I curled chicken wire around the bottom of my largest feeder (it generally has the most activity) so that it covers the ground that surrounds the feeder with the most discarded seed shells and other debris. I don't worry about diseases being transmitted anyway though, since most of my bird feeders are pretty clean and grass covers the ground below the feeders. I think keeping mass amounts of seed shells and droppings accumulating from under the feeders is a good idea though.
