You can get a high iron fertilizer for petunias and it would work but you can buy liquid iron in a quart bottle and dilute it. Just be careful as it stains if you spill it on the concrete your shoes, clothes etc...spray it on and also pour it on the soil.. since it is a potted plant I am suspecting it is irony deficiency and not a disease.
I leave my dahlias in, but I mulch them heavily. Every three years, I divide up the tubers in early spring. This year, as I am expecting a cold winter, I will pull them up and store over the winter and divide them out in spring. I will try @Sjoerd technique to store them.
