Soil temp? Lots of rain lately? Are the plants in-ground or in containers? Mulched?

If you will search "yellow leaves" here you'll find that 95% of the time it is due to excess watering by the gardener or excess rain thanks to Mom Nature and/or too cold and wet soil. The leaves are dying.

If you can, let the plants dry out. If in containers move them out of the rain access, if in the ground dig a shallow trench to divert the water and pull back mulch and pray for sun - something we haven't seen much of here lately.

The yellow leaves will die and fall off but new growth will follow.