The key is crop diversity because brassica plants (cabbage, broccoli, brussel sprouts, etc) are not helped by mycorhizal fungi at all, and in abundance it might

The key is crop diversity because brassica plants (cabbage, broccoli, brussel sprouts, etc) are not helped by mycorhizal fungi at all, and in abundance it might even be detrimental to them. Some mycorhizal fungi types will destroy some species of plants in order to benefit their species of choice, going so far as to tap directly into other plant roots to use them as nutrition.