woke up to this...please help..pics

I have 5 plants in week 5 of veg on an ebb and flow table with a 400 watt MH above. I have been cycling my flood and drain table every 4hours since I started growing these. I woke up this morning and one of my blue rhino(the biggest plant i have)has one big fan leave that has completely wilted and even turned several shades darker, The pictures make it look way healthier than it actually looks in person. It is almost black on the wilted part.
my ph is 5.8
temp 75 humidity 40%
I am running my nutes at 15-10-5 using general hydro flora series
ppm 975@.5 conversion

I noticed some powdery mildew on lower leaves of some of the other plants and plan on using a sulfer burner to combat that. is this root rot? deficiency? will a sulfer burner hurt the plant anymore??

sorry for the book Im just concerned about the plant. This all happened suddenly overnight

milk bath for mildew. 10 to one any milk. safe and effective. did you spill something on your plants? klna looks like contact burn... im no expert just wanted to answer ya

yo that happened to me recently. my lights turn on at 11ish and i forgot to open the grow door to vent them, when i finally got around to it it was already 1:30 and the temp was over 100 so i just attributed it to heat stress(but i could be wrong). Have you figure out what the problem was yet?