If rust appears after corn has started to fill, it will not affect production. It will be a problem when the corn is still young. Fungicides containing mancozeb used early can control it. The product I know that contains mancozeb is Dithane. The label is in the link below. It is important to read and follow label instructions.
Cultural controls would be to plant rust resistant varieties, time plantings to avoid the cool wet weather and planting early in the season if you live in a northern climate. Since spores blow in on the wind it is difficult to control the disease coming from outside.
If the infections are severe and you want to avoid having to spray, then crop rotation or using disease resistant cultivars are the best solution. Rust can also be found in alternative hosts, so alternative hosts also need to be controlled. Newer strains of rust have arisen that make it much harder to control the infections without resorting to sprays. Fungicides work best sprayed as a preventive, most fungal infections, once established are difficult to control. Most of the resistant corn varieties that are resistant are GMO.
https://www.dowagro.com/usag/prod/060.htm
https://vegetablemdonline.ppath.cornell. ... n_Rust.htm