Hi - The speckling symptom on the leaves is indicative of a spider mite infestation. Spider mites are very tiny (like specks of ground pepper) and you would see them on the undersides of the leaves. They suck on the plant sap, which results in the yellow spots you see on the upper surfaces. Spider mites tend to be a problem mostly under very hot and dry conditions.

Here is our web page about spider mites on vegetables and what you can do.
https://extension.umd.edu/hgic/topics/spider-mite-vegetables

Christa