Looks like anthracose to me, which comes from fungus splashing up from the soil. Iron deficiency is unlikely, especially with composted manure. My guess is the leaves near the soil are most affected.

Keep the leaves as dry as possible, make sure you have a thick mulch to soften the rain and separate plants from the soil, keep the plants upright and supported, and avoid sprinkling -- always better to water directly at the base, preferably through drip irrigation (you can search for a simple method to do so with upside down 2 liter pop bottles.

You could spray a fungicide, but it won't help the affected leaves -- it will just prevent further spread if the wet conditions are not solved.

Plants are about 3ft tall. They are caged in home made rebar cages. The ground around them is covered in lawn mower clippings that are dried out. Most of this yellow spotting is toward the bottom but some is also showing near the top on two plants. I have not watered in three or four days but have seen no wilting on the beans, cukes, onions or the peas.