it will allready have spread in fact. Pythium, the bacteria that causes root rot tends to spawn when temperature fluctuate. Basically, getting it during and because of heat issues and then solving those heat issues have spurred it so spawn trillions of spores which will be floating arround in your res. The rest of the plants will get it, and at 72 F you're borderline on res temps at best. Do a comeplete res change with regular water and get 35% H202 from a hydro sto lre. Use 1.7ml/L in your res and run it for a day at least, then swap res again and try nutes. If you have some $ get some hygrozyme at the hydro store too and add some at half strength. The H202 will kill surface bacteria and sterilize the inside of your system. The hygrozyme will eat any necrotic tissues and get rid of any dead roots and dead bacterias.
Root rot is the plant plague. You don't ever get "just a little bit" of root rot in a common res system. It will spread and kill all of your plants, or at the best stunt them horribly and lower their production very significantly. If you have a mother and clones and your only at the beginning of teh grow, restart. Starting fresh without any root rot will work best, and since you have now solved the temp issue if u use h202 it shouldn't come back again. Obviously we don't have any pictures, but if it's not super advanceded you can probably save these plants as long as you get some bacteria fighting shit in there now! Don't underestimate root rot, it is the worst thing that can happen to hydro plants unfortunately.
good luck
