Some of my sugar snap pea plants (Super Sugar Snap and Cascadia) are turning yellow. About 15%, here and there. The new leaves are affected, and also the stems that support those leaves. It's the whole leaf, not just the veins or between the veins.

I've ruled out over-watering and under-watering, as I have sandy soil and monitor soil moisture. I doubt there are nutrient issues, so I suspect cold nights (low thirties). Hopefully it's not some incurable disease. The plants are about seven inches tall (planted late).

I'm in inland Southern California. Has anyone encountered this problem?