maybe if @Aqua Man is available he will help out..
Hey brother hope all is well.
OP can give give details of the following (please answer all of them fully)
Source water ppm?
Nutrients used specifically how much of each and in what order are you mixing them?
Any other additives used?
When and where did this start on the plant?
Pot size?
Volume of water given per fertigation and frequency?
Hiw are you removing runoff?
How do you determine when to water?
Light make, model and distance?
Assuming thus is an autoflower?
Temp and humidity day and night?
Air exchange equipment and if not 24/7 the the intervals your using.
I know it's alot and there may be more but garbage info in means garbage info out so details matter