The answer to your question is actually quite simple. If there was a sudden decrease of Fungi the environment would be filled with more dead things. Not just that, but it would also smell terribly. Just think of it like this: Fungi is basically like garbage collectors. They will recycle the stuff that you dont need and or want. ~Hope this helped! :)
If there was a sudden decrease of fungus in a specific area, the impact this would have on the plant populations within this environment is that it would be filled with more dead things and would smell like garbage.
