28 plants started from seed in early March have led to very healthy 4' tall plants by early June. Had what I thought were Aphids (small black and white flies) early on but never so many for me to be overly concerned. What seems like overnight on most of my plants I'm seeing signs of this. Most older leaves look like the 2nd and 3rd picture. ONly a few leaves on one plant look like the 4th picture. I could go into detail about soil, growing conditions, weather, etc. but I'll just post some pictures and let me know if you have any questions. Thanks.

Looks like aphids. Are you seeing a larger population of ants too. Ants actually farm aphids. Lots of products out there to kill them, Strong squirt of the hose will help as well. I treated for ants as well as aphids when I got them outside, years ago.