Probably environmental stress. Heat & recently planted = water shortage in the top growth.

another way of saying what jean said.. is that individual cells burst due to some stress ... and a few days later.. those cells browned ...

those leaves will not recover.. but we watch the new ones.. and see if they are coming out clean.. and that would rule out bugs ...

if these came from a greenhouse.. with filtered light.. and were slammed into full sun in high heat.. they were not properly hardened off to garden culture ... aka full beating down sun ....

plus.. on top of that.. the roots were totally disturbed in the planting process ...

i have been known to hold stock.. for days.. or weeks waiting for a break in temps.. or rain.. or whatever..

never forget.. just because you buy it.. does NOT mean you have to run home and plant it immediately .. wait until the circumstances are a bit better..

ken

Thanks to both of you. I think you're right in that the plants are stressed and I'll be watching them carefully. Since my initial post, I've seen (and smashed) a few striped cucumber beetles on them.
PS I sowed & grew them in the greenhouse and hardened them off in the cold house. They were watered and pruned before being set out on a partly cloudy day, but the soil was pretty dry and I know I could've watered more thoroughly.
Thanks again for the input.
Janine