Lettuce Ring Spot

Does anyone know anything about this please? All I know is that its a fungus. I've just removed all the remaining lettuce most of them had ring spot and severe slug damage. I'm thinking both of these problems have been aggravated by the nice little moist environment under and between the lettuce? I followed the directions, I thinned them to 12 inches apart but obviously after growth theyre all tightly packed together again.

Anyway, my main question is can I compost the affected lettuce? And can I eat the unaffected leaves of an affected plant?