Thanks for writing. Try to give your collard greens a spot with full sun and good air movement to help deter fungal leaf spotting diseases from getting established. If you grow collard greens every year, it can also be helpful to move them to different parts of the garden each year so the diseases don't get a chance to build up in the soil.
It can also help a lot to remove affected leaves when you first see spots form; that may stop the disease from spreading to other leaves.
As a worst-case scenario, you can also try spraying fungicides labeled for use on edible plants. Do this in during especially wet periods in spring before you see any disease. Fungicides work to prevent disease, not kill it once it's infecting your plants.
---Justin, Senior Garden Editor, BHG.com