might be a root zone problem. Heat won't cause those spots. if you don't let top 1" dry, start doing so.. 20% runoff waste and repeat. avoids buildup, keeps roots happy. I am not familiar with your ferts, but if you can look into their labels and deduce PPM of what you feed is in a normal range, this is my best guess.. and just a guess. do google and look for more info in addition to questions here.
(also, not elgible for the GotM contest, so if you like my solution, do give the 'answer' to someone else, please)