I think I have found an explanation for the symptoms. I remember to have seen similar symptoms, caused by volatiles that came from "fresh" plastics. These volatiles, plasticizers (weekmakers in Dutch) can cause symptoms of burning in leaves. I believe that the introduction of the plastic bags with oyster mushrooms is the reason that the leaves are burnt. This could easily be tested by sowing two pots with garden cress (tuinkers), covering them individually with a plastic bag, and putting a piece of the plastic from the mushroom bags in one of them. Garden cress is fast growing and sensitive, after a week the differences should show.
See also: http://www.agrocontrol.nl/UserFiles/File/Groen%20Agro%20Control%20nieuwsbrief%202011%20okt.pdf