I've put in some bush beans

[China_cat]

and they'd been coming up beautifully, about 8 inches high now with several sets of leaves. However, the leaves are developing light brown spots that are thin and starting to tatter, some on the edges of the leaves. I've checked very carefully and can find no insects or sign of insects. They are in partial shade according to the package directions, drainage is good. The few pictures I took don't show the problem adequately.

Any idea of what this is? How I can fix it? Prevent it? Stop it from possibly killing off my beans?