Been studying plant-fungal interactions for about 10 years, including a master's degree I dropped out of and never actually finished* so I'm full of fungus facts i don't really get to use ever.
*Actually did all the course work and lab work but didn't finish my thesis in time
There's some amount of really basic, "I (plant) don't need shit from you, I'm going to direct my sugar somewhere else... oh really? I'll send my nutrients somewhere else then" going on, but I know there's more complex signalling going on too. For example, there are "cheater" (or "parasitic") mycorrhizae that somehow convince their host to give them sugar without actually giving them anything in return (and can even reroute the carbon and stuff they get to other plants they're connected to). I'll have to read up on all that again. Good question!