Hi, 

It looks like they are just stressed plants. Are all the leaves exhibiting these symptoms? A bit of chlorosis maybe.

Are they watering regularly and deeply? Fertilizing regularly or with a slow release granular? They could try to feed with a liquid fertilizer and make sure to water regularly to see if that will help with symptoms. It won't necessarily mean these yellow leaves will change but it can put out more growth that should be darker green. 

If that doesn't seem to help is the pH correct for the garden soil? It may not warrant mailing in a soil sample and waiting for results, but making sure it is getting the appropriate nutrients with fertilizer and that the soil in the garden bed is rich and fertile would be good. Maybe testing it for the next rotation of crops would be helpful. 

Emily