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I know some people who are well knowledged in mushrooms and such, no one has suggested that mushrooms are toxic to other plants. They don't steal nutrients, they have extremely shallow root systems and they don't release toxins into the soil, unless they're cut open or broken. Even still those toxins are usually only harmful to humans and some other mammals/animals.
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I don't think the cause of your plants dying is the mushrooms, it was possibly coincidence. Ive seen all different kinds of mushrooms growing in my backyard, around my plants, in pots, in my seedling tray, under trees, in grass patches around the home etc. Ive also seen about 30 different toxic types of mushrooms growing in the wild around other plants, around and under trees. Very often they will grow in shaded, moist areas, around root systems of other plants. No time have I ever seen a dead plant in the vicinity or any negative affects come by it.