Yellowing Where Stem/Internode Meet. Pics Attached

Platinum Bubba kush vegging under T5's/R/O... 3 week veg in 4x4 rock wool AN:
Sinsi grow A n B
B-52
Tarantula
Pihrana
Voodoo
Sensi Cal
1/2 drops of super thrive

+ small amount of Nitrozime

20 gal rez... 80 deg and 40 hum... PH 5.7

Everything looked great, Green as all hell and now this all of a sudden this.. Any ideas? Nit def? Roots are white as hell out the bottom of the cubes but this yellow shit just popped up on the upper most leaves.





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