Is it only on lower leaves or higher up on the plant? If only lower, this might be normal and nothing to be concerned about.

Others who have more expertise in deficiency will have more to contribute.

Jenn

Jenn is right - you are developing interveinal chlorosis - you can Google it for lots of info -which is a nutrient issue. If only on the older bottom leaves it is common as they age and die. if higher up the plant it indicates magnesium deficiency (consider a mild dose of diluted epsom salts). Your plants appear to be in containers so regular nutrient supplements are needed to replace what leaches out each time you water.

Dave

Before anybody commented, I read the post and thought it is some sort of Nutes deficiency but refrained from commenting. I had a mortgage lifter ,last season that displayed the same look, while others next to it were fine. I kept regularly spraying with fungicide but nothing happened. The newer growth/leaves were fine.
I would take Dave's advice and won't worry too much.

Sey