When my corn allergy became more sensitive, I found that I started reacting to mushrooms purchased from the grocery store. But I could eat them if I grew them myself, or purchased them from a cultivator at the farmers’ market who grew his mushrooms on sawdust.
It’s been suggested by another mushroom cultivator  that I could just be allergic to Agaricus mushrooms, but I react to shiitakes grown on corn medium and don’t react to shiitakes grown on a straw medium, so there’s evidence that the medium does matter, at least for me. I suspect the reaction is more due to cross contamination rather than allergens actually making it into the fruiting body.

Portobellos, buttons, and criminis are the same species of mushroom in different stages of maturity and they have to grow on compost. That can mean just about anything, as long as it’s rotted. I don’t know what the smaller operations will find to be useful or cheap for compost but I am positive that the BIG guys use some kind of corn medium because I react to the mushrooms, and actually within the last year or so I have begun to have airborne reactions from being on the same block as some of the bigger agaricus growing operations.

Other mushrooms like oyster mushrooms, shiitakes, morels, basically the “weirder” mushrooms all grow on cellulose, so typically wood cellulose and grains such as barley and rye, but could also be corn cobs, kernels, or husks.

The mushrooms will not be rinsed with anything generally as water will make them slimy but I imagine that they could *possibly* be sprayed with some kind of preservative. I have no evidence that anyone does so, it just seems possible. The trays the mushrooms come in could contain corn fibers. If the mushrooms are covered with plastic, the plastic could be corny or could be dusted with corn starch.

A nice article on how different types of mushrooms are grown. 

Another possible issue if you are purchasing mushrooms from some vendors is the baskets they come in: Those green composite baskets cause problems for a lot of people.

Cross Contamination With Gluten and Other Allergens

For a long time I was able to eat mushrooms from a farmers market vendor who grows theirs on barley and sawdust. Unfortunately I began to have a gluten (but not corn) reaction to the mushrooms, intermittently. I think the issue is cross contamination with the growth medium. Not sure if the cross contamination is handling/storage of the mushrooms and growth medium together, or if it’s coming through in the mycelium. I don’t think that it’s in the actual fruiting body of the mushroom, but I do think that it’s possible the mycelium still contains traces of gluten, especially in a short growth cycle where it’s possible the mycelium of the mushroom is not completely consuming the grain and associated gluten.

Growing Your Own Edible Mushrooms

I’m busy right now now with other priorities so I’m just not eating mushrooms, but I do plan to begin growing my own soon. In the past I have had great luck with mushroom growing kits and sawdust spawn from Fungi Perfecti.  The Oyster Mushroom kits are darn easy even for the total newbie, but they also do *some* phone tech support if you need it. Generally it’s best to hit up the blogs and forums, and perhaps even buy a book or two to get you started.

Note that I have not looked at the “corn safety” of the plastic sheeting or plastic bags of their kits, just know that they were fine for me when I was medium-sensitive. I bet they would be happy to answer questions about their materials. When I got more sensitive I was already pretty experienced at growing mushrooms. Rather than getting a pre-made kit,   I just put together my own safe materials for a fruiting chamber and got some sawdust spawn to colonize my own safe growth medium.

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