteflies. Mine had them bad so I sprayed with Sevin. Horticultural oil or Safer Soap would work. For long term protection might want to go with a systemic like orthene or Bayer granules. Steve in Baltimore County.

I didn't hear anything that would indicate an insect infestation. Sounds more like chlorosis to me, or too heavy soil, poor root development, too much water, etc.
I'd make sure the site drains well for the tubers and feed with bone meal, tomato fertilizer or Espoma's Bulbtone.

Nitrogen promotes foliage development and you do want that, but in a bulb or tuber, you don't want a high N fertilizer - it will rot them out. You want a fertilizer that promotes root and flower production - a higher Phosphorous and Potassium level than Nitrogen.

This year I'm trying something new, in the garden I amended with Sweet Peet and composted wood chips to improve drainage and porosity and double layered Bulbtone down in the planting holes. I also grow 60 gallon containers of dahlias on the deck where my mix is very porous this year - I'm using 40% composted wood mulch, 40% Sweet Peet and the remainder is ProMix - a peat based soil-less container mix that holds water. I double layered the Bulbtone in there also to give the roots something to grow to. And just getting ready for first extra fertilizer application - I will use Mighty Plant 2 times, July and August then stop because it has a lot of N. According to Dahlias.org, you stop the high N fertilizers in August because a tuber that has been over-fertilized will have a greater tendency to rot over the winter. I've done this in the past with good results.

OK but better to treat for insects first. By the time you go through all the chlorosis remedies the plants may well be dead. Steve in Baltimore County