United States v. Gordon, No. 3:04CR00023, 2010 WL 4974567 (W.D. Va. Dec. 2, 2010):
The Brady rule states that suppression by the prosecution of evidence favorable to an accused (both exculpatory evidence and impeachment information) violates due process where the requested evidence is material either to guilt or to punishment. The rule applies at trial and exists to preserve the fairness of a trial verdict and to minimize the chance that an innocent person would be found guilty. The guilty plea context, however, does not present the same fairness concerns because the defendant's guilt is admitted. Accordingly, the prosecution's failure to disclose favorable, material impeachment information prior to a guilty plea does not establish a constitutional violation on which the defendant can invalidate his plea (the Ruiz rule).
In other words, Brady applies to all favorable trial evidence while Ruiz applies only to impeachment information prior to a guilty plea. A circuit split exists regarding whether whether the Brady right to exculpatory information, in contrast to impeachment information, might be extended to the guilty plea context. See United States v. Moussaoui, 591 F.3d 263, 286 (4th Cir. 2010).