Why Was Brock Poisoned?

At first, Jesse believed that Brock was given the ricin that was intended for Gus Fring; he thought that Walt stole the ricin and gave it to Brock as a way to punish Jesse for getting too close with Gus. ... In the sad reality, Jesse was right; Walt did cause Brock's illness as a way to turn Jesse against Gus.

How did Jesse find out that Walt poisoned Brock?

In the season 5 episode Confessions Jesse realises that Saul and Huell took his ricin cigerette on the orders of Walt (Jesse finds this out by threatening them with a gun) which causes him to also realise that Walter was behind the whole false story of Gus poisoning Brock when it had been him all along.

Who poisoned Brock in Breaking Bad Season 4?

Jesse stopped trusting Walt after he poisoned Brock

One of the biggest turning points occurs when Jesse finally figures out it was Walt who poisoned Brock, not Gus Fring. Walt's treachery causes Jesse to stop trusting him and permanently damages their relationship for the rest of the series.