To Much water? Not enough fertilizer? Fungus disease? Hot temps?
I would spray them with a general fungicide and give them a shot of fertilizer with at least a nitrogen number of ten or above if you have not been giving them much nitrogen. Hot weather can make leaves turn yellow but usually from the bottom of the plant. We just pull those leaves off and get additional air circulation to the plant.

I agree with huey......strip off the bottom leaves and give the plants a shot of high nitrogen fertilizer. If they don't green up, try a fungicide.
DB

thanks to both of you. unfortunately, it's not just the bottom leaves and it seems limited to just one variety that i bought this year, so i'm pretty sure it's a bug or a fungus. i bought 2 kinds...one just a figaro mix, which are fine and "sunburst" which are beautiful, but yellowing on all the leaves. i'm going to try bayer's 3 in one rose and flower which is supposed to aid blooming (no real problem here), and control bugs and fungus. wish me luck. they will either be better when i get back from vacation or they will be near death........