I think its leaf spot fungus (LSF-Rust), or PH . However, It doesn't look like any other deficiencies are present that would lead me to believe its PH. Now if it's LSF, the rust you're seeing is a fungi that can be confusing and present itself as a nute lock out or nute burn issue, but it's typically a ventilation problem. The air in your grow is not being replaced well enough. This is usually a situation that involves inadequate exhaust, over abundant intake, and high humidity. Stagnant air and high humidity. Can't know for sure though with info given. If your PH has been ok, then maybe look into LSF.