You might try giving them some additional water, as the bulbs begin to enlarge they will need extra moisture, and all those mixed composts don't always hold water really well. 

You could do a little investigating on the plants that are showing yellow leaves. Pull one that looks the worst and look for Allium Leafminer, which is becoming a more common pest in MD. 

If no sign of pests, just continue to monitor the condition and if it gets worse get back in touch with us.  From the photos, it appears that the most yellowing is on the older leaves, which could be nutrient deficiency or environmental stress. 

Additional info on growing onions can be found here: https://extension.umd.edu/resource/onions

Good luck. 

Hi again, 

I adjusted our watering schedule and didn’t see much of a change, so I pulled a few onions and unfortunately I think we have allium leaf minors as you suggested. I attached a few photos here.

Should I go ahead and pull all of our onions? Will the soil be affected, or will it be okay to plant something else in this space as long as it’s not in the onion family? 

Thank you,

Danielle 

Oh bummer.  You have a couple of options.  If most of the crop has not been affected, you could pull the ones that are and discard before the larva turn into adults and treat the unaffected ones with an insecticide or apply floating row cover to exclude the adult pest.  Additional information on Allium Leaf Miner. 

You most certainly could pull and discard everything and replant with anything that is not in the allium (onion) family. Since it is still pretty early in the growing season, you have sevearl options for what you could grow if you choose this option.  You should probably not plant anything in the allium family this fall either, as the pest could become active again in Sept. 

General info on Allium Leaf Miner. 

Good luck and reach out with additional questions.