How do I prevent rust on my hollyhock leaves?

It looks like you've got a fungus on your plants. According to this article this will not kill the plants and the recommended treatment is organic but prevention is the key. Read this and see if it helps:

http://homeguides.sfgate.com/hollyhock-diseases-48998.html

Good luck!

http://homeguides.sfgate.com/homemade-spray-control-rust-plants-32703.html

As mentioned by another commentor, rust is a fungus. I use "safers Fungicidal Soap" when rust spots appear on my plants. It is environmentally friendly, safe for animals and children. I am not sure what your watering schedule is but also try watering in the morning not evening and water at the base of the plant not on the leaves and blooms. This helps to prevent fungus from forming on your plants.

I sprinkle cornmeal around the base of my Hollyhocks. That seems to help quite a bit.

We were on a garden tour and theirs were beautiful. They used systemic rose food.