One of my dahlia plants (a Camano Sitka) is suddenly turning yellow. It's one of ten dahlias being grown in grow bags in the same raised bed, in the same potting mix, getting the same amount of water, yet this one dahlia is suddenly going yellow. The other dahlias are all a nice, dark green, but this one is bordering on chartreuse. Any suggestions on why and what to do about it?

The attached photo shows my yellowing dahlia next to another dahlia, showing the color difference.