I believe fungi that produce mycotoxins are found everywhere in small amounts not just on grains. Some mycotoxins (e.g. penicillin) are beneficial to humans.
Usually problems only occur when a fungus population gets large enough that the amount of mycotoxin produced is harmful to health. I believe the JAMA 2002 besides discussing concerns about long term exposure to low levels of mycotoxins in developing countries also discusses the guidelines the FDA has in place regarding some of the more problematic mycotoxins.
At the moment there is nothing in the research literature suggesting a link exists between mycotoxins and IBS.
There is evidence to support the notion that wheat alone - not just gluten can be a problem for some IBSers. Wheat contains fructans - long chains of fructose molecules that are known to be a problem for some GI disorders including IBS. Usually wheat doesn't have to be eliminated from the diet but the daily consumption has to be reduced. Over the years many people who have visited this board have tried the GF approach. Reports although sketchy suggest many didn't get much relief.
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS