Mychoriza shows up best when the soil is dry. When wet it's almost transparent so you won't see it, even if it is present. Also be aware that, even though pines are adapted to pair with mych, they are not obligated. Mych in the wild is just an adaptation to help source nutrients when they are scarce or insoluble. Pines also have normal roots so, when nutrients are readily available, they can access all their needs directly from the soil.
Yellow needles after a heat wave is most likely dehydration. Pines don't usually show us they are thirsty by wilting like broad leaf trees and the foliage is tough so damage does not show immediately. Random yellowing is usually the tree getting rid of the least productive foliage to reduce transpiration - older foliage, any foliage in too much shade and sometimes needles taking the brunt of the hot sun are usually first to go.
Of course, the problem is that overwatering causes root damage which then leads to dehydration and the same symptoms as above.
day knocked over one of my original JBP from this thread. It's roots were partially exposed for an unknown number of hours. I put it back in it's pot. The next day, it happened again. Same tree.
