OS X Mavericks has a global "Correct spelling automatically" setting under System Preferences > Keyboard > Text

I have this option unchecked. However, SourceTree's Commit Message textfield continues to automatically correct my spelling (even after restarting the app).

Red lines under misspelled words are great. However, I do not want the blue lines and iOS-like automatic spelling correction.

Other apps respect this global setting. If you change the global setting and then restart the app, the new spelling correction mode will take effect. This does not occur in SourceTree.

I am aware that the spelling correction can be turned off for SourceTree individually. But that's not a fix for this issue of not respecting the global defaults.

> In Lion this is no longer a system-wide option. Right-click on the commit text area, select Spelling and Grammar, and alter the Check / Correct options as appropriate. SourceTree 1.4 onwards remembers these settings and they're remembered independently of any other application.

https://answers.atlassian.com/questions/58370/how-to-disable-spelling-correction-in-sourcetree-commit-messages

If it was missing from Lion, the global setting is definitely back in 10.9 Mavericks. SourceTree should use it.