I recently had this question myself and found it hard to find anything specifically addressing mushrooms. However, I was able to tweak another solution. Get some pickling lime (kalk) and some RO/DI H2O and make a THICK slurry (think sour cream) then place some on the mouth of the mushrooms (used a kid's medicine syringe, get 'em at local Rx). Depending on your flow you may want to adjust it while doing this. The slurry needs to remain on the mushroom as long as possible and I was absolutely blown away at how well it did stick (day+. . .even with my flow up and running, biggest problem was dripping, not blowing). Anyway, for the most part all of my targets were gone within a day or two, a couple budded but were re-attacked successfully. All said and done probably whacked 30 shrooms in a week.
NOTE: Kalk affects Alk, Ca, and PH so don't go crazy. Just do a few a day or whatever, just please monitor your parameters if you do this.
HTH,
Gerald
