It's hard to determine at that stage. I've seen that before sort of. The version i saw was more advanced than that is though. The 2 times ive seen that before it was deemed to be pH fluctuation in the soil, but in my case it was water culture. The other was nutrient burn. The nutrient burn was more brownish though whereas the pH fluctuations were a more greyish in colour. That's all i can tell you from my end.