Jesse assumes it was ricin,he doesn`t know Walt has a lily of the valley plant.In fact,i think you might have just hit on something for the next episode.Think about it,we saw Jesse getting ready to torch Walts house,but in the flash forward in episode 9 (first one of this series) the house is in tact,just empty,so i`m wondering if Jesse doesn`t torch the place.Maybe he goes outside and see`s the lilly of the valley plant and then ..................well,who knows.