Rusty water will work, but I might be inclined to look at the problem another way. what is causing it? If in fact you have an iron deficiency, i.e. interveinal to total chlorosis (yellowing) of new growth, then I might look at two things, is there enough organic material in the soil, and how is the ph or acidity of the soil?

A picture here would be good.

And what have you treated for that might be causing holes in the leaves? I found two hatchling slugs high up on dahlia leaves today. Still baiting for slugs routinely around here.

You might just use iron phosphate - Sluggo is a brand or Garden Supply has their own - which will kill slugs. Maybe it will even give the extra nutrients you need. All of my dahlias are green and pretty good looking, except 2 that were planted together looking chlorotic. I just fertilized with Mighty Plant and Messenger tonight and will see if that helps.