It looks like it's really hot where you're at...no? The file: IMG_1916 is a good example of what heat burn looks like.
So, you can do 2 things. Add Epsom Salts (as your Mag. source) and also add SILICA . You can get it in liquid form...Bontanacre makes one called "Silica Blast" or something like that. But, there are also other ones too. DynaGrow's "Pro-TeKt" is another good one. Anyway, that will help with the heat tolerance.
Also, a couple-few inches of mulch is also KEY. That will help to keep you roots cool. They like it cool...homey.