When my brugs do that, I give them a shot of 1 tbl epsom salts mixed with a gallon of water. The next week do it again. By then the leaves usually come back.

Do you have spider mites? I noticed this morning that one of my dahlias has spider mites on it. I sprayed it, and gave it the epsom salt treatment, and now wait for it to reguvenate.

You also mentioned that you'd had a lot of rain. This can cause the leaves to yellow. Hopefully the rains will leave you, and come and visit us. We haven't had rain for such a long time......the grass is brown and crisp to walk on. My watering bill is ridiculously high.

Jroot, is it the magnesium in epsom salts that works magic on the dahlia yellowing?

If you just water your gardens, it will surely rain. Based on recent experience. 3.5 inches of rain in just several days. Maybe you could just pretend to water and fool the rain gods.