The mishna on the previous amud had stated that anything can be used for an eruv, except for water and salt (melach). The gemara later says that mushrooms and truffles also can't be used.
The question we raised was this: Rashi on the mishna says that the reason water and salt can't be used for an eruv is because they provide no nourishment, and a dirah must be a place of eating. The reason we can't buy water and salt with ma'aser goes along with the same reasoning that water and salt are components that come from themselves. Anything else, like bread, has a source from wheat/flour/water, etc. Water and salt don't have a source. By someone who makes a neder against eating, they can still have water and salt because it provides no nourishment.
Aside from the obvious question about water not providing nourishment, what common denominator do mushrooms have with the water and salt that we can't use them for an eruv? True, mushrooms can grow on rocks and don't have roots, but they still need some moisture to be able to grow.
Ritv"a seems to have a weak answer that since people don't commonly eat mushrooms, they're included with the water and salt.
GR"A has a much more shocking answer. If you look in the hagahot haGR"A, he adds in, "water, salt, etc." He therefore adds in many more things than just water and salt. (our maggid shiur said that this is an example of the GR"A arguing on many rishonim, who all keep the girsa at water and salt.)