[Sir_Ed]

But Brock wasn’t poisoned by the ricin, he was poisoned by the Morning Glory plant in Walt’s back yard, so why would Jesse think it was the ricin cigarette?

Ed

[E. Pluribus Unum]

Don't you remember Jesse telling his girlfriend that he thought Brock had been poisoned with ricin after he discovered that his ricin cigarette was gone? That's when the Tall and Taller FBI guys questioned him the first time. He beat up Walt accusing him of it, and Walt convinced him that Fring must have poisoned Brock because Fring had a history of allowing children to be killed. It wasn't until later that the doctors figured out that he had been poisoned by Lilly of the Valley.

Don't you remember the scene where Jesse went insane looking for the ricin cigarette? Walt created a dummy with salt instead of ricin in the vial. He went through an elaborate charade of "helping" Jesse find it when they miraculously "found" the dummy ricin cigarette in the Rumba robot vacuum cleaner?

Don't you remember Walt hiding the real ricin vial that he had stolen from Jesse in the electrical outlet in their bedroom?