Flea Beetles, A common problem this time of year. It has come to my attention that they are afoot. If your leaves have what looks like little shot holes you might/probably have flea beetles. They seem to attack the lower leaves first. Here is a link ot a site that gives you good information as to what they looks like, how they overwinter and what to do about it.
http://www.ext.colostate.edu/pubs/insect/05592.html
flea beetle damage
Notes From The Tomato Lady 
Hi, I hate those little flea beetles, I have them every year in my poly tunnel as I grow mustard……to eat not to deter them from eating my other veggies. I know that they dont do loads of damage but I hate seeing all those little holes in my leaves. Have you tried DE power……it does work and its organic.
What is DE power? I’ve used a product called Garden Safe by Shultz and it seems to work pretty well. I also have some diatomaceous eath that I am going to try.