Another photo.

Perhaps heat stress?
It's been quite warm here during the past few days.
Hot sun that hits a black plastic pot can heat the soil enough to kill roots.

Oh, and how long have they been in the pots?

Thanks for the response! They've been in pots since early spring 2013. They are already a few years old and produced for me last year. I did water them during our heat wave last week. They are in resin gray pots and drain well.