Pest issues would be my first guess from the photos - maybe spider mites. Possibly squash bug nymphs. The large mottled leaf in the first pic especially and the black spotted on in the second. Have you ruled out any pests?

Next best guess would be one of the bacterial diseases like bacterial speck or spot.

The new growth leaves look ok so if they were mine I'd carefully inspect for bugs (squash bugs and eggs and spider mites on the underside of the leaves), be sure to remove all the damaged/infected leaves close to the base, cover the cuts with soil, and consider giving them a spray of fungicide if new symptoms show on the new growth.

Dave

I think Dave covered all of your questions but just wanted to point out that you look like you have a zucchini ready to pick (maybe 2?) in the first photo. Hard to tell how big they are but they taste way better when small-medium in size.

Thanks. Yes I have one about ready to pick. Funny enough the worst looking bush is the only one with fruit ready to pick. I'll inspect the plants again and see if I find anything.

A follow up to my original post, it seems like two bushes are really only affected by this. They happen to be the same variety which I direct sowed myself into cell packs back in end of April and transplanted. They were slow to grow and they didn't seem too healthy to be honest. The other varieties I planted are doing really well (some were purchased seedlings, others were also direct sown but different variety). So I am hoping this won't spread to the other bushes. I'll be yanking them out this weekend if they don't improve. I'll also spray with neem.

I inspected the undersides of the leaves (which I normally do anyways on a regular basis), and I didn't find any eggs. I couldn't see any other pests.

Somewhat related... the only real pest I have in my garden this year (so far at least) has been roly polys. I know that most folks think they are harmless, but they are decimating my pole beans before they set their second set of leaves. I caught them in the act a few times. I've had to use cellpacks and sow pole beans away from the beds to try to thwart them. They would latch on the newly emerged bean sprouts and go to town. They even chewed down some newly planted basil seedlings. In a few weeks when the japanese beetles show up I'll have something else to complain about.

Interesting about roly polys. But yes, they are not a pest for mature plants, but they can go to town on seedlings.