There are zillions of fungi that infect plants. Many are specialized pathogens that feast on one particular plant or tree species. Smut, known scientifically as Ustilago, has a particular taste for corn. Corn smut is a really horrible looking black blob that grows right on cobs of living corn. It's a delicacy in Mexico.
Fungal infections can grow fast inside trees.
Fruiting bodies do produce fruit, of a sort - they make more spores. These spores go on to establish new sites of decay. They're not always easy to see, either.