This year I'm having some problems with my bean seedlings. They are about 2 weeks old and the tops of some are wilting and falling compleately off. On some others that this is happeneing to, the stem looks as if it has been pinched in. They are standing up right and haven't been touched or bent at all but this has happened to several now. Is this some sort of fungus, too much watering?

Thanks for any help

Toller