I have very similar looking leaves on some beans and did a tissue test.  Low in K.  The soil tests low in K in the spots also.  Tested the whole field last fall and tests showed lots of K.  I have been trying to help them along for the past 6 weeks with foliar feeding 2 gallons of 5-15-15 every 2 weeks.  It has helped some as the check strip I left looks way worse, but they still don't have that nice dark green color to them.

Where are you located??