spicy vegetables

Question

a)shulchan aruch says that a spicy vegetable such as tznon can turn an aino ben yomo knife into ben yomo (by making the pagum taam lishvach). are there any vegetables that we use today that are considered spicy or sharp ie onions, garlic or is it only tznon - what really is tznon? i read a translation that said black radish - is it correct? b)since shulchan aruch says you can taste the vegetable to see if tastes like milk and we know it never really tastes like milk because we have a good dishwasher, can we use the opposite knife lchatchila and not worry. c)what about a knife that was used to cut non-kosher and then you cut an onion with it. is this a problem?