Are you by any chance watering them with water from a water softener? Looks like serious salt burn.

If it's not water from a softener or from fertilizer burn, my immediate thought was lack of water.
Imo, once a week in those pots, in high heat is not enough. They are frying.

You're in zone 9, but you don't say what state. That could make a big difference.
Here in zone 9, California, we've had very hot and very dry weather the last few months, especially the last few weeks, with temps in the mid to high 90*F, some days being quite windy. Wind and high temps will suck the moisture right out of the pot within hours.
In that kind of heat I'm watering every 3rd to 2nd day, and checking the soil moisture everyday.

In those black pots, they can dry out very quickly, and even when moist the pot can get very hot and literally steam the roots.
In the nursery they get around this by either watering everyday, or if they don't the plant will often burn like this. Then you won't see them, as the staff will take them off the floor and throw them in the dumpster. I know as I work at a nursery. We often burn them too from lack of water.

We get credit on dead plants from growers and we write them off. So just because we use the black plastic pots does not mean they are a good pot to use indefinitely. They are meant to be temporary until planted properly.

When I have to leave them in the black pots for any length of time, I either slip them into a lighter colored pot or wrap some thick paper or cardboard around them. Anything to get the direct sun off the pot.

Imo, you need to monitor you soil moisture more closely and likely water more often, and somehow shield the pots themselves from direct sun. (not the trees, just the pots them selves.)

And I'd either get them in the ground asap, if that's your plan, or re-pot them with a good porous mix (search this forum for container soil recipes/mixes you'll find a lot of good info on what to use), and into a larger, lighter colored pot, if keeping them in a container is your plan.

I water mine daily in a fast-draining soil. Once a week in hot weather (as mentioned) is not enough. Basically, what BarbJP said is right. Search for 5-1-1 or gritty mix for a better potting medium. Also, when I moved plants into larger containers it helped regulate temperature.