If you don’t have a growlog please answer these questions.
What strain are you growing? Is it Auto, Fem or Reg?
Are you growing Indoor or outdoor?
Are you using nutrients? Which ones? How much?
White widow
Reg
Auto
Out door
What Medium? nutrition? Plant age? How much/ often you water?
What does the whole plant look like? Because its hard to say forsure on a leaf. Is it soft or dry? If its soft then its possible over watering, which can cause nutrition lockout. Hence why its a yellow color.
Without more info it could be a long number of things.