Multiple diseases/deficiencies can cause yellowing leaves. The most common in my garden is chlorosis; a nutritional disease caused by soil problems, including high pH, or absolute iron deficiency. Sometimes simply adding iron isn't enough; pH should be corrected. Magnesium supplementation also helps.

Try this link for a start: Yellow Leaves

Are your plants in the ground or in containers?

Perhaps a soil test is in order.

Good luck!